easa - Module 5

PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS

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MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS (DCAM PART 66 CATEGORY B1.1/B2)

For Training Purposes Only

Issue 2 Revision 0 Oct 2011 Page i


WARNING


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This document is intended for the purposes of training only. The information contained herein is as accurate as possible at the time of issue, and is subject to ongoing amendments where necessary according to any regulatory journals and documents. Where the information contained in this document is in variation with other official journals and/or documents, the latter must be taken as the overriding document. The contents herein shall not be reproduced in any form without the expressed permission of ETD


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TABLE OF CONTENT 5.1 ELECTRONICS INSTRUMENT SYSTEM (DCAM Ref 5.1) Level 3……………………………………………………………………………….……….. 1

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5.1.1 Introduction………………………………………………………………………………………………………………………………………………………. 1 5.1.2 Integrated Digital Avionics System…………………………………………………………………………………………………………………………….. 4 5.1.3 Electronics Flight Instruments System (EFIS)………………………………………………………………………………………………………………… 6 5.1.4 Electronic Centralized Aircraft Monitoring (ECAM)……………………………………………………………………………………………………………14 5.1.5 Engine Indicating and Crew Alerting System (EICAS)………………………………………………………………………………………………………. 16 5.1.6 Flight Management System (FMS)…………………………………………………………………………………………………………………………….. 18 5.1.7 Cockpit Layouts…………………………………………………………………………………………………………………………………………………...20


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5.2 NUMBERING SYSTEMS (DCAM Ref 5.2) Level 2………………………………………………………………………………………….…..……………. 24 5.2.1 Decimal System………………………………………………………………………………………………………………………………………………….. 24 5.2.2 The Binary System………………………………………………………………………………………………………………………………………………. 26 5.2.3 The Octal System…………………………………………………………………………………………………………………………………………………38 5.2.4 Hexadecimal System……………………………………………………………………………………………………………………………………………..44 5.2.5 Binary Coded Decimal……………………………………………………………………………………………………………………………………………49 5.2.6 Adding……………………………………………………………………………………………………………………………………………………………...52 5.2.7 Subtraction………………………………………………………………………………………………………………………………………………………... 52 5.3 DATA CONVERSION (DCAM Ref 5.3) Level 2…………………………………………………………………………….…………………………………..56 5.3.1 Digital and Analogue Computers………………………………………………………………………………………………………………………………. 56 5.3.2 Digital Bus………………………………………………………………………………………………………………………………………………………… 59 5.3.3 Typical Digital System Inputs…………………………………………………………………………………………………………………………………… 60 5.3.4 Typical Digital Systems Outputs………………………………………………………………………………………………………………………………...62 5.3.5 Digital to Analogue / Analogue to Digital Conversion………………………………………………………………………………………………………... 64


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5.4 DATA BUSES (DCAM Ref 5.4) Level 2…………………………………………………………………………………………………………………………90 5.4.1 What is Data Bus?.................................................................................................................................................................................................... 91 5.4.2 About ARINC……………………………………………………………………………………………………………………………………………………... 93 5.4.3 Data Encoding……………………………………………………………………………………………………………………………………………………. 93 5.4.4 ARINC 429………………………………………………………………………………………………………………………………………………………... 97 5.4.5 MIL-STD-1553B………………………………………………………………………………………………………………………………………………….. 104 5.4.6 ARINC 629………………………………………………………………………………………………………………………………………………………... 106 5.4.7 Ethernet…………………………………………………………………………………………………………………………………………………………… 122


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5.5 LOGIC CIRCUITS (DCAM Ref 5.5) Level 2……………………………………………………………………………………………………………………..128 5.5.1 Logic Gates……………………………………………………………………………………………………………………………………………………….. 128 5.5.2 Electronic Gates in Circuit……………………………………………………………………………………………………………………………………… 138 5.5.3 Gate Application in Aircraft Systems……………………………………………………………………………………………………………………………150 5.5.4 Tri-state Devices…………………………………………………………………………………………………………………………………………………. 154 5.6 BASIC COMPUTER STRUCTURE (DCAM Ref 5.6) Level 2…………………………………………………………………………………………………156 5.6.1 Central Processing Unit…………………………………………………………………………………………………………………………………………. 157 5.6.2 Memories…………………………………………………………………………………………………………………………………………………………. 160 5.6.3 Magnetic Memory Types…………………………………………………………………………………………………………………………………………162 5.6.4 Solid State Memories……………………………………………………………………………………………………………………………………………. 172 5.6.5 Storage Capacity………………………………………………………………………………………………………………………………………………….179 5.6.6 Highway Structure……………………………………………………………………………………………………………………………………………….. 179 5.6.7 Instruction Words………………………………………………………………………………………………………………………………………………… 180 5.7 MICROPROCESSORS (DCAM Ref 5.7) Level 2………………………………………………………………………………………………………………181 5.7.1 Microprocessor (CPU) Structure and Operation……………………………………………………………………………………………………………… 181


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5.8 INTEGRATED CIRCUITS (DCAM Ref 5.8) Level 2…………………………………………………………………………………………………..……….184 5.8.1 Adders…………………………………………………………………………………………………………………………………………………………….. 184 5.8.2 Flip-Flops…………………………………………………………………………………………………………………………………………………………. 192 5.8.3 Counters…………………………………………………………………………………………………………………………………………………………... 197 5.8.4 Decoders………………………………………………………………………………………………………………………………………………………….. 200 5.8.5 Encoders………………………………………………………………………………………………………………………………………………………….. 210

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5.9 MULTIPLEXING (DCAM Ref 5.9) Level 2…………………………………………………………………………………………………………………..…..216


5.9.1 Multiplexers……………………………………………………………………………………………………………………………………………………….. 216 5.9.2 De-multiplexer……………………………………………………………………………………………………………………………………………………. 220 5.10 FIBRE OPTICS (DCAM Ref.5.10) Level 2…………………………………………………………………………………………………………………….224 5.10.1 Definition of Fibre Optics………………………………………………………………………………………………………………………………………. 224 5.10.2 Fibre Optic Data Links…………………………………………………………………………………………………………………………………………. 224 5.10.3 Fibre Optic Systems……………………………………………………………………………………………………………………………………………. 226 5.10.4 Advantages and Disadvantages of Fibre Optics……………………………………………………………………………………………………………. 228 5.10.5 Frequency and Bandwidth…………………………………………………………………………………………………………………………………….. 230 5.10.6 Fibre Optic Concepts……………………………………………………………………………………………………………………………………………232 5.10.7 Transmission of Light Through Optical Fibres………………………………………………………………………………………………………………. 236 5.10.8 Basic Structure of an Optical Fibre…………………………………………………………………………………………………………………………… 242 5.10.9 Propagation of Light along a Fibre…………………………………………………………………………………………………………………………… 244 5.10.10 Fibre Types……………………………………………………………………………………………………………………………………………………. 253 5.10.11 Properties of Optical Fibre Transmission……………………………………………………………………………………………………………………260 5.10.12 Choice of Fibres………………………………………………………………………………………………………………………………………………..274 5.10.13 Joining Optical Fibre………………………………………………………………………………………………………………………………………….. 275 5.10.14 Aircraft Fibre Optic Networks………………………………………………………………………………………………………………………………… 279 5.10.15 Applications in Aircraft Systems……………………………………………………………………………………………………………………………...284 5.10.16 Fibre Optic Testing……………………………………………………………………………………………………………………………………………. 287


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5.11 ELECTRONIC DISPLAYS (DCAM Ref.5.11) Level 2……………………………………….………………………………………….……………………288 5.11.1 Light Emitting Diodes (LEDs)…………………………………………………………………………………………………………………………………..288 5.11.2 Liquid Crystal Displays (LCDs)………………………………………………………………………………………………………………………………...290 5.11.3 Cathode Ray Tube (CRT)………………………………………………………………………………………………………………………………………294 5.11.4 Principle of the Colour CRT…………………………………………………………………………………………………………………………………… 302

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5.12 ELECTROSTATIC SENSITIVE DEVICES (DCAM Ref.5.12) Level 2………………………………….…………….…………………………………….304 5.12.1 What is an Electrostatic Discharge?....................................................................................................................................................................... 304 5.12.2 Typical Electrostatic Voltages…………………………………………………………………………………………………………………………………. 306 5.12.3 ESDS Equipment and Material Handling……………………………………………………………………………………………………………………. 310


5.13 SOFTWARE MANAGEMENT (DCAM Ref.5.13) Level 2……………………………..……………………………………………………………..………316 5.13.1 Introduction……………………………………………………………………………………………………………………………………………………… 316 5.13.2 Requirements…………………………………………………………………………………………………………………………………………………… 317 5.14 ELECTROMAGNETIC ENVIRONMENT (DCAM Ref.5.14) Level 2………………………………………………………………………………………..320 5.14.1 General…………………………………………………………………………………………………………………………………………………………...320 5.14.2 Lightning versus Static Discharge………………………………………………………………………………………………………………………….… 322 5.14.3 Electromagnetic Interference (HIRF)………………………………………………………………………………………………………………………… 324 5.14.4 Lightning Strikes……………………………………………………………………………………………………………………………………………….. 325 5.14.5 Protection………………………………………………………………………………………………………………………………………………………... 326 5.14.6 Maintenance…………………………………………………………………………………………………………………………………………………….. 326 5.14.7 Static……………………………………………………………………………………………………………………………………………………………... 327 5.15 TYPICAL ELECTRONIC/DIGITAL AIRCRAFT SYSTEMS (DCAM Ref.5.15) Level 2……………………………………………………..……………334 5.15.1 On Board Maintenance and BITE……………………………………………………….……………………………………………………………………. 334 5.15.2 ARINC Communication Addressing and Reporting System (ACARS)…………………………………………………………………………………… 342 5.15.3 Electronic Centralised Aircraft Monitors (ECAM)…………………………………………………………………………………………………………….344 For Training Purposes Only

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5.15.4 Electronic Flight Instrument Systems (EFIS)…………………………………………………………………………………………………………………354 5.15.5 Engine Indicating and Crew Alerting System (EICAS)……………………………………………………………………………………………………. 372 5.15.6 Fly-By-Wire……………………………………………………………………………………………………………………………………………………… 396 5.15.7 Flight Management Systems (FMS)………………………………………………………………………………………………………………………….. 416 5.15.8 Global Positioning Systems (GPS)…………………………………………………………………………………………………………………………… 422 5.15.9 Inertial Reference Systems (IRS)…………………………………………………………………………………………………………………………….. 438 5.15.10 Traffic Alert Collision Avoidance System (TCAS)…………………………………………………………………………………………………………. 464 5.15.11 Integrated Modular Avionics (IMA)………………………………………………………………………………………………………………………….. 476 5.15.12 Cabin Systems………………………………………………………………………………………………………………………………………………… 480 5.15.13 Information Systems………………………………………………………………………………………………………………………………………….. 496

f o g y n r i r a t e e e i n r i p g o n r P SE A M For Training Purposes Only

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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1 ELECTRONIC INSTRUMENT SYSTEM (EIS) 5.1.1 Introduction In a modern aircraft, the cockpit crew is provided with many different information. To handle this amount of data with a minimum of manpower and ’in time’; it is necessary to reduce the number of indicators in the cockpit, i.e. no longer using a separate indicator for each system/sensor.


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Figure 1.1 Cockpit of Conventional Design (Separate indicator for each system/sensor)


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) This is achieved by displaying all data from the various systems on some few ‘multi-purpose indicators’. A system which handles the flow and display of data of many or all aircraft systems is called an ’integrated digital avionic system’. The advantages of such a system are - it provides a better overview over the actual condition of each connected system - it reduces the flight crew’s workload - less time (and money) is required for maintenance - it saves electrical power consumption - the weight is reduced (wiring, units).


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Figure 1.2 Cockpit with EFIS (Multi-purpose Indicators)



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) A digital avionic system replaces the former instrumentation by an electronic flight instrument system (EFIS) and/or an engine indication and crew alerting system (EICAS). This includes the change from old analogue equipment to modern systems, because digital outputs are required.


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Figure 1.3 ‘Fully Integrated’ Cockpit Design with EFIS and EICAS



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1.2 Integrated Digital Avionics System The new generation of aircraft avionics is highly integrated. It saves electrical power and weight. It provides high accuracy and reliability. The level of integration varies (whether only the main avionic systems are combined or all), depending on the type of aircraft.

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An essential function of an integrated digital avionic system is the exchange of information between subsystems and/or between line replaceable units (LRU) within a subsystem. Note: A line--replaceable unit (LRU) (also called ’black box’) is a unit, which can be replaced at flight--line level in a short time.

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To incorporate a fully integrated digital avionic system and to ensure the required information exchange, a digital data bus is required. This bus is used to provide a 2-way interface between various computers, sensors and indicators. The interface between computers and/or external devices (e.g. transceivers, receivers) is accomplished via the digital data bus. Data may travel ’one way’ or in 2 directions, depending on the system design.


Figure 1.4 Conventional and Integrated Designs



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) Typically, the data bus is a serial bus on which the data are transmitted sequentially, i.e. one word after the other. A serial bus is commonly used for longdistance data transmissions (more than 50 m) as required in large aircraft. The bus is made up of a twisted pair of wires which are shielded and jacketed. The shielding is grounded at all terminal ends and breakouts to keep bit distortion at a low level. Shield grounding and high voltage spike protection within the individual data--receiving components ensure accurate transmissions.

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Transmission of data within micro-computers, between micro-computers and external devices as well as between other components can be accomplished with 8-, 16-, 32- and 64-bit digital data words, depending on the system layout.

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Some of these information are in the form of discrete data. Typically, these data are formed by switching between +28 V DC and open (or between ground and open). Discrete data are carried on a single wire.


This type of information is used for annunciators, warnings and wherever simple ‘condition information’ is sufficient. This is a small portion of the total information interchange.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1.3 Electronics Flight Instruments System (EFIS) A basic EFIS system is more than an Electronic Flight Director with Electronics Attitude and Direction Indicators (EADIs) or Primary Flight Displays (PFDs) and Electronic Horizontal Situation Indicators (EHSIs) or Navigation Displays (NDs) as display units. A typical EFIS system consists of two or more display units and their associated drive and control units. 5.1.3.1 Electronic Attitude and Direction Indicator The electronic attitude direction indicator (EADI - see Fig. 1.5) is normally comprises: • • • • • • • •

an attitude indicator a fixed aircraft symbol pitch and bank command bars a glide slope indicator a localizer deviation indicator a slip indicator flight mode anunciator various warning flags


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The aircraft' s attitude relative to the horizon is indicated by the fixed aircraft symbol and the flight command bars. The pilot can adjust the symbol to one of three flight modes. To fly the aircraft with the command bars armed, the pilot simply inserts the aircraft symbol between the command bars. The command bars move up for a climb or down for descent, roll left or right to provide lateral guidance. They display the computed angle of bank for standard-rate turns to enable the pilot to reach and fly a selected heading or track. The bars also show pitch commands that allow the pilot to capture and fly an ILS glide slope, a pre-selected pitch attitude, or maintain a selected barometric altitude. To comply with the directions indicated by the command bars, the pilot manoeuvres the aircraft to align the fixed symbol with the command bars. When not using the bars, the pilot can move them out of view. The glide slope deviation pointer represents the centre of the instrument landing system (ILS) glide slope and displays vertical deviation of the aircraft from the glide slope centre. The glide slope scale centreline shows aircraft position in relation to the glide slope.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) The localizer deviation pointer, a symbolic runway, represents the centre of the ILS localizer, and comes into view when the pilot has acquired the glide slope. The expanded scale movement shows lateral deviation from the localizer and is approximately twice as sensitive as the lateral deviation bar in the horizontal situation indicator. The selected flight mode is displayed in the lower left of the EADI for pitch modes, and lower right for lateral modes. The slip indicator provides an indication of slip or skid indications.


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Figure 1.5 A typical EADI display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1.3.2 Electronic Horizontal Situation Indicator The electronic horizontal situation indicator (EHSI) assists pilots with the interpretation of information provided by a number of different navigations aids. There are various types of EHSI but essentially they all perform the same function. An EHSI display (see Fig. 1.6) can be configured to display a variety of information (combined in various different ways) including: • • • • • •

heading indication radio magnetic indication (RMI) track indication range indication wind speed and direction VOR, DME, ILS or ADF information.


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Figure 1.6 A typical EHSI display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1.3.3 Flight Director Systems The major components of a flight director system (FDS) are the electronic attitude and direction indicator (EADI) and electronic horizontal situation indicator (EHSI) working together with a mode selector and a flight director computer.

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The FDS combines the outputs of the electronic flight instruments to provide an easily interpreted display of the aircraft' s flight path. By comparing this information with the pre-programmed flight path, the system can automatically compute the necessary flight control commands to obtain and hold the desired path. The flight director system receives information from the: • • • • •


attitude gyro VOR/localizer/glide slope receiver radio altimeter compass system barometric sensors

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The flight director computer uses this data to provide flight control command information that enables the aircraft to: • • • • •

fly a selected heading fly a predetermined pitch attitude maintain altitude intercept a selected VOR track and maintain that track fly an ILS glide slope/localizer



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) The flight director control panel comprises a mode selector switch and control panel that provides the input information used by the FDS. The pitch command control pre-sets the desired pitch angle of the aircraft for climb or descent. The command bars on the FDS then display the computed attitude to maintain the pre-selected pitch angle. The pilot may choose from among many modes including the HDG (heading) mode, the VOR/LOC (localizer tracking) mode, or the AUTO APP or G/S (automatic capture and tracking of ILS and glide path) mode. The auto mode has a fully automatic pitch selection computer that takes into account aircraft performance and wind conditions, and operates once the pilot has reached the ILS glide slope. Flight director systems have become increasingly more sophisticated in recent years.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1.3.4 Primary Flight Display (PFD) The typical EFIS PFD is a multicolour cathode ray tube (CRT) or liquid crystal display (LCD) display unit that presents a display of aircraft attitude and flight control system commands including VOR, localizer, TACAN (Tactical Air Navigation), or RNAV (Area Navigation) deviation together with glide slope or preselected altitude deviation. Various other information can be displayed including mode annunciation, radar altitude, decision height and excessive ILS deviation.


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Figure 1.7 EFIS primary flight display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1.3.5 Navigation Display (ND) Like the EFIS PFD, a typical EFIS ND takes the form of a multicolour CRT or LCD display unit. However, in this case the display shows the aircraft' s horizontal situation information which, according to the display mode selected, can include compass heading, selected heading, selected VOR, localizer, or RNAV course and deviation (including annunciation or deviation type), navigation source annunciation, digital selected course/desired track readout, excessive ELS deviation, to/from information, distance to station/waypoint, glide slope, or VNAV deviation, ground speed, time-to-go, elapsed time or wind, course information and source annunciation from a second navigation source, weather radar target alert, waypoint alert when RNAV is the navigation source, and a bearing pointer that can be driven by VOR, RNAV or ADF sources as selected on the display select panel. The display mode can also be set to approach format or en-route format with or without weather radar information included in the display.


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Figure 1.8 EFIS navigation display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1.3.6 Display Select Panel (DSP) The display select panel provides navigation sensor selection, bearing pointer selection, format selection, navigation data selection (ground speed, time-togo, time, and wind direction/speed), and the selection of VNAV (if the aircraft has this system), weather, or second navigation source on the ND. A DH SET control that allows decision height to be set on the PFD is also provided. Additionally, course, course direct to, and heading are selected from the DSP. 5.1.3.7 Display Processor Unit (DPU)

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The display processor unit provides sensor input processing and switching, the necessary deflection and video signals, and power for the electronic flight displays. The DPU is capable of driving two electronic flight displays with different deflection and video signals. For example, a PFD on one display and an ND on the other.


5.1.3.8 Weather Radar Panel (WXP)

The weather radar panel provides MODE control (OFF, STBY, TEST, NORM, WX, and MAP), RANGE selection (10, 25, 50, 100, 200 and 300 nm), and system operating Controls for the display of weather radar information on the MFD and the ND when RDR is selected on the MFD and/or the DSP. 5.1.3.9 Multifunction Display (MFD)

The multifunction display takes the form of another multicolour CRT or active-matrix LCD display unit. The display is normally mounted on the instrument panel in the space provided for the weather radar (WXR) indicator. Standard functions displayed by the unit include weather radar, pictorial navigation map, and in some systems, check list and other operating data. Additionally, the MFD can display flight data or navigation data in case of a PFD or ND failure. 5.1.3.10 Multifunction Processor Unit (MPU)

The multifunction processor unit provides sensor input processing and switching and the necessary deflection and video signals for the multifunction display. The MPU can provide the deflection and video signals to the PFD and ND displays in the event of failures in either or both display processor units.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1.4 Electronic Centralized Aircraft Monitoring (ECAM) Technical information concerning the state of an Airbus aircraft is displayed using the aircraft' s electronic centralized aircraft monitor (ECAM— see Fig. 1.9). This normally takes the form of two CRT or LCD displays that are vertically arranged in the centre of the instrument panel. The upper (primary) display shows the primary engine parameters (N1/fan speed, EGT, N2/high pressure turbine speed), as well as the fuel flow, the status of lift augmentation devices (flap and slat positions), along with other information. The lower (secondary) ECAM display presents additional information including that relating to any system malfunction and its consequences.


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Figure 1.9 A320 ECAM displays located above the centre console between the captain and first officer



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) Test your understanding Figure 1.10 shows a flight deck display.


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Figure 1.10

(a) (b) (c) (d) (e)

Identify the display. What information is currently displayed? Where is the display usually found? What fan speed is indicated? What temperature is indicated?



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1.5 Engine Indicating and Crew Alerting System (EICAS) In Boeing aircraft the equivalent integrated electronic aircraft monitoring system is known as the engine indicating and crew alerting system (EICAS). This system provides graphical monitoring of the engines of later Boeing aircraft, replacing a large number of individual panel-mounted instruments. In common with the Airbus ECAM system, EICAS uses two vertically mounted centrally located displays (see Fig. 1.11).

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The upper (primary) EICAS display shows the engine parameters and alert messages whilst the lower (secondary) display provides supplementary data (including advisory and warning information).


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3)


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Figure 1.11 EICAS display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1.6 Flight Management System (FMS) The flight management system (FMS) fitted to a modern passenger aircraft brings together data and information gathered from the electronic flight instruments, aircraft monitoring and navigation systems, and provides outputs that can be used for automatic control of the aircraft from immediately after take-off to final approach and landing. The key elements of an FMS include a flight management computer (FMC), control and display unit (CDU), IRS, AFCS, and a system of data buses that facilitates the interchange of data with the other digital and computerized systems and instruments fitted to the aircraft.

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Two FMS are fitted, one for the captain and one for the first officer. During normal operation the two systems share the incoming data. However, each system can be made to operate independently in the event of failure. By automatically comparing (on a continuous basis) the indications and outputs provided by the two systems it is possible to detect faults within the system and avoid erroneous indications. The inputs to the FMC are derived from several other systems including IRS, EICAS, engine thrust management computer, and the air data computer. Figures 1.12 and 1.13 shows the FMC control and display units fitted to an A320 aircraft.





f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 1.12 Captain’s FMS CDU

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Figure 1.13 A320 cockpit layout



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) 5.1.7 Cockpit Layouts Major developments in display technology and the introduction of increasingly sophisticated aircraft computer systems have meant that cockpit layouts have been subject to continuous change over the past few decades. At the same time, aircraft designers have had to respond to the need to ensure that the flight crew are not overburdened with information and that relevant data is presented in an appropriate form and at the time that it is needed.

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Figure 1.14 shows how the modern EFIS layouts have evolved progressively from the basic-T instrument configuration found in non-EFIS aircraft. Maintaining the relative position of the instruments has been important in allowing pilots to adapt from one aircraft type to another. At the same time, the large size of modern CRT and LCD displays, coupled with the ability of these instruments to display combined data (for example, heading, airspeed and altitude) has led to a less-cluttered instrument panel (see Fig. 1.15). Lastly, a number of standby (or secondary) instruments are made available in order to provide the flight crew with reference information which may become invaluable in the case of a malfunction in the computer system.


Basic T'flight instrument configuration

Basic EFIS flight instrument configuration

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Enhanced EFIS flight instrument configuration

Figure 1.14 Evolution of instrument layouts





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Figure 1.15 Captain' s flight instrument and display layout on the A320



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) Test your understanding 1. Identify each of the Boeing 767 flight instruments and displays shown in Fig. 1.16. 2. Classify the flight instruments in Question 1 as either primary or standby.


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Figure 1.16



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC INSTRUMENT SYSTEM (DCAM 5.1 L3) NOTES


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) 5.2 NUMBERING SYSTEMS Most digital transmission in today' s modern aircraft uses various types of number systems. Typically these would be decimal, binary, octal and hexadecimal, or various forms of these systems. 5.2.1 Decimal System

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This is the system in everyday use, there are 10 digits (0 to 9 inclusive), so it has a ‘base’ or ‘radix'(number of digits used in the system) of 10. Taking the number 72306 can be written as:

(7 × 10,000) + (2 × 1000) + (3 × 100) + (0 × 10) + (6 × 1)

(

) (

) (

) (

) (

= 7 × 10 4 + 2 × 10 3 + 3 × 10 2 + 0 × 101 + 6 × 10 0 = 72306

)


Each digit is effectively multiplied by a power of 10.

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Note that to write 19.526 in powers of 10 then:

(1× 10 ) + (9 × 10 ) + (5 × 10 ) + (2 × 10 ) + (6 × 10 ) 1

0

−1

−2

−3

1 1 1 = .006 = .02 + 6 × = .5 + 2 × 10 100 1000 = 10 + 9 + .5 + .02 + .006 = 10 + 9 + 5 × = 19.526

Note that 100 = 1, in fact any number to the power of nought = 1. Proof, using the number 3 i.e. 30 = 1

1=

9 32 = 2 = 3 2 × 3 − 2 = 3 2+ ( − 2 ) = 3 0 9 3

3 was used because it is an easy number to show you that any number to the power of nought is equal to 1. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) It can be seen that the decimal system is based on successive powers of 10, the number with the smallest value (100) is known as the least significant digit (LSD) and the number with the highest value is known as the most significant digit (MSD). The disadvantage of the decimal system for use in a digital computer is that the circuits which would be used, e.g. transistors, would have to have 10 discrete levels at collector current. For example 0mA (milliamps) = 0, 1mA = 1, 2mA - 2, 3mA = 3 etc. Such a system would be extremely difficult to operate because: a) Any variation of power supply would cause errors. b) Component tolerance would have to be virtually zero, and be unaffected by temperature variations. c) Component values will change with age.

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Any errors from the above may cause an error increment of one or two, giving an incorrect output (e.g. instead of 8 [correct reading] it might be 7 or 9).


Precision is important, and to expect a circuit to be infallible in distinguishing between 10 different magnitudes of current is a bit much.

Where accuracy and speed are important it would be better to use a system which has just two states. Reliance is high because the circuit is either HIGH (voltage level) or LOW (voltage level) or ON and OFF and component characteristics variations are unimportant. The system that is the basis of today' s digital processing is the "two states" BINARY SYSTEM.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) 5.2.2 The Binary System This has a base or radix of 2. As in the decimal system, we can represent any number in successive powers of 2. For example:

27 = 2 4 + 2 3 + 21 + 2 0 = 16 + 8 + 2 + 1 further expanded


= 1 × 2 4 + 1 × 2 3 + 0 × 2 2 + 1 × 21 + 1 × 2 0 ↓ 1

↓ 1

↓ 0

↓ 1

↓ 1

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means the binary number for 27 = 110112

To avoid confusion between systems with different radix this would be written as 110112 to identify it as a binary number. What about the fraction expressed as powers of 2?

1 = 0.5 2 1 .012 = 2 − 2 = 2 = 0.25 2 1 .0012 = 2 −3 = 3 = 0.125 2 1 .00012 = 2 − 4 = 4 = 0.0625 2 .12 = 2 −1 =



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) Example Change 17.75 decimal to binary.

17.75 = 2 4 + 2 0 + 2 −1 + 2 −2 expanded gives

= 1 × 2 4 + 0 × 2 3 + 0 × 2 2 + 0 × 21 + 1 × 2 0 + 1 × 2 −1 + 1 × 2 −2 ↓ ↓ ↓ 1 0 0 17.75 = 10001.112

↓ 0

↓ 1

↓ ⋅ 1

↓ 1


Note a binary digit is termed a bit.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) 5.2.2.1 Binary/Decimal Equivalents The table below shows the relationship between Binary and Decimal numbers up to Decimal 21, but it could obviously be continued for larger numbers. Decimal Number 0 1 2 3 4 5 6 7 8 9 10

24 0 0 0 0 0 0 0 0 0 0 0

5-bit binary number (Word) Decimal Number 23 22 21 20 0 0 0 0 11 0 0 0 1 12 0 0 1 0 13 0 0 1 1 14 0 1 0 0 15 0 1 0 1 16 0 1 1 0 17 0 1 1 1 18 1 0 0 0 19 1 0 0 1 20 1 0 1 0 21 BINARY/DECIMAL EQUIVALENTS


24 0 0 0 0 0 1 1 1 1 1 1

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5-bit binary number (Word) 23 22 21 1 0 1 1 1 0 1 1 0 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 0 0 1 0 1 0 0 1 0

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20 1 0 1 0 1 0 1 0 1 0 1

From the table it can be seen that the binary number is longer than the decimal number but because of the very fast switching speeds of modern circuits this does not present a problem. Also because of the reliability of the two-state system, the practical advantages gained by using binary numbers are considerable.

ACTIVITY: Write the successive powers of 2 for the following decimal numbers and then expand to finally give the binary number. a) 19 b) 29 c) 15.125 d) 22.0625 This method is OK but when you get larger numbers it becomes much more difficult, to convert from decimal to binary, the successive division by two may be employed, the ' remainder'of any division (which must be either 0 or 1) is then recorded successively in a separate column. The following examples show the method used. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2)

Example 1 Convert 796 to binary 2 2 2 2 2 2 2 2 2 2

796 398 199 99 49 24 12 6 3 1 0

remainder remainder remainder remainder remainder remainder remainder remainder remainder remainder


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) Example 2 Convert 217 in binary form 2 2 2 2 2 2 2 2 2

217 108 54 27 13 6 3 1 0

remainder remainder remainder remainder remainder remainder remainder remainder


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2)

NOTES


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) CONVERSION OF DECIMAL TO BINARY ACTIVITY: Convert the following decimal numbers to binary a) 846 b) 317 c) 147 You should practice converting the smaller number using powers of 2 and perhaps speed it up a bit. Example


Convert 4510 to binary

Write down the successive powers of 2

(2 ) (2 ) (2 ) (2 ) (2 ) (2 ) 5

32

4

16

3

2

1

0

8

4

2

1

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Then make up the number, i.e.

(1 × 32) + (1 × 8) + (1 × 4) + (1× 1) = 45 So

32 16 8 4 2 1 1 0 1 1 0 1

The binary number is 1011012



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) ACTIVITY:

So the idea is to write down the powers of 2 and put 1' s in the powers you need to make up the number. Try these: a) 47 b) 32 c) 21


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) The following example shows you how to convert decimal fractions to binary. You use successive multiplication by two, recording the carries and then reading DOWN the carries column. Convert 0.615 to binary form

READ DOWN And write from left to right to give binary fraction: 0.1001112

0.615 x 2 1 230 x 0 460 x 0 920 x 1 840 x 1 680 x

2 2 2 2 2 2 ----- until required 1 360 x accuracy is obtained



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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) 5.2.2.2 Conversion of a Mixed Decimal to a Binary The next example shows how to convert a mixed decimal number to binary. Note that it must be treated in two parts as shown. Example Convert 14.625 to binary Separate into whole and fraction parts – i.e.: 14.625 = 14.000 + 0.625 (WHOLE) (FRACTION)


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Recombine the whole fraction parts to give: 14.62510 = 1110.1012



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) Conversion of Binary to Decimal

Assume we have a binary number e.g., 1011012

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The easy way to convert this is to write the powers of 2 above each bit position starting from left and working towards the right e.g.:

32 16 8 4 2 1 the powers added 1 0 1 1 0 1


So it can be seen that the number is: 32+8+4+1=4510

(there are no 16s and no 2s but one of each of the other values – 32, 8, 4 & 1)

Examples: 1.

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Convert 1101101 binary to decimal

Again, write the powers of 2 above each bit position.

64 32 16 8 4 2 1 1 1 0 1 1 0 1 So 64+32+8+4+1=10910



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) 2.

Convert 1101.1 binary to decimal

Again, write the powers of 2 above each bit position.

8 4 2 1 .5 1 1 0 1 .1 So 8+4+1+.5=13.510 3.

Convert 1001110.112 to decimal


64 32 16 8 4 2 1 .5 .25 1 0 0 1 1 1 0 .1 1 =64+8+4+2+.5+.25 =78.7510

ACTIVITY:

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Convert to decimal

a) 110011 b) 1110011 c) 1011.1 d) 1100.001



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) 5.2.3 The Octal System In the binary system the number of bits in a word can be quite lengthy and problems can occur such as the high possibility of an error in manipulating so many digits. The octal system helps lessen these difficulties, being more compact and easily converted back to decimal or binary.

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The system uses the base or radix 8, this means of course, to convert from decimal to octal we divide by 8 then record the remainders as before and read upwards to get the octal number.

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To convert this number to binary split each octal number into it' s three figure binary number and join together.


Example 1

Convert 796 to octal and then convert octal to binary

To convert from binary to octal, start from the right and group into threes, if the final group does not have three bits then add noughts to make up to the three.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) Example 2 Convert 10101002 to octal


Example 3


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) To convert from octal to binary (just a recap) the reverse procedure is used. Example - convert 12638 to binary


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) ACTIVITY: Convert the following binary numbers to octal: a) 101010100 b) 1110100000 c) 111010001 Convert the following octal numbers to binary: a) 426 b) 5625 c) 65217


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) Fractions in decimal to octal are dealt with in a similar manner to decimal to binary except multiplication by 8 is used. Example Convert .9062510 to binary then octal.


to convert to octal

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or we could have used the binary number split into threes 0.11101 starting after the decimal point going left to right

The reverse procedure is used to obtain a binary fraction from an octal fraction. For Training Purposes Only



Convert the following binary numbers to octal. a) 111.111 b) 101.1 c) 110.0111 Convert the following octal numbers to binary a) .64 b) .77 c) .43


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) 5.2.4 Hexadecimal System This system has a base or radix of 16 and is used again where large binary numbers are handled to cut down possible errors. Since we have only ten different digit symbols (0 to 9 inclusive) six other symbols have to be used these are the letters A to F inclusive. The table below shows the three numbering systems already considered and the hexadecimal system.

Decimal Octal Binary Hexadecimal 0 0 0000 0 1 1 0001 1 2 2 0010 2 3 3 0011 3 4 4 0100 4 5 5 0101 5 6 6 0110 6 7 7 0111 7 8 10 1000 8 9 11 1001 9 10 12 1010 A 11 13 1011 B 12 14 1100 C 13 15 1101 D 14 16 1110 E 15 17 1111 F 16 20 10000 10


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COMPARISON OF NUMBERING SYSTEMS



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) Conversion from Decimal to Hexadecimal Convert 76210 to Hexadecimal


Conversion from Hexadecimal to Binary Convert 2BC16 to Binary

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2BC16 is equal to 10101111002




Convert the following Binary numbers to Hexadecimal a) 11100010 b) 1111111 c) 111001 Convert the following Hexadecimal codes to Decimal a) 2D b) 1AF c) 21A


Convert the following Decimal numbers to Hexadecimal a) 1632 b) 494 c) 5174

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Convert 17816 to Decimal, Binary and Octal Convert EF16 to Decimal, Binary and Octal



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) Fractions in Hexadecimal Convert 0.9062510 to Hexadecimal

to convert to Binary 0.E816

f o g y n r i r a t e e e i n r i p g o n r P SE A M E 8 1110 1000 = 0.111010002

Group into four digits

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Convert the following Hexadecimal numbers to Binary a) 0.A8 b) 0.F6

Just comparing the length of a binary number to Octal or Hexadecimal 111100001111101100011.00010011012 =7417543.04648 =1E1F63.13416



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) ACTIVITY: Convert the following decimal fractions to Hexadecimal a) 0.6250 b) 0.81250


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) 5.2.5 Binary Coded Decimal BCD There are several forms of this system but we shall concentrate on the 8421 code. It is used in display read-out systems, decoders and counters.


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It is called an 8421 code as each digit is weighted from left to right 8421 in powers of 2.

You might be thinking that this is the same as the binary code, however, with numbers from 10 upwards each number is represented by the 4 bit code. Example 1110 to BCD Is 0001 0001 leaving a space between each group of four digits.

Converting from BCD to Decimal is again quite easy For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) 10000101 Split into groups of four


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By way of a summary and to enable a comparison to be made between a number representation in the various systems and codes, consider the decimal number 347.

OCTAL HEXADECIMAL

533 15B

When a number such as decimal 347 is converted into any binary form the corresponding group of binary digits is known as a WORD. Each word is formed of a number of BITS (BINARY DIGITS) and this represents the word length.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) ACTIVITY: Convert the following decimal numbers to BCD a) 94 b) 529 c) 2947 Convert the following BCD numbers to decimal a) 011100001001 b) 001101100100


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) 5.2.6 Adding The rules are similar to those when adding decimal numbers, e.g. 5+5 = 0 and carry 1 to the next higher ' power'column, and 1+1 in binary results in 0 carry 1. Rules 0+0=0 0+1=1 1+0=1 1+1=0 with 1 to carry Example: add 1011 and 1110


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5.2.7 Subtraction

Again similar to rules for decimal subtraction except 0-1 = 1 borrow 1 Rules

0-0=0 1-0=1 1-1=0 0-1=1 borrow 1

Example: subtract 10101 from 11011



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) Multiplication Rules 0x0=0 0x1=0 1x0=0 1x1=1 Example: multiply 1100 x 11


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Multiplication in a computer is achieved by repeated addition (e.g. in decimal 2 x 4 is computed as 2+2+2+2=8). Division

Example: divide 111100 by 110

Check this by converting binary 1010 to see if the answer is correct.

Computers cannot divide; they carry out division by repeated subtraction, which is in itself done by an addition process.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) Positive and Negative Numbers The computer needs to distinguish between positive and negative numbers. For storage purposes only there is usually an additional bit added which identifies whether the number is positive or negative, e.g.: ‘0'for positive numbers ‘1’ for negative numbers Example - using an 8 bit binary word the sign bit is added on the front. Decimal - 4.5

binary 100100.100


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As mentioned, this is a convenient method for storing numbers but does not allow direct subtraction of one number from another. By inverting the number and adding 1 we get the negative of the number. This process is called TWO' s COMPLEMENT. The Twos Complement process involves inverting each bit in a word and adding 1.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS NUMBERING SYSTEMS (DCAM 5.2 L2) Example: Find the negative binary number of +5 decimal minus +7 decimal Note negative numbers have a 1 in the most significant bit position whilst the positive number has a 0. When numbers are represented as negative, subtraction is achieved by addition (adding a negative number being the same as subtracting a positive number) e.g.


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This means that both addition and subtraction can be done by the same circuits in a computer which considerably reduces the hardware involved.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) 5.3 DATA CONVERSION

5.3.1 Digital and Analogue Computers

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With new technological advances in computer development, an ever increasing number of digital computers are being used in the aviation industry. In addition to performing the traditional avionics functions, the digital computers are being used in new areas, such as thrust management and engine

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controls. With the use of these digital Systems, the flight crew’s workload is reduced. Other important considerations in the use of digital computers are the reduction of system weight and size; increase in data handling speed, and improved system reliability. This contributes to an increase in overall aeroplane


efficiency.

5.3.1.1 Analogue Computers

Analogue computers are characterized by the processing of analogue signals. An analogue signal is an electrical signal whose amplitude varies continuously with time. Transducers, such as temperature and fuel sensors, provide voltage and current outputs proportional to the quantity being monitored. An analogue computer operates on a principle of creating a physical or electrical analogy using mathematical formulas. Variables, such as temperature or fuel flow, are represented by the magnitude of a physical phenomenon, such as voltage or current. The computing process is accomplished by designing hardware to perform prescribed functions. To change algorithm most often requires redesign of the circuit. Hence, analogue computers are not easily used in more than one operating environment. An analogue computer designed for the 727 aeroplane would not be easily converted for use in 777 aeroplanes



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2)


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) 5.3.1.2 Digital Computers

Digital is defined as incremental or stepped. A digital computer processes information and outputs an incremented or stepped output. This output can be used to control an on-off type device or to communicate with another digital device. The mathematical formulas used to process the information and

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develop an output are stored in a program in the computer’s memory. Should it become necessary to change these formulas, the computer program is simply modified. The ability to change a program, within a digital computer allows one computer to be used in many applications or on many airframe types


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) 5.3.2 Digital Bus

Digital devices usually need to communicate with each other. One device will send information to another which will process this data and possibly send it to another device. The path or circuit over which these devices communicate is called a Digital Bus. This Digital Bus may be used to connect several

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sources to any of several destinations. For example, airspeed information is transmitted from the Air Data Computer to several destinations over a Digital Bus called an ARINC 429 Bus.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) 5.3.3 Typical Digital System Inputs

One type of input used by digital systems is the discrete input. This is a high/low or on/off input used by the digital computer to indicate certain conditions taking place within the aeroplane. An example of a discrete input would be an input from the air/ground switch indicating whether the aircraft is on ground

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or in the air. The digital computer would use this indication within its calculations. A fire detection input would be Digital inputs send digitally encoded data using a rapid series of pulses encoded in one several forms. For example, the Air Data Computer sends a digital output representing airspeed to the

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computer on a Digital Bus. This digital input, called ARINC 429 information in this case, is a series of high and low levels or pulses which the computer uses for calculation using airspeed.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) 5.3.4 Typical Digital Systems Outputs

Digital computer also utilize discrete as outputs and may use these discrete outputs to turn on and off other devices. For example, the computer controlled stall warning light in the cockpit is illuminated by a high level output from the computer. Other examples may include illuminating the fire light with a high kevel output and turning on the fire bell circuit with a low level output.

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Digital outputs are generated from a computer in the form of a high and low levels and pulses. The series pulses can represent, for example, barometric altitude and is sent from the Air Data Computer to the cabin pressurization system on a Digital Bus. This digital information is a type of ARINC information.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) 5.3.5 Digital to Analogue / Analogue to Digital Conversion

While digital computers process information faster and more efficiently than analogue computers, they do have somewhat of a disadvantage in that they only understand 1’s and 0’s. The real world is a analogue in nature. Temperature, for example, does not change in discrete steps. It is a continuously

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varying quantity. In order for digital computers to use temperature information, the analogue quantity must be converted to a digital representation of temperature. Aeroplane control surfaces do not move in discrete steps but rather in continuous motion. A digital computer may be able to determine where

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a control surface must be positioned, but the signal to the surface must be in analogue form to drive the surface. The circuits used to interface digital computers to the analogue world are referred to as Digital to Analogue Converters (DAC) and Analogue to Digital Converters (ADC).


DAC converters change the digital data words of a digital computer to an equivalent analogue signal as either a voltage or current source. There are many different types of DAC converters but each one has this same basic description.

ADC converters change the analogue signals received from sensors to digital data understood by the digital computer. Many different types of ADC converters exist. The type used depends on the type of analogue signal and what the digital computers need to know about the signal.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) 5.3.5.1 Digital to Analogue Converters (DACs)

Principle

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Each DAC circuit has a specific purpose based on its use within the system but the general purpose of all DACs is to provide an analogue signal output based on the digital value represented in the digital computer. This analogue signal is either a voltage or current output but the principles involved are essentially the same.


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The basic principle of a DAC converter is to divide the analogue output into a series of small steps. The number of steps depends on the number of bits used in the data to be converted. If the data consists of 8 bits then the output is divided into 256

(28)

steps. The size of each step would be 0.0195 volts

(5v/256 steps). If an 8 bit, 5 volt converter is driven by a simple counter then the output of the converter would be a series of 256 steps of 0.0195 volts. As the counter progress from 0 to 255, the converter output increases from 0 volts to 4.98v then drops to zero when the counter rolls over. Also, note that the maximum output voltage is not 5 volts.

This is due a fact that each digital input bit is weighted according to its position within the binary input. The least significant bit (LSB) has a weight of 5v/256 = 0.0195 volts, the next most significant bit has a weight of 5v/128 = 0.039 volts, the next has a weight of 5v/64 = 0.078 volts, and so on, with the most significant bit (MSB) having a weight of 5v/2 = 2.5 volts. If you add all the individual bit weights, you get a 4.98 volt maximum output when the DAC input is 1111 1111.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) Binary Weighted Ladder Starting from V1 and going through V3, this would give each input voltage exactly half the effect on the output as the voltage before it. In other words, input voltage V1 has a 1:1 effect on the output voltage (gain of 1), while input voltage V2 has half that much effect on the output (a gain of 1/2), and V3 half of that (a gain of 1/4). These ratios are the same ratios corresponding to position weights in the binary system. The op-amp is used as a summation device, to sum the weighted inputs of the digital information.

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If we drive the inputs of this circuit with digital gates so that each input is either 0 volts or full supply voltage, the output voltage will be an analogue representation of the binary value of these three bits.

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The disadvantage of this circuit is that a high precision is required of the resistors, especially the higher values (± 0.5%). This makes it difficult to mass produce.






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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) R-2R Ladder The R-2R ladder network is another type of DAC. Each bit of the binary input controls a solid state switch which connects either a reference voltage or a ground to the resistors. The ladder is constructed of resistors of only two values, R or 2R rather than binary weighted. In this type of network the actual resistor values are not as critical as in the binary weighted ladder. Also since the resistance values can be small; it is much easier to implement this in a solid state device.

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The inverting input of the op-amp is at virtual earth. Current flowing in the elements of the ladder network is therefore unaffected by switch positions. The selected resistor values ensure that the relationship:


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The current ITOTAL, is the sum of the currents switched to the inverting input of the op-amp from the R-2R ladder by the digitally controlled switches Do - D3. ITOTAL is given by the relationship:

The output voltage VOUT is the voltage across the op-amp feedback resistor which is by Ohms law:





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) Some uses of DACs

Waveform Generators

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The analogue output required from a digital computer which is not always a steady output level. Sometimes a particular wave form is required, such as a sawtooth or ramp. A ramp or sawtooth wave can be easily implemented by using simple R-2R ladder and a counter.

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A binary counter used to drive a R-2R ladder causes the ladder to output a sequence of steps of different voltage levels. As the counter reaches its


maximum value, it returns to zero or is said to "roll over' . When the input word rolls over to zero the output also returns to zero. While this output is not a pure ramp due to the fact that the binary words generate steps, most analogue systems are slow enough in their reaction times that they react the same as if the signal was continuous. High quality R-2R ladders have relatively short response times (2 microseconds typical). For an 8 bit device this makes the rise time from 0 to 10 volts in the order of 0.5 microseconds or a frequency of 2 MHz.





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Programmable Gain Amplifiers

DACs can also be used to provide gain control of an analogue signal. This arrangement may be required, for example, to control the speed of an AC motor by varying the input voltage level to the motor. This can be accomplished quite simply by applying an AC voltage as VR to a ladder network.

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A simple-motor controller is shown. A 5 volt AC reference signal is applied as VR to an 8-bit DAC. The data word is generated by the computer


to select the desired motor speed by setting the output level. Remember for an 8-bit device this output can be divided into 256 steps, so very fine control can be established with this simple method. One minor problem with this simple motor control is the fact that the computer does not know if the motor speed is correct. To check this, there must be a feedback from the motor. This feedback would be analogue so we need an analogue to digital converter.

Almost all "real world" applications are analogue in nature. Therefore, analogue to digital converters (ADCs) are quite common in computer systems, and especially in those systems dedicated to monitoring or controlling "real world" events. An ADC converts a continuous voltage signal, or analogue signal into a multi-bit digital word.





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5.3.5.2 Analogue to Digital Converters (ADCs) Principle

There are seven techniques normally used for the conversion process. They are: •

Flash

•

Pipeline

•

ramp generation,

•

successive approximation

• • •

f o g y n r i r a t e e e i n r i p g o n r P SE A M integration

charge balancing sigma delta

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Successive approximation and integration are the most common in use. Both the ramp generation and successive approximation type converters require DACs as part of the circuit but these are usually built into the ADC chip.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) Flash ADC

Illustrated in figure 3.23 is a 3-bit flash ADC with resolution 1 volt. The resistor network and comparators provide an input to the combinational logic circuit, so the conversion time is just the propagation delay through the network - it is not limited by the clock rate or some convergence sequence. It is the fastest

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type of ADC available, but requires a comparator for each value of output (63 for 6-bit, 255 for 8-bit, etc.). Such ADCs are available in IC form up to 8-bit and 10-bit flash ADCs (1023 comparators) are planned. The encoder logic executes a truth table to convert the ladder of inputs to the binary number output.


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The flash converter requires 2n - 1 comparators, where n is the number of bits. So a 4 bit flash ADC will require 15 comparators. The flash converter is sometimes called a parallel converter because the conversion takes place in a single cycle. For this reason also, it is the fastest type of ADC, but it does have the most complicated architecture.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) Pipeline ADC

The ADC pipeline architecture effectively overcomes the limitations of the flash architecture. A pipelined converter divides the conversion task into several consecutive stages. Each of these stages consists of a sample and hold circuit, an m-bit ADC (e.g., a flash converter), and an m-bit DAC. First the sample

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and hold circuit of the first stage acquires the signal. The m-bit flash converter then converts the sampled signal to digital data. The conversion result forms the most significant bits of the digital output. This same digital output is fed into an m-bit digital-to-analog converter, and its output is subtracted from the

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original sampled signal. The residual analog signal is then amplified and sent on to the next stage in the pipeline to be sampled and converted as it was in the first stage. This process is repeated through as many stages as are necessary to achieve the desired resolution. Pipelined converters achieve higher


resolutions than flash converters containing a similar number of comparators. This comes at the price of increasing the total conversion time from one cycle to p cycles.

Ramp Generation ADC

A ramp generation ADC, sometimes called a counter ADC, compares the unknown input voltage to a DAC connected as a ramp generator. As long as the unknown input is greater than the ramp signal the counter continues. As soon as the ramp exceeds the unknown voltage input the counter stops and the ramp is held at the fixed level. The count is then read by the computer. The clear line clears the binary counter, setting its output to zero. The counter counts up with each clock pulse as long as the comparator output is high. The comparator output is high as long as the output of the DAC (Vo) is less than the analogue input (VI). When Vo exceeds VI, the comparator output goes low. This in turn stops the counter and Vo remains greater than VI. The digital output word now represents the input voltage. The fact that Vo is slightly greater than VI is of little consequence since the step size is 1/256 times the full range. The circuit will hold this digital value until the binary counter is cleared or until VI is greater than Vo again. The ramp generation ADC converter is really working as a peak detector. This can pose some problems if the computer needs to sense variations of VI over short time periods. Therefore, the ramp generation ADC works best in applications where the analogue signal level varies at a slow rate, such as a temperature sensor.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) An improvement of the ramp generation ADC converter is to use an up-down counter (sometimes called a Tracking ADC). The up-down counter eliminates the need for the clear line and the AND gate. The computer output is used to control the counter either up or down as required to track the input signal. This is sometimes called a tracking or servo counting ADC.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) Successive Approximation Register (SAR) ADC

The successive approximation ADC (also known as a Bit-Weighted converter) is similar to the ramp generation ADC except that a programmer is used to control the DAC ladder network. A successive approximation converter provides a fast conversion (second only to the Flash converter) of a momentary

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value of the input signal. It works by first comparing the input with a voltage which is half the input range. If the input is over this level it compares it with three-quarters of the range, and so on. Twelve such steps give 12-bit resolution. While these comparisons are taking place the signal is frozen in a sample

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and hold circuit. After A-D conversion the resulting bytes are placed into either a pipeline or buffer store. A pipeline store enables the ADC to do another conversion while the previous data is transferred to the computer. Buffered ADCs place the data into a queue held in buffer memory. The computer can


read the converted value immediately, or can allow values to accumulate in the buffer and read them when it is convenient. This frees the computer from having to deal with the samples in real time, allowing them to be processed in convenient batches without losing any data.

The successive approximation method is much faster than the counting type methods. For an 8bit word, the successive approximation takes 8 clock cycles while the counting type method could take up to 256 (28) clock cycles.

The Successive-Approximations Register (SAR) architecture can be thought of as being at the other end of the spectrum from the flash architecture. While a flash converter uses many comparators to convert in a single cycle, a SAR converter conceptually uses a single comparator over many cycles to make its conversion. An unknown input voltage, VI, is applied to the comparator. The output of the comparator controls the operation of the programmer. The digital word representing VI is determined as follows: T1 - bit 7 is set, VD greater than VI

T2 - bit 7 is reset, bit 6 is set, Vo greater than VI T3 - bit 6 is reset, bit 5 is set, Vo less than VI T4 - bit 4 is set, Vo less than VI For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) T5 - bit 3 is set, Vo greater than VI T6 - bit 3 is reset, bit 2 is set, Vo greater than VI T7 - bit 2 is reset, bit 1 is set, Vo greater than VI T8 - bit 1 is reset, bit 0 is set, Vo less than VI

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The final digital word is 00110001. After this word is stored by the processor, the programmer is instructed to start the sequence over again.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) Integration Type ADC

There are various forms of integration type ADC: single slope and dual-slope. The dual-slope method is by far the most common. Basically, dual-slope converters allow a capacitor to charge from the unknown analogue input voltage for a fixed period of time. The charge on the capacitor at the end of this

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time depends only on the input voltage level. At the end of the fixed charge time, a known reference voltage of opposite polarity is gated to the capacitor such that it will discharge back to zero. The time required for discharge is counted. The final count is the digital equivalent of the analogue input level.

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The advantages of the integration methods are accuracy and noise immunity. However, the conversion process usually takes between 10 and 50


milliseconds. This is quite a long time in the computer world. It is ideal however for use in a digital multimeter which requires a fine resolution but can sacrifice high speeds.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) Initial conditions on the circuit are that both Clear and Timer lines are high. This condition causes an open on the input to the integrator, clears the integrator by discharging the capacitor through S3, and clears the counter (sets all output bits to 0). The processor starts the conversion by setting both Timer and Clear lines low. This results in the input voltage (VI) being applied to the integrator. S3 is now open so VI is integrated and the integrator output goes negative as the capacitor charges. The negative integrator output causes the comparator output to be high which allows the counter to count the clock pulses.

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When the signal to be measured fluctuates slowly, as in the case of a temperature monitored by a thermocouple, an integrating converter is best. By averaging the signal the converter helps reduce unwanted signal contamination (noise).


This converter reduces noise but is slower than the successive approximation type. It lets the input signal charge a capacitor for a fixed period and then measures the time for the capacitor to fully discharge at a fixed rate. This time is a measure of the integrated input voltage, which reduces the effects of noise.

Since the counter was set to zero at clear, the counter starts counting from zero to 256 (all 1' s). When the counter counts one more pulse it goes to zero. This transition from all 1' s to all 0' s is sensed by the processor which in turn causes the timer pulse to be set high. S2 then switches the integrator input from V, to VR. VR is the opposite polarity of V, so the capacitor starts to discharge. As the capacitor discharges the integrator output returns to zero. When this occurs the comparator output goes low which stops the counter. The time required to discharge the capacitor is proportional to the input voltage since VR is a constant and the charge time is a constant (256 counts). The processor starts the sequence over again by setting the clear pulse once every 512 clock pulses.

The major disadvantage of the single-slope integrating ADC is that it becomes inaccurate as the circuit drifts (an inevitable result of age and temperature). This is irradiated in the dual-slope version.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) Charge Balancing ADC

In a Charge Balancing Converter the input signal again charges a capacitor for a fixed period, but in this case the capacitor is simultaneously discharged in units of charge packets. (Meaning that if the capacitor is charged to more than the packet size it will release a packet, if not a packet cannot be released.)

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This creates a pulse train. By counting the pulses coming out of the capacitor, the system determines the input voltage. (A charge balancing converter is also known as a voltage to frequency converter.)

Sigma-delta ADC


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This converter digitizes the signal with very low resolution (1-bit) and a very high sampling rate (MHz). By over sampling, and using digital filters, the resolution can be increased to as many as 20 or more bits. Sigma-delta converters are especially useful for high resolution conversion of low-frequency signals as well as low-distortion conversion of signals containing audio frequencies. They have good linearity and high accuracy.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA CONVERSION (DCAM 5.3 L2) NOTES


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) 5.4 DATA BUSES

From the computers within the aircraft, systems information has to be transmitted to the components, this may be in the form of electrical or fibre optic signals and are known as data buses. Aeronautical Radio Incorporated (ARINC) is a corporation made up of scheduled airlines, transport companies, aircraft manufactures and foreign flag airlines.

One primary activity of ARINC is to produce specifications and reports for the purpose of: 1. Indicating to manufacturers the group opinion concerning requisites of new equipment.


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2. To channel new equipment designs in a direction which will result in maximum standardisation.

There are many ARINC specifications in existence for example: o

ARINC 573 - Specification for digital flight data recording systems

o

ARINC 561 - Specification for the Inertial Navigation System

o

ARINC 429 - Specification for transfer of digital data between avionic Components

o

ARINC 629 - An improvement on the ARINC 429 system


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) 5.4.1 What is Data Bus?

A bus is a collection of wires through which data is transmitted from one part of a computer to another. You can think of a bus as a highway on which data

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travels within a computer. When used in reference to personal computer components, the terms bus usually refers to internal bus. This is a bus that connects all the internal components to the CPU and main memory.

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However when used in reference to aircraft, it is the data highway which links one computer to another within the aircraft for example, the Flight


Management Computer and the Air Data Computer.

All buses consists of 2 parts •

An address bus

•

A data bus

The data bus transfers actual data whereas the address bus transfers information about where the data should go. On aircraft bus, the two parts are incorporated within a single data ‘word’. A bus can be either serial or parallel. A serial bus requires less wiring, but is slower. A parallel bus required one wire for each bit within the data word, but is much faster.

A bus can enable communication between a single computer to a single LRU only, known as single source-single sink; or a single computer to multiple LRUs, known as single source-multiple sinks, or multiple computers to multiple LRUs known as multiple sources-multiple sinks. The latest databus systems are multiple sources-multiple sink (ARINC 629 and MIL-STD-1553B).



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Data Bus Systems On aircraft in general, there are unidirectional and bidirectional data bus architectures. The accompanying communications protocols will then support one or the other system. The simpler and to this day the most prevalent architecture on-board commercial airplanes is the unidirectional (simplex) data bus represented by ARINC 429. The system consists of a single transmitter and a number of receivers, each monitoring the line and listening for messages relevant for them. Communication back to the transmitter, if needed, is performed by a separate transmitter, receiver(s), and wires. Due to its simplicity, the system is robust, fault-tolerant, and extremely simple to design and implement. The disadvantage is a lot of labor-intensive wiring, which is also expensive and heavy, requires higher power consumption and leads to high operational costs of the aircraft.

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More advanced data bus architectures such as ARINC 629 are bidirectional (duplex). ARINC 629 is applied on Boeing 777 and some Boeing 737 models. Here, any member or user of the system can transmit, receive, or both on a common, shared data bus. As the implementation of the physical layer is comparatively expensive, market penetration did not happen. More than 30 years ago, especially military avionic systems, which previously used discrete wiring, were upgraded to interface with the MIL-STD-1553B bidirectional data bus. This led to a significant weight saving, and the labor to run wiring harnesses could be reduced by thousands of hours. However, the bus control became more complex, and the engineering effort to integrate the system was no small task.


Bus Traffic Control

Every non-100% time-triggered, Time Division Multiple Access (TDMA) based databus must be able to arbitrate data bus transmissions to ensure that only one transmitter is operational at a time and that the receivers are waiting for messages. And, unlike the unidirectional bus, a lot of thought has to go into the system design and integration. There are two approaches commonly used for traffic control of bidirectional buses: central control and distributed control. The advantage of the central control or command/respond approach is that only one bus component has control of the bus traffic (master). All users are directed by the bus controller (master). If the data bus architecture has to change, only the bus controller (master) has to be modified to support the new configuration. But the most significant weakness of such architecture in an aircraft environment, where the bus controller or network failure could have catastrophic repercussions, is that the bus controller (master) represents a single point failure. When it fails, the entire system fails. Therefore, many systems based on MIL-STD-1553B are equipped with redundant bus controllers (distributed control), only one having control of the network at a time. In a fully distributed control system, all members of the network are in charge of their own access. Single fault tolerance can be achieved when appropriate fault handling mechanisms are implemented as well as true partitioning is in place (both within and between Line Replaceable Units, LRUs) to prevent fault propagation. So if any member fails or happens to operate erratically, the rest of the system will not be affected and will continue operating correctly. The weakness of the distributed control is exactly the opposite of the central control strength. It generates higher complexity in bus access sharing, and a change to the system configuration may require a change to every user of the system.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) 5.4.2 About ARINC

Aeronautical Radio Incorporated (ARINC) is a major company that develops and operates systems and services to ensure the efficiency, operation, and performance of the aviation and travel industries. It was set-up in 1929 by four major airlines to provide a single licensee and coordinator of radio

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communications outside the government. Only airlines and aviation-related companies can be shareholders, although all airlines and aircraft can use ARINC' s services.

5.4.3 Data Encoding


Parity Checking

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In communications, parity checking refers to the use of parity bits to check that data has been transmitted accurately. The parity bit is added to every data unit (typically seven or eight bits) that are transmitted. The parity bit for each unit is set so that all bytes have either an odd number or an even number of set bits. Assume, for example, that two devices are communicating with even parity (the most common form of parity checking). As the transmitting device sends data, it counts the number of set bits in each group of seven bits. If the number of set bits is even, it sets the parity bit to 0; if the number of set bits is odd, it sets the parity bit to 1. In this way, every byte has an even number of set bits. On the receiving side, the device checks each byte to make sure that it has an even number of set bits. If it finds an odd number of set bits, the receiver knows there was an error during transmission. The sender and receiver must both agree to use parity checking and to agree on whether parity is to be odd or even. If the two sides are not configured with the same parity sense, communication will be impossible. Parity checking is the most basic form of error detection in communications. Although it detects many errors, it is not foolproof, because it cannot detect situations in which an even number of bits in the same data unit are changed due to electrical noise.

Parity checking is used not only in communications but also to test memory storage devices. Many PCs, for example, perform a parity check on memory every time a byte of data is read. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Binary Encoding Formats

There are several ways in which binary 1 and 0 can be represented by voltage levels on a pair of wires. The few which are used on aircraft data bus types will be shown here:

Bipolar Return to Zero (BPRZ)

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In Bipolar Return to Zero encoding, a Hi level (logic 1) is given by a positive voltage on one wire followed by a return to null (zero). A Lo level (logic 0) is


given by a negative voltage on the other wire followed by a return to null.

BPRZ requires a clock to maintain the bit cell period.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Harvard Bi-Phase Harvard Bi-Phase uses a two phase voltage level to represent a high (logic 1) and a single phase voltage to represent a low (logic 0). If two lows occur sequentially, the voltage level will toggle from high to low or low to high, but remain in that state for the time period of the bit cell. Harvard Bi-Phase requires a clock to maintain the bit cell period.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Manchester II / Non Return to Zero (NRZ)

Manchester II NRZ uses the change from a high to a low voltage to represent a high (logic 1) and a change from low to high to represent a low (logic 0). Manchester II NRZ requires a clock to maintain the bit cell period.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) 5.4.4 ARINC 429

ARINC 429 is the most commonly used data bus for commercial and transport aircraft. ARINC 429 employs unidirectional transmission of 32 bits word over two wires (twisted pairs) using bipolar RZ format. The protocol has five fields in each word which enables it to transmit the data and the address

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information within the same bus transmission. Messages are repeated at specified intervals with typical applications sending groups or frames of messages.

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The commonly used word formats such as BNR, BCD, Discrete data, and other formats. ARINC 429 is a specification, which defines how avionics


equipment and systems should communicate with each other. They are interconnected by wires in twisted pairs. The specification defines the electrical and data characteristics and protocols, which are used.

ARINC 429 employs a unidirectional (simplex) data bus standard, sometimes known as Mark 33 Digital Information Transfer System (DITS). Messages are transmitted at a bit rate of either 12.5 kilobits per second or 100 kilobits per second to other system elements, which are monitoring the bus messages. Transmission and reception is on separate ports so that many wires may be needed on aircraft, which use a large number of avionics systems.

ARINC 429 Usage

ARINC 429 has been installed on most commercial transport aircraft including; Airbus A310/A320 and A330/A340; Bell Helicopters; Boeing 727, 737, 747, 757, and 767; and McDonnell Douglas MD-11. Boeing has installed a newer system specified as ARINC 629 on the 777, and some aircraft are using alternate systems in an attempt to reduce the weight of wire needed and to exchange data at a higher rate than is possible with ARINC 429. The unidirectional ARINC 429 system provides high reliability at the cost of wire weight and limited data rates. Military aircraft generally use a high-speed, bidirectional protocol specified in Military Specifications MIL-STD-1553.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2)

The ARINC 429 System First of all we are going to consider the ARINC 429 Specification for data transfer. As an example let us consider the Air Data Computer which will have on its output side an ARINC 429 transmitter (TX). One of its outputs (among many) on the data bus will be altitude information which will be picked up by the ARINC 429 Receiver (RX) in the Digital Altimeter.


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Figure 4.4 SIMPLIFIED ARINC 429 SYSTEM

Figure 4.5 DITS DATA BUS

The data bus is a pair of twisted wires with shielding. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) The ARINC 429 Digital Information Transfer System (DITS) is a self-clocking, self-synchronising system called "bipolar return to zero". The data is transmitted in binary bit form. The ' 1' s and ‘0' s are represented by high voltage levels (+10v) and low voltage levels (-10v) respectively for one half of the clock cycle i.e., each pulse returns to zero in the middle of a clock pulse.

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Figure 4.6 shows the principle of this transmission. The data is transmitted in groups of 32 bits in serial fashion i.e., one bit at a time. Some data buses are designated as LOW SPEED (12.5k-14k bits/sec) or HIGH SPEED (100k bits/sec). Signals only flow in one direction on the bus.


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Figure 4.6 DITS TRANSMISSION CHARACTERISTIC



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Word Format This 32 bits word has five basic parts:

1. LABEL 2. SOURCE/DESTINATION IDENTIFIER (SDI) 3. DATA FIELD 4. SIGN STATUS MATRIX (SSM) 5. PARITY BIT (P)


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Figure 4.7 ARINC 429 DIGITAL DATA WORD



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) The LABEL. The first eight bits of the word (octal coded), identifies the information contained in the data e.g., airspeed, exhaust gas temperature etc. There are a large number of words being transmitted on the bus and the receiver decodes the labels and selects only those words it requires.

The SOURCE/DESTINATION IDENTIFIER.

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This is used to identify the source or destination of a word and are bits 9 and 10 in the word i.e., which of a number of installations the word is coming from or needs to be directed to.


The DATA FIELD

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Contains the specific data related to the label e.g., how many knots for airspeed, value of exhaust gas temperature etc. For a binary word BNR this is contained in bits 11-28; and for a binary coded decimal (BCD) word it is in bits 11-29. Any bits not used are filled with logic ‘0' s these are known as pad bits and have no data significance.

The SIGN STATUS MATRIX,

Bits 29, 30, 31 for a binary word BNR and 30, 31 for a BCD word identifies the characteristics of the word e.g., north or south, positive or negative, east or west and its status e.g., no computed data, failure warning or functional test or normal operation.

The PARITY BIT.

ARINC 429 uses odd parity i.e. the total number of logic ' 1’s in the word must be an odd number, if it is not an odd number, the parity bit is set to 1. This is used in the system to check for errors, if on receiving a signal it does not contain an odd number of ' 1' s then there is something wrong with the transmission and a fault signal would be generated.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) To enable the receiver to identify the beginning of a transmission, the data word is synchronised by a minimum 4 bits time gap. Figure 4.8 and Figure 4.9 shows two examples of an ARINC 429 transmission, one using binary BNR and the other BCD.

Binary Word Example


BCD Word Example

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Figure 4.8

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Figure 4.9

The transmitters of the system, which are embedded in the system equipment, are capable of interfacing with up to 20 receivers.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) NOTES


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) 5.4.5 MIL-STD- 1553B This is a United States Military Standard and has been adopted as a NATO standard (STANAG 3838). It is a multiple source data transmission system in that transmission can from more than one source. It is a half duplex system in that data transfer can take place in either direction on a single line but not in both directions on that line simultaneously. The basic configuration is shown in the following diagrams. The data bus may be a twisted pair or fibre optic cable with a maximum length of 100m.

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Control of the bus is effected by a Bus Controller (BC) which is connected to a number of Remote Terminals {RT), up to a maximum of 31, via the data bus (central control). These remote terminals are connected to the aircraft subsystems. Data is transferred at a rate of 1MHz using the Manchester bi-


phase digital format (see ARINC 629). The data word size is 20 bits, with an actual word size of 16 bits, with SYNCH waveform and parity taking up four bits. Between the Bus Controller (BC) and Remote Terminals (RT) there are various transfer formats.

However, we shall look at transfer of data from RT to BC. 1. BC sends a transmit signal to RT.

2. RT replies after a short time with a status word, this is followed immediately with one or more data words up to a maximum of 32. 3. Transmission of one word takes 70µs.

Transfer from one RT to another RT:

1. BC sends command to one RT.

2. Transit command sent to other RT.

3. The sending RT sends a status word followed by the data word up to a maximum of 32 words. 4. The receiving RT sends a status word to the BC.

ARINC 629 has been developed from the MIL-STD-1553B the BC being replaced by the Data Autonomous Transmission and Communication (DATAC) where each LRU has a serial interface module which controls the timing of the LRU data onto the bus when the LRU' s are not transmitting. For Training Purposes Only



f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 4.10 1553B DATA BUS LAYOUT

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Figure 4.11 1553B WORD FORMATS

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Figure 4.12 1553B TYPICAL DATA TRANSACTION



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) 5.4.6 ARINC 629 In the ARINC 429 system data only flows in one direction so if we wish to send data back to the receiving element another data bus is required. Also with the advent of more digital systems on aircraft another data transmission system was required which was faster and bidirectional. The ARINC 629 fulfils these requirements and is used, as well as ARINC 429, on the Boeing 777.

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The ARINC 629 data bus is an unshielded, twisted pair of wires bonded and terminated at both ends. Data is sent and received at a rate of 2 megabits per second. The system has three parts:


a) Data bus cable

b) Current mode coupler c) Stub cable





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Figure 4.13 AR1NC 629 - TYPICAL ARRANGEMENTS



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Data Bus Cable

Up to 120 terminals can be connected to the bus which can be up to 100 metres long. The bus operates a 2MHz which allows 100,000 20 bit words to be transmitted each second, The cable is 20 AWG (American Wire Gauge) twisted pair of wires bonded together continuously along their length, at the end of each bus cable are 130 ohm impedance matched resistors.

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Many LRU' s receive data at the same time. Communication is bidirectional and on B777, the maximum number of LRU' s on one bus is 46. All the data on the bus is available to all LRU' s on the bus. Each LRU has a serial interface module and a terminal controller which controls the timing of its LRU data onto


the bus when other LRU' s are not transmitting (distributed control).

Current Mode Coupler

This connects the data bus cable to the stub cable. These are found in panels in the electronics compartment arranged in rows so that the bus cable can run through them.

Figure 4.14 and Figure 4.15 shows a coupler, which uses the inductive principle ie, transformer coupling, as can be seen the coupler has two parts, the lower part in which the data bus cable is carefully positioned through the wire guides to give controlled routing and protection. The upper part has the electronics for putting data on and taking data off the bus and sending via the receptacle to the LRU stub cable. The housing is waterproof. The diagram shows a typical current mode coupler panel.




f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 4.14 CURRENT MODE COUPLER

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Figure 4.15 CURRENT MODE COUPLER PANEL



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Stub Cables

The stub cables are for bidirectional data movement between LRU and current mode coupler. The stub cables also supply power from the LRU' s to the current mode couplers. The stub cable has four wires, two to transmit and two to receive. These cables are in the normal aircraft wiring bundles.

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Figure 4.16 shows the arrangement and layout of a stub cable. These cables can be up to 40 metres long. All the data bus cables on the 777 are inside the current mode coupler panels except the left and right bus cables. The diagrams show the basic layout.


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These long runs of cables will have production break splices done at the factory, and you can see the coupler connector to the coupler panel.





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Figure 4.16 ARINC 629 STUB CABLE



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Data Structure The data is transmitted in groups called messages. Messages are comprised of word strings and up to 31 word strings can be in a message. Word strings begin with a label followed by up to 256 data words. Each label and data word is 20 bits. Figure 4.17 shows the complete structure of the ARINC 629 message.


GENERAL - ARINC 629 DATA BUS CABLE

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BUS CABLE

Figure 4.17 (A)





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COMPLETE ARINC 629 MESSAGE STRUCTURE

Figure 4.17 (B)



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) A particular parameter is defined by its position in the labelled word string and it is the responsibility of the system designer to define labels and parameters. The table below compares the ARINC 429 to the ARINC 629 data word. ARINC 429 requires a different word for each type of information it sends. ARINC 629 uses a word string that has a label followed by information that can have up to 256 types of data i.e., more information on the bus hence faster transmission. ARINC 629 deals with standards for this bus and it is also referred to as the Digital Autonomous Terminal Access Communication (DATAC) bus.

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To keep the data bus to a twisted pair of cables it was necessary for the databus to be self-clocking. Manchester II bi-phase is the form of self-clocking used in ARINC 629. See Figure 4.18.

f o g y n r i r a t e e e i n r i p g o n r P SE A M ARINC 429 LABEL PARAMETER 230 231 233 234 235 323 330 331 332 333 362 364 363

DATAC LABEL WORD POSITION PARAMETER AIR DATA TRUE AIRSPEED 230 1 TRUE AIRSPEED TOTAL AIR TEMP 2 TOTAL AIR TEMP STATIC AIR TEMP 3 STATIC AIR TEMP BARO CORRECTION (mB) #1 4 BARO CORRECTION (mB) #1 BARO CORRECTION (in of Hg) #1 5 BARO CORRECTION (in of Hg) #1 AIR DATA FLIGHT PATH ACCEL 330 1 FLIGHT PATH ACCEL BODY WAY RATE 2 BODY WAY RATE BODY LONGITUDINAL ACCEL 3 BODY LONGITUDINAL ACCEL BODY LATERAL ACCEL 4 BODY LATERAL ACCEL BODY NORMAL ACCEL 5 BODY NORMAL ACCEL ALONG TRACK HRZ ACCEL 6 ALONG TRACK HRZ ACCEL VERTICAL ACCEL 7 VERTICAL ACCEL CROSS TRACK HRZ ACCEL 8 CROSS TRACK HRZ ACCEL DATA IDENTIFICATION/ RECOGNITION COMPARISON TABLE – EXAMPLE





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Figure 4.18 MANCHESTER II BIPHASE ENCODING PRINCIPLE



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) The data buses between LRU' s and LRM' s (line replacement modules) may be of different types and data maybe transferred at different speeds. For example: ARINC 629 ARINC 429 ARINC 453 ARINC 717

sends and receives data at 2 megabits per second. one way buses data sends and receives data at low speed (12 kilobits per second and 14 kilobits per second) and high speed (100 kilobits per second) is a one way bus that sends data at 1 megabit per second. is a one way bus that sends and receives data at 128 words per second. is a one way bus that sends and receives data at 128 words per second. is a one way bus that sends and receives data at 9600 bits per second is a one way bus that sends and receives data on a casual wire at 20 kilobits per second. two way bus that sends and receives signals at 10 megabits per second. two way bus that sends and receives signals at 10 megabits per second. two way bus that sends and receives signals at 10 megabits per second. fibre optic data bus sends and receives signals at 100 megabits per second.

f o g y n r i r a t e e e i n r i p g o n r P SE A M ARINC 618 RS-422 RS-232

10 base T RS-485

10 base 2

ARINC 636

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As you can see there is a considerable amount of data being sent and received at varying speeds and using a variety of data bus systems. You do not need to remember all these data bus systems but you should appreciate the layout of a bus system on a modem aircraft.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Protocol

This is the formal code of behaviour of a computer/ data bus system. It governs the sequence which computers/terminals write data onto the data bus. In basic terms it means that when one computer/terminal is writing data onto the bus the rest are quiet and, if necessary, reading that data off the bus. It can

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be either Basic Protocol (BP) or Combined Protocol. The BP can be operated in periodic mode or aperiodic mode. We shall consider the BP modes.

Timing


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All operations on a data system are governed by a timing signal. On the ARINC 629 system the timing of messages is such as to ensure that the required repetition rate is maintained and there are no collisions between messages. Three timers are used to ensure periodicity, collision free access and equal access opportunity, these are: •

Transmit Interval (TI). This is common to all terminals and can be set in the range of 0.5 to 64 milliseconds. TI controls the minimum interval between periodic transmissions.

•

Terminal Gap (TG). Each terminal has a unique time value assigned to it. This is in the range from 4 to 128 microseconds. The TG is the parameter which ensures clash free access.

•

Synch Gap (SG). This is common to all terminals and is set to a value larger than the largest TG. SG is the basic parameter which ensures that all the terminals have equal access opportunity.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) ‘Periodic’ Mode

Figure 4.19 shows the operation in periodic mode. This is used when there is a sufficient bus data rate to allow all messages to be transmitted at the same rate (the highest rate). The TI is set to the inverse of the TG to allow collision free access. There is spare time on the TI and the terminals generally transmit in a set order (power-up order). The SG does not feature in this mode.


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Figure 4.19 BUS ACTIVITY - PERIODIC MODE



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) ‘Aperiodic’ Mode

If there is insufficient bus capacity for all the messages to be transmitted at the maximum rate then the aperiodic mode is used. Each terminal is given an equal opportunity to transmit one message on the bus during a defined period. This is achieved by assigning to each terminal a unique terminal gap (TG) value and by determining a Synchronisation Gap (SG) that is longer than the longest TG.

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Figure 4.20 shows the operation of a simple three terminal system. The terminal with the shortest TG starts to transmit (terminal 1). All other terminals reset their timers. When the first terminal has finished transmitting it will have to wait until the bus has been quiet for the duration of the SG before it is given the


opportunity to transmit again. So effectively now the terminal with the shortest TG that has not transmitted it' s information is terminal 3 (looking at figure 4.20, terminal 3' s TG is shorter than terminal 2' s TG so terminal 3 will transmit.) This process will repeat itself until all the terminals have transmitted in order of their shortest TG.

When all terminals have transmitted the bus goes quiet while SG timers operate, and the cycle will start again with the shortest TG terminal transmitting first. This cycle time is called a Minor Frame (MIF) and the shortest MIF possible (equal to the synchronization gap, and the sum of the messages and their terminal gaps) is the Minimum Frame Time (MFT).




f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 4.20 BUS ACTIVITY – TIMING

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Figure 4.21 DATA BUSES



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) 5.4.7 Ethernet History of Ethernet Engineers Bob Metcalfe and D.R. Boggs developed Ethernet beginning in 1972. Industry standards based on their work were established in 1980 under the IEEE 802.3 set of specifications. Generally speaking, Ethernet specifications define low-level data transmission protocols and the technical details manufacturers need to know to built Ethernet products like cards and cables.

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Ethernet technology has evolved and matured over a long time period. The average consumer can generally rely on the off-the-shelf Ethernet products to work as designed and to work with each other. Ethernet Technologies


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Traditional Ethernet supports data transfer at the rate of 10 Megabits per second (Mbps). Over time, as the performance needs of LAN have increased, the industry created additional Ethernet specifications for Fast Ethernet and Gigabit Ethernet. Fast Ethernet extends traditional Ethernet performance up to 100 Mbps and Gigabit Ethernet up to 1000 Mbps speeds. Ethernet cables likewise are manufactured to any of several standard specifications. The most popular Ethernet cable in current use, Category 5 or CAT5, supports both traditional and Fast Ethernet. The Category 5e (CAT5e) cable supports Gigabit Ethernet. Quad cables are used in modern commercial aircraft to transport data of the high speed Ethernet. Ethernet data communication is used on the Airbus A380 in the Network Server System (NSS) and in the Avionics Data Communication Network (ADCN). In the Ethernet Network the quad cable guarantee failure free data transmission with up to 100 Mbps over a distance of up to 100 meters between a very high number of users. Quad cables are usually delivered as complete harnesses. One harness is composed of a shielded quad cable with four internal copper wires and a contact (pin or socket) on each end.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Types of Ethernet Often referred to as Thicknet, 10Base5 was the first incarnation of Ethernet technology. The industry used Thicknet in the 1980s until 10Base2 Thinnet appeared. Thicknet and Thinnet used coaxial cable but Thinnet is thinner and more flexible cabling. The most common form of traditional Ethernet, however, is 10Base-T. 10Base-T offers better electrical properties than Thicknet or Thinnet, because 10Base-T cables utilize unshielded twisted pair (UTP) wiring rather than coaxial. (Note: The number shows the data transport rate in Mbps and the letter defines the cable type used as the physical transport medium) Fast Ethernet

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In the mid-1990s, Fast Ethernet technology matured and met its design goals of (i) increasing the performance of traditional Ethernet while (ii) avoiding the need to completely re-cable existing Ethernet networks. Fast Ethernet comes in two major varieties: • •


100Base-T (using unshielded twisted pair cable) 100Base-FX (using fiber optic cable)

By far the most popular of these is 100Base-T, a standard that includes 100Base-TX (full duplex), 100Base-T2 and 100Base-T4.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Ethernet Topology, Protocol and Devices Traditional Ethernet employs a bus topology, meaning that all devices or hosts on the network use the same shared communication line. Each device possesses an Ethernet address. Sending devices use Ethernet address to specify the intended recipient of massages.

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Data sent over the Ethernet exists in the form of frames. An Ethernet frame contains a header, a data section, and a footer having a combined length of no more than 1518 bytes. The Ethernet header contains the addresses of both the intended recipient and the sender. Data sent over the Ethernet is automatically broadcast to all devices on the network. By comparing their Ethernet address against the address in the frame header, each Ethernet device tests each frame to determine if it was intended for them and reads or discard the frame as appropriate.

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Devices wanting to transmit on the Ethernet first perform a preliminary check to determine whether the medium is available or whether a transmission is currently in progress. If the Ethernet is available, the sending device transmits onto the wire. It’s possible, however, that two devices will perform this test at approximately the same time and both transmit simultaneously. By design, the Ethernet standard does not prevent multiple simultaneously transmission. These so-called collisions, when they occur, cause both transmissions to fail and require both sending devices to re-transmit. Ethernet uses an algorithm based on random delay times to determine the proper waiting period between re-transmissions. In traditional Ethernet, this protocol for broadcasting, listening and detecting collisions is known as CSMA/CD (Carrier Sense Multiple Access / Collision Detection).


Some newer forms of Ethernet do not use CSMA/CD. Instead, they use the so-called full duplex Ethernet protocol, which supports point-to-point simultaneously sends and receives with no listening required.





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Figure 4.22 TRADITIONAL ETHERNET



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) Avionics Full Duplex Switched Ethernet (AFDX) A problem with the standard ethernet is, that if you have many computers you may get long waiting times to detect a silent medium that allows you to transmit the information. Therefore if you have time critical data transmissions like in the avionics system, you need to divide the ethernet network by switches. In the Avionics world this is called the AFDX switch. Now all computers can transmit data at any time and the switch transfers the data only to the receiver who needs the information. Here for example from computer 1 to computer 3. At the same time computer 5 could send data to computer 6. A collision is not possible, because the switch always buffers the data until the specific line is free. If for example Computer 1 and computer 5 want to send data at the same time to computer 3, than the AFDX switch buffers both data and transmit the data after each other.


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Figure 4.23 FAST ETHERNET



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES & ELECTRONIC INSTRUMENT SYSTEMS DATA BUSES (DCAM 5.4 L2) NOTES


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) 5.5 LOGIC CIRCUITS 5.5.1 Logic gates

Introduction

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Devices used in logic networks control the flow of information through the system and are therefore known as logic gates since the ' gates'are opened and closed by the binary inputs in order to perform a logical function.


Logic gates are the basic building blocks from which many different kinds of logical outputs can be obtained. The gates we shall consider are the AND, NOT, OR, NAND, NOR and XOR gates. They are made up of electronic components and the output can be represented by Boolean algebra (named after George Boole (1815 -1864).

Logic gates have binary inputs of 1 or 0 and they may represent (in a circuit) ON, CLOSED (logic 1) or OFF, OPEN (logic 0). We shall be using the American symbols for the gates (there are British Standard symbols but are not in common use in the aircraft industry).



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) AND Gate

This gate can have two or more inputs and only one output. It will give an output if all inputs are on. If any one input is not available the output will be zero. The symbol for a 2 input AND gate is shown below. The AND gate can be made up electrically by two switches in series. The lamp will only light when

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switches A AND B are both made. If any one switch is open the lamp will not light. The operation of the logic gate can be described by means of a TRUTH TABLE.

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•

When switch A is open (logic 0) and switch B is open (logic 0) there is no output the lamp (logic 0).

•

When switch A is made (logic 1) and switch B is open (logic 0) there is still no output to lamp (logic 0).

•

When switch A is open (logic 0) and switch B is made (logic 1) - still no output to lamp (logic 0).

•

When switch A is made (logic 1) and switch B is made (logic 1) there is an output to the lamp (logic 1).

f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 5.1

Where A and B are the inputs and S is the output. Only 2 inputs are shown but there may be more A, B, C, D etc. You must remember the symbols and the truth table.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) ACTIVITY:

Sketch a circuit for a three input AND gate and draw it' s symbol and derive it' s truth table. The Boolean expression for this gate is written A.B = S. The dot means AND, and the expression is read as "A AND B equals S" (in some books the output is called z).


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) OR Gate

This can have two or more inputs and will give an output if any one input is logic 1.


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An OR gate circuit can be made up by two switches in parallel. The lamp will light if switch A OR B is closed.

So the truth table will be:

The Boolean expression is:

A + B = S The + means OR and the expression is read as "A OR B equals S". For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) NOT Gate

This gate has one input and one output.


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This gate produces an inversion of the input signal, so when the input is A the output is NOT A, which is symbolised by a bar on top of the A = A. So the output of this gate is the opposite to it' s input.

So input logic 1; output logic 0. Input logic 0; output logic 1. The truth table:




PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) NAND Gate

This is short for NOT AND and works similar to a NOT gate except that it has more than one input.


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The bubble on the end of what is an AND gate has the same function as in the NOT gate - it inverts the signal, except that in this case more than one input is involved. In this gate when A is 0 AND B is 0 then the output is 1. In the AND gate this would be 0. So the NAND gate is an inverted AND gate.

The truth table is:


A⋅ B = S

The bar over A.B gives NOT AND and is read as “NOT (A AND B) EQUALS S”



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) NOR GATE

This is short for NOT OR and is simply a negated OR gate.


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Again an input A = 0 and B = 0 would, for a OR gate, give 0 as an output, but for the NOR gate it would give a 1 as an output.

The truth table is:

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A+ B = S

The bar over A+B gives NOT OR and is read as “NOT (A OR B) equals S”.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) XOR Gate

You may have noticed that the OR gate gives an output when A OR B = 1 and when A AND B = 1. The XOR gate only gives an output when A OR B are 1 not when A and B are 1, so it is exclusively an OR gate and will not work under the AND function. It is read as a two syllable word x then or.


The truth table is:

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The Boolean expression is

AB + AB = S Which is read as "NOT A AND B OR A AND NOT B EQUALS S".



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) It should be appreciated that for all the gates so far discussed we have assumed logic 1 is positive (+5 volts) and logic 0 is zero (0 volts). This is called POSITIVE LOGIC and is the notation most frequently used.

However, NEGATIVE LOGIC may be used, and this means that logic 0 is positive (+5 volts) and logic 1 is zero (0 volts). We shall use positive logic throughout this book. To consolidate your knowledge of logic gates we shall put a few together to make up some simple logic circuits.

Note: Bubbles on inputs to gates negates or inverts the signal.


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With reference to logic circuit 1, what is the output logic level S if A = logic 1 B = logic 0 and C = logic 1?

With A = 1 and B = 0 (remember this 0 is changed to 1 by the bubble) the output of the first gate = 1. The input to the second gate is 0 (the 1 from gate 1 inverted, and input C = 0) so the output is 0.

What would be the Boolean expression for this circuit? The output of gate 1 is A.B (read as "A AND NOT B"). The output of gate 2 is A. B. C (read as "NOT [A AND NOT B] AND C").



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2)

ACTIVITY:

Study the following circuits and determine the logic level of the output from each.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) 5.5.2 Electronic Gates in Circuit

The earliest examples of logic mechanisms were those using mechanical levers (the key in a lock for example - AND logic}, gear wheels etc. Other systems used low-pressure highly filtered air in accurately engineered logic gates to control machines. These devices were slow, heavy and prone to failure due to their moving parts.

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Electrics have played a part in logic circuits as shown in the drawings above on AND and OR circuits. Micro switches, suitably wired are still used on many aircraft to perform logic functions. With the introduction of electronics, solid state circuitry can perform the most complex logic functions with the advantages


of minimal size and weight; very low power consumption, and extremely fast operation.

The simplest logic using discrete components was the diode-resistor logic, the diagram below shows an AND gate.

When A or B = 0 then current will flow through the resistor and the diodes. This means that all the voltage is dropped across the resistor and no voltage is on the output line, so the output is 0V (logic state 0). When A and B are logic 1 (+5 volts), no current flows, voltage is the same both sides of resistor R and output is +5v (logic state 1).



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) The diagram below shows an OR gate.


When A or B are logic 1 current will flow and S will be high logic 1.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) The next advance was resistor transistor logic and diode transistor logic. The diagrams below show some typical circuits.


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Today since the integrated circuit (IC) became possible the logic gates work by transistor-transistor logic (TTL), metal oxide semiconductors (MOS) and complementary metal oxide semiconductor (CMOS) group of families.

The integrated circuit is a complete electronic circuit, transistors, diodes, resistors, capacitors, all made from and on one ' chip'of silicon, typically 5mm square and 0-5mm thick.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) The following diagram shows a typical IC with it' s plastic case partly cutaway to show the ' chip' .


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Note the metal pins - for inserting into a suitable IC socket or to be soldered into a PCB (Printed Circuit Board). Note also the metal connections from the chip to the pins. The reason for this form of construction is so as to allow the chip to be connected to other circuits. It is too small, in it' s original form, to be handled and/or to be connected to anything.

The scales of integration refer to the number of gates contained in a single IC package: •

Small scale integration (SSI) - containing not more than 11 gates.

•

Medium scale integration (MSI) - containing up to 100 gates.

•

Large scale integration (LSI) - containing between 100 to 1000 gates.

•

Very large scale integration (VLSI) - containing over 1000 gates.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) The diagrams below show the some TTL and CMOS sates.

f o g y n r i r a t e e e i n r i p g o n r P SE A M TTL AND GATE

CMOS NOR GATE


TTL NAND GATE

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CMOS NAND GATE


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) Properties of TTL and CMOS

TTL uses bi-polar transistors along with diodes and transistors formed to microscopic dimensions on a slice of silicon (chip). TTL must have a steady 5V dc supply, while CMOS will work on dc voltages between 3V and 15V and usually requires much less power. CMOS uses unipolar Field Effect Transistors

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(FET) with metal-oxide-silicon technology; this lends itself to VLSI as they take up less room on a chip, compared to the TTL. CMOS has much higher input impedance. One important point with CMOS is that if static electric charges are allowed to build up on it' s input pins, these voltages can break down the

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thin layer of silicon oxide insulation between the gate and the other electrodes of MOSFET' S and this will destroy the IC. Anti-static protection is important.


Gate operating parameters include:

a. Speed of operation - the time that elapses between the application of a signal to an input terminal and the resulting change in the logical state at the output terminals.

b. Fan in - number of inputs coming from similar circuits that can be connected to the gate without adversely affecting it' s performance. c. Fan out - the maximum number of similar circuits that can be connected to it' s output terminals without the output falling outside the limits at which logic levels 1 and 0 are specified.

d. Noise margin - this is maximum noise voltage (unwanted voltage) that can appear at it' s input terminals without producing a change in output state.

e. Power dissipation - as in any circuit, supply voltage multiplied by the current (Power = V x I) gives the power in the circuit and this heat must be dissipated.

f.

Speed of operation - the time that elapses between the application of a signal to an input terminal and the resulting change in the logical state at the output terminals.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) Typical figures for TTL and CMOS are shown below. Speed of Operation Fan in Fan out Noise Margin Power Dissipation Standard TTL

9nS

8

10

0.4V

40mW

CMOS

30nS

8

50

1.5V

0.001mW

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If you look back at the diagrams for the TTL AND gate and the TTL NAND gate you will see that the NAND gate uses fewer components and is therefore cheaper to produce.


This also applies to the NOR gate, i.e. it is cheaper to produce than the OR gate.

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NOT GATE (INVERTING)

AND GATE

OR GATE



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) NAND gates can be connected together to form any of the other basic gates -thus reducing production costs by manufacturing one gate only. The following drawings show how these gates can be formed.

f o g y n r i r a t e e e i n r i p g o n r P SE A M NOR GATE

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NAND GATE



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) The following drawings show the pin connections of ICs for different gate configurations. There is no need to remember them but it does give a good idea of how the chip (with the gates in) is connected - although the chip itself is so small that it looks like a piece of silver metal 4 or 5 mm square.

f o g y n r i r a t e e e i n r i p g o n r P SE A M PIN CONNECTIONS TTL NOR GATE

PIN CONNECTIONS TTL AND GATE


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PIN CONNECTIONS CMOS NOR GATE

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PIN CONNECTIONS CMOS AND GATE


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) It is sometimes useful to know other logic gate representations. The drawings below show some alternative logic gate symbols.

AND GATE

f o g y n r i r a t e e e i n r i p g o n r P SE A M OR GATE

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NAND GATE

NOR GATE

NOT OR INVERTING GATE



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) At first sight it might appear that they are not equivalent by looking at the Boolean expressions. However using De Morgans rules we can prove they are the same.

De Morgans rules state:

A + B = A⋅ B A⋅ B = A + B


Taking rule 1; NOT A OR B - NOT A AND NOT B. To apply the rule - split the bar and change the sign.

A + B split the bar A + B and change the sign A ⋅ B

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Taking the first AND gate output A.B output of equivalent circuit A ⋅ B = split the bar A + B change the sign A ⋅ B . Double bar over a letter removes the bar, i.e. second bar negates the single bar so the output is A.B, same as the AND gate. This procedure can be used with all the equivalent circuits. However, much more importantly, to convert the AND gate into it' s OR gate equivalent, remove bubbles where there are any, add bubbles where there are none. No bubbles on AND gate so add bubbles on all three connections.

Look at the NAND gate and shown that the OR gate has two bubbles on the input (none on the NAND) and none on the output (none on the NAND). So WHERE THERE ARE BUBBLES REMOVE THEM, WHERE THER ARE NO BUBBLES PUT SOME IN, this will give the equivalent circuit.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) ACTIVITY:

Draw the equivalent circuit of these gates.


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We now need to look at how these gates are used in aircraft circuits. You will need to be able to interpret these diagrams and explain how an output is arrived at.

You should be able to describe the operation of a logic circuit if an unexpected logic input is present. Logic gate circuitry is extensively used in aircraft schematic diagrams for all aircraft systems including airframe systems, engine systems, and all avionic systems.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) 5.5.3 Gate Application in Aircraft Systems Take-Off Warning System

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It can be seen that it consists of two OR gates and an AND gate, with logic states sent from 7 parameters. When throttle lever is pushed forward, the switch at that position is made (advance) and there is logic state 1 to OR gate 1.

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Five other parameters are sensed and logic states sent to OR gate 2. If all the other parameters are in the take-off position, i.e.


1.

Slats not fully extended.

2.

Flaps in take-off position ie less than 25°.

3.

Spoiler handle in retract position.

4.

Horizontal stabiliser in the green band (correct angle of incidence for take-off).

Then the inputs to OR gate 2 are all logic 0 and it' s output to the AND gate is logic state 0.

The aircraft on the ground (weight switch) gives another logic 1 to the AND gate, which has at this time 2 logic 1' s and a logic 0.

If either of the four inputs go out of the take-off position e.g. flaps greater than 25°, then the flap input signal to logic gate 2 is logic 1 which makes the input to the AND gate logic 1. The AND gate now has three logic 1' s which now gives an output to the warning circuits (CONFIG light and aural warning).



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) The following schematic diagram shows a take-off warning circuit


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) Landing Gear Lever “Disagree” Circuit The below diagram is of an undercarriage "gear disagree" indication circuit. Systems 1 and 2 sense any disagreement between the landing gear position and the landing gear selector lever position. So if a disagreement is detected then the ' gear'light illuminates.


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LANDING GEAR LEVER "DISAGREE" CIRCUIT

ACTIVITY:

Explain the operation of the circuit.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) ACTIVITY: To further enhance your ability to interpret logic diagrams, study the next drawing. It is a take-off warning circuit to warn the pilot if the aircraft is in the incorrect configuration for takeoff. Draw the logic circuit for the diagram.

It is important that you look at the logic schematic diagrams for your aircraft and work out how the gates are used.


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TAKE-OFF WARNING CIRCUIT



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) 5.5.4 Tri-state Devices

A tri-state device is a modification of the NOT gate. It includes an extra input called the Control Gate. The NOT gate actually has three output possibilities: •

If the control line is ON and the input is 1, the output is 1.

•

If the control line is ON and the input is 0, the output is O.

•

If the control line is OFF, the output should have no effect on the bus.


This is not the same as either a 0 or a 1. It is "disconnected", like a loose wire. We call such a thing a "tri-state device".


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS LOGIC CIRCUITS (DCAM 5.5 L2) Another version of a tri-state device is shown in fig 5.33 (b):


(Different models of tri-state devices differ in whether the data, or control, or both, or neither, is inverted.)

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Tri-state devices are connected to the output of logic circuits, just before the logic circuit connects to a bus. In this way, several logic circuits can be connected to the same bus. Thus, when the control line is off, the output does not interfere with other outputs all tied together on the bus. So at all times we will have exactly one item outputting to the bus, by switching the control line on for that item' s tri-state device and switching the control line off for all other items'tri-state.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) 5.6 BASIC COMPUTER STRUCTURE

A basic computer consists of the following: Central Processing Unit (CPU), Memory, Address Bus, Control Bus, Data Bus, Input/Output ports and power connections. Figure 6.1 shows the basic layout.


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Figure 6.1 BASIC COMPUTER LAYOUT

These elements are known as the hardware elements of a computer.

The computer' s function is to execute the instructions in the program. The program, it' s procedures, and documentation is known as the software. On aircraft this will be mainly in the form of pre-loaded programs but in some systems the program is on disc, which can be changed e.g., Flight Management System Navigational Data Base.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) 5.6.1 Central Processing Unit (CPU) This executes the individual instructions that are in the program and may also be called a microprocessor. This is a single integrated circuit on a chip. Its components include: •

The Register section

•

Arithmetic Logic Unit (ALU)

•

Control and Timing section


The Register Section

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The register section consists of a number of storage locations (shift registers) where a piece of data is kept, or one of the registers used by the microprocessor in carrying out the different operations.

These storage elements or temporary memory store a single byte or word; a byte is typically an 8 bit (binary digit) word e.g., 10011101. Arithmetic Logic Unit (ALU)

This unit performs the arithmetic and logical operations. All calculations are performed in binary, including addition, subtraction, multiplication and division. Adders can be made using a number of logic gates where the basic principles of addition apply i.e., 0+0=0 1+0=1 0+1=1

1+1=1 carry 1



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) For subtraction the following method using "two' s complement" may be used. To obtain the two' s complement of a number (in binary form): i.

invert the binary number i.e., change all the ' 0' s to ‘1’s and all the ' 1' s to ‘0’s

ii.

then add 1

Example: Subtract 5 from 9 using a word length of 4 bits.

STEP 1: Write down in binary form the equivalent of binary 510 and 910.


510=0101

910=1001

STEP 2: Subtract binary 5 from binary 9. Binary 5 is

0101

Invert gives

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1010

Add 1

1

1011 this is the two’s complement

Gives

STEP 3: Now we ADD the two’s complement of 5 to binary 9 9

1001

-5

1011

(1)0100

Using a word length of four bits [the carry one (1) is ignored], the answer is 0100 which is decimal 4.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) Multiplication can be achieved by repetitive adding:

A x B = A + A + A + ……. (B times)

Division is achieved by repetitive subtractions: A ÷ B = A - B - B - B - …… Until the remainder is zero or a smaller number than B.

Timing and Control Section


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This co-ordinates the internal operation of the microprocessor and controls the operation of the ALU and registers to the desired action specified when an instruction is performed. The clock in this section is a crystal controlled oscillator producing pulses typically 120MHz to 200MHz (200,000,000 pulses per second). The program counter within the section counts the pulses and initiates the next step in the program and points to the address of the next instruction.

The microprocessor communicates with the memory to access, store and transfer data to the data highway or bus.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) 5.6.2 Memories

The purpose of the memory is to store the program instructions and data. These are recalled at appropriate times by the CPU while it is performing its functions. Memory can be basically divided into two types: a. Magnetic. b. Solid state.

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Magnetic memory is typically used where bulk-data is maintained on a long-term basis. In this type of memory, the data can be stored as a presence (or


absence) of a magnetized area in the storage medium.

Solid state memory can be used for transient or permanent data storage. In this type of memory, the data is stored as a voltage level. Access to the memory elements is directed by the address bus. Information transfer is accomplished using the bi-directional data bus.

Memory devices are the individual elements of the computer memory which can store Logic "1" and Logic "0" bits, in such a manner, that a single bit or group of bits (words) can be stored and retrieved. The memory can be physically part of the computer. For example, the computer-on-a chip has built-in memory. Although this memory is usually small, it does provide the computer with the necessary storage for instructions and data.

Memory devices may be on the same circuit card as the computer, in the same line replaceable unit (LRU), or in a separate LRU. In the case of aircraft computer systems, the memory is usually within the same LRU. There are many memory devices in use. The most common are the magnetic tape, magnetic disc and solid state memory.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) General Characteristics

Many devices have been used for storage of data in computers. Some of these devices have found wide usage while others have seen only limited application. Because of the diversity between the various memory types, it is difficult to define a set of standard characteristics which apply to all. There are some characteristics, however, which are generally applied to memory devices.

Volatile vs Non-Volatile Memories


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The selection of a memory type for a given computer system can be based upon the need for memory retention when system power is removed. If the memory is of a type that loses its data when power is removed, it is termed volatile memory; memory that retains data when power is removed is termed non-volatile memory. Volatile memory is made temporarily non-volatile by using a dedicated battery to provide enough power to the device to hold its memory.

Magnetic vs Solid State Memories

In general, most memory devices are either classified as magnetic or solid state. Memory devices using magnetic storage of data are typically employed where bulk data is maintained on a long-term basis. In this type of device, a binary "1" is noted by the presence of a small magnetized area in the storage medium. A binary "0" has no magnetized area or a magnetized area of opposite flux.

Solid state memories are used for transient storage of data and for permanent storage of data/instructions. Binary data is stored in these devices as a voltage level on a capacitor or at the output of a latch circuit.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) Destructive vs Non Destructive Readouts

In configuring a computer system, the designer must provide additional circuitry for memories whose data is lost upon readout. These types of memories are referred to as having destructive readouts. Their use requires that the system temporarily store the data readout then write that data back into the

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memory before processing it. Memory devices which do not lose data upon readout are referred to as having nondestructive readouts.

5.6.3 Magnetic Memory Types


Magnetic Tape

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The magnetic tape is a flexible plastic tape, with a uniform coating of magnetic material on one surface. The tape is usually stored on a plastic or metal reel, and when the tape is used, it is unwound off the supply reel past a read/write head and is wound onto a take-up reel. As the tape passes the read/write head, the computer controls the reading and writing of the magnetic bits from parallel tracks on the tape. These tracks extend from one end of the tape to the other. The read/write head is divided into the same number of sections as there are tracks on the tape being used. One track on the tape is usually reserved for the clock or timing pulses to allow synchronization of the tape with the computer operation.

Magnetic tapes can be used to store the computer program instructions and data. By means of a bi-directional tape drive, the computer may access the stored information anywhere on the length of the tape. Depending upon the length of the tape and the speed of the drive mechanism, memory access times can be very long.

Magnetic tapes can also be used as a storage medium for permanent record or backup. If a program is on a magnetic disc, for example, a copy of the program can be recorded onto the magnetic tape for backup. This is stored in a library. If anything should happen to the disc, the copy tape can be used to reprogram another disc. As revisions to the program are made, revisions are also made to the stored backup tape. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2)


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Figure 6.2 The Magnetic Tape Head



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) In a typical digital flight data recorder, aeroplane operating parameters are recorded for later use during performance evaluation or during accident investigation. The flight data recorder uses a magnetic tape. The magnetic tape is made of Kapton (similar to Mylar, except for higher thermal properties). The width is 1/4 inch, the length is 388 ft. The tape is self-lubricating. The tape format is 8 track bi-directional. The tape has a recording capacity of 25 hours. Beginning-of-tape (BOT) and end-of-tape (EOT) sensors are provided to reverse direction of motion and to switch to the next track.


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Figure 6.3 Magnetic Tape Used in Digital Flight Data Recorder



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) Magnetic Floppy Disc

The magnetic floppy disc is a flat magnetically coated disc. Digital information is stored on the disc in magnetic tracks. Generally, there are 40 to 80 tracks on a disc. The tracks are concentric about the disc center and each track is divided into pie-shaped sectors. Each sector can store between 128 and 512

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bytes of information and is accessible by using read/write heads. The disc is rotated to allow the read/write heads to store or to read the information on any of the sectors. There are several sizes of magnetic discs, ranging from 3 1/2 inches to 14 inches in diameter. Memory capacity on these discs varies,

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depending on disc size and formatting. The most common floppy disc is the 3 1/2 inch floppy. Usually it is formatted to hold 1440K bytes of information. Discs can be flexible (floppy discs) or rigid (hard discs). Rigid or hard discs can be stacked in such a manner that one head or multiple heads on an access


arm can extend between pairs of discs to read or write on disc surfaces above and below the arm.

The flexible disc is usually packaged inside a more rigid plastic envelope or jacket. Head access to the disc is provided through a slot in the envelope. A sliding guard protects the magnetic disc when not in use. A drive spindle makes contact with the disc through a hole in the center of the envelope. This allows the spindle to spin the disc at high speeds inside the envelope. A typical 3 1/2 inch disc spins at around 300 rpm when data is read or written. The advantage of magnetic discs is their rapid access time. The access for magnetic tape is in seconds (or minutes) while disc access time is in milliseconds. Size is a disadvantage due to the mechanical components needed to drive and read the discs.

Figure 6.4 A Magnetic Floppy Disk (diskette)



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) Magnetic Hard Disk Drive (HDD) A hard disk is a sealed unit containing a number of platters in a stack. Hard disks may be mounted in a horizontal or a vertical position. In this description, the hard drive is mounted horizontally. Electromagnetic read/write heads are positioned above and below each platter. As the platters spin, the drive heads

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move in toward the center surface and out toward the edge. In this way, the drive heads can reach the entire surface of each platter.

On a hard disk, data is stored in thin, concentric bands. A drive head, while in one position can read or write a circular ring, or band called a track. There

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can be more than a thousand tracks on a 3.5-inch hard disk. Sections within each track are called sectors. A sector is the smallest physical storage unit on a disk, and is almost always 512 bytes (0.5 kB) in size.


The structure of older hard drives (i.e. prior to Windows 95) will refer to a cylinder/head/ sector notation. A cylinder is formed while all drive heads are in the same position on the disk. The tracks, stacked on top of each other form a cylinder. This scheme is slowly being eliminated with modern hard drives. All new disks use a translation factor to make their actual hardware layout appear continuous, as this is the way that operating systems from Windows 95 onward like to work.

To the operating system of a computer, tracks are logical rather than physical in structure, and are established when the disk is low-level formatted. Tracks are numbered, starting at 0 (the outermost edge of the disk), and going up to the highest numbered track, typically 1023, (close to the center). Similarly, there are 1,024 cylinders (numbered from 0 to 1023) on a hard disk. The stack of platters rotates at a constant speed. The drive head, while positioned close to the center of the disk reads from a surface that is passing by more slowly than the surface at the outer edges of the disk. To compensate for this physical difference, tracks near the outside of the disk are less-densely populated with data than the tracks near the center of the disk. The result of the different data density is that the same amount of data can be read over the same period of time, from any drive head position.




f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 6.5 The Internal Working of an HDD

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Figure 6.6 Parts of a Hard Drive ( with Two Platters)



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) The disk space is filled with data according to a standard plan. One side of one platter contains space reserved for hardware track-positioning information and is not available to the operating system. Thus, a disk assembly containing two platters has three sides available for data. Track positioning data is written to the disk during assembly at the factory. The system disk controller reads this data to place the drive heads in the correct sector position. A sector, being the smallest physical storage unit on the disk, is almost always 512 bytes in size because 512 is a power of 29 (2 to the power of 9). The number 2 is

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used because there are two states in the most basic of computer languages - on and off. Each disk sector is labeled using the factory track-positioning data. Sector identification data is written to the area immediately before the contents of the sector and identifies the starting address of the sector.

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The optimal method of storing a file on a disk is in a contiguous series, i.e. all data in a stream stored end-to-end in a single line. As many files are larger


than 512 bytes, it is up to the file system to allocate sectors to store the file' s data. For example, if the file size is 800 bytes, two 512 k sectors are allocated for the file. A cluster is typically the same size as a sector. These two sectors with 800 bytes of data are called two clusters. They are called clusters because the space is reserved for the data contents. This process protects the stored data from being over-written. Later, if data is appended to the file and its size grows to 1600 bytes, another two clusters are allocated, storing the entire file within four clusters.

If contiguous clusters are not available (clusters that are adjacent to each other on the disk), the second two clusters may be written elsewhere on the same disk or within the same cylinder or on a different cylinder - wherever the file system finds two sectors available. A file stored in this non-contiguous manner is considered to be fragmented. Fragmentation can slow down system performance if the file system must direct the drive heads to several different addresses to find all the data in the file you want to read. The extra time for the heads to travel to a number of addresses causes a delay before the entire file is retrieved.

Cluster size can be changed to optimize file storage. A larger cluster size reduces the potential for fragmentation, but increases the likelihood that clusters will have unused space. Using clusters larger than one sector reduces fragmentation, and reduces the amount of disk space needed to store the information about the used and unused areas on the disk. Most disks used in personal computers today rotate at a constant angular velocity. The tracks



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) near the outside of the disk are less densely populated with data than the tracks near the center of the disk. Thus, a fixed amount of data can be read in a constant period of time, even though the speed of the disk surface is faster on the tracks located further away from the center of the disk.

Modern disks reserve one side of one platter for track positioning information, which is written to the disk at the factory during disk assembly. It is not

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available to the operating system. The disk controller uses this information to fine tune the head locations when the heads move to another location on the disk. When a side contains the track position information, that side cannot be used for data. Thus, a disk assembly containing two platters has three sides that are available for data.


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Figure 6.7 Sectors and Clusters



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) Optical Laser Disc

One of the developments in memory storage is the optical laser disc or Compact Disc (CD). This method of storage uses a technique which allows a large volume of information to be stored in a much smaller area. The 12 centimeter disc currently stores over 600 megabytes (600MB or 600 000 000 bytes).

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This is enough data for an average 24 volume encyclopedia. This small compact size is used in the smaller computers (or home computers). Other laser disc systems will be used with large mainframe computers. For instance, one manufacturer produces a system which stores one trillion bytes of information on four 14-inch laser discs. This is approximately equivalent to 400 billion-typed pages.


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There are several configurations for optical laser disc. One configuration is called CD-ROM (Compact Disc, Read Only Memory). This performs a similar function as a Read Only Memory microcircuit. A second configuration is called WORM (Write-Once, Read Many times) often referred to as CD-R. This optical disc can be written onto only once but read many times. Once data has been written onto the disc it can not be erased or written over. Currently, the capacity for a WORM is up to 650 Meg. A third configuration is the rewritable CD-RW. These discs allow the user to write, read, erase and rewrite up to 650MB of information on the disc. There are two methods currently used to organize an optical disc. The first method is called Constant Linear Velocity. In this method the disc is organized much like an old vinyl record played on a stereo. The second is called Linear Angular Velocity where the data is arranged on the disc in concentric rings, much like the tracks on a floppy disc. Information is stored on the disc as "pits" or "bumps" which are only 0.83 to 3.04 microns long and 0.50 microns (0.002 inches) wide. Each "track" is 1.6 microns apart. The Digital Versatile Disc (DVD) is the same size as a standard CD but by using different data encoding techniques, has a capacity of up to 4.7 Gigabytes (4.7GB or 4 700 000 000 bytes) . Like the CD they are available in both DVD-R and DVD-RW versions but due to the variety of encoding methods compatibility can be a problem. Both CDs and DVDs are available as double sided discs, with double the capacity, for specialist applications.

In all configurations a laser disc has a special Dye-polymer coating which responds to the wavelength of the recording laser to create either "bumps" or "pits". When reading the disc, the absence or presence of these pits is detected by a photodetector. The photodetector compares a beam of light being sent from the recording laser to a beam being returned from the disc. This is done by using a beam splitter in the laser disc system. The beam splitter For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) sends part of the light to the detector. The other part of the light is sent to the laser disc, reflected back to the beamsplitter and reflected onto the photodetector. The photodetector can now compare the two beams of light. An erasable Laser disc system uses a second laser to heat the dyepolymer layer and remove the bump or pit created by the recording laser.


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Figure 6.8 The Optical Laser Disc



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) 5.6.4 Solid State Memories Semiconductor or solid state memory chips have been used for small capacity memories. However, the development of high capacity semiconductor memory chips, with the ability to store millions of bits within a single integrated circuit (IC), has greatly modified the whole technology of building computer

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memories. Semiconductor memories have made possible the building of faster, more compact, lower power consumption memories for computers.

The two principal types of solid state memory devices are the: •

Read Only Memory (ROM).

•

Random Access Memory (RAM)


Classifications of Solid State Memory

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Up to this point, you have learned some of the general functions of the CPU, the physical characteristics of memory, and how data is stored in the internal storage section. Now, we will explain yet another way to classify internal (primary or main) storage. This is by the different kinds of memories solid state memory: read-only memory, random-access memory, programmable read-only memory, and erasable programmable read-only memory.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) Read-Only Memory (ROM)

In most computers, it is useful to have often used instructions, such as those used to bootstrap (initial system load) the computer or other specialized programs, permanently stored inside the computer. Memory that enables us to do this without the programs and data being lost (even when the computer

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is powered down) is called read-only memory. Only the computer manufacturer can provide these programs in ROM and once done, they cannot be changed. Consequently, you cannot put any of your own data or programs in ROM. Many complex functions such as routines to extract square roots,

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translators for programming languages, and operating systems can be placed in ROM memory. Since these instructions are hard wired (permanent), they can be performed quickly and accurately. Another advantage of ROM is that your computer facility can order programs tailored for its needs and have them


permanently installed in ROM by the manufacturer. Such programs are called microprograms or firmware.

Programmable Read-Only Memory (PROM)

An alternative to ROM is programmable read only memory (PROM) that can be purchased already programmed by the manufacturer or in a blank state. To implant programs, or data, into a PROM a programming machine (called a PROM programmer) is used to apply the correct voltage for the proper time to the appropriate addresses selected by the programmer. As the PROM is simply an array of fusible links the programming machine essentially blows the various unwanted links within the PROM leaving the correct data patterns, a process which clearly cannot be reversed.

By using a blank PROM, you can enter any program into the memory. However, once the PROM has been written into, it can never be altered or changed. Thus you have the advantage of ROM with the additional flexibility to program the memory to meet a unique need. The main disadvantage of PROM is that if a mistake is made and entered into PROM, it cannot be corrected or erased. Also, a special device is needed to "burn" the program into PROM.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) Erasable Programmable Read-Only Memory (EPROM)

The erasable programmable read-only memory (EPROM) was developed to overcome the drawback of PROM. EPROMs can also be purchased blank from the manufacturer and programmed locally at your command/activity. Again, this requires special equipment. The big difference with EPROM is that it

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can be erased if and when the need arises. Data and programs can be retrieved over and over again without destroying the contents of the EPROM. They will stay there quite safely until you want to reprogram it by first erasing the EPROM with a burst of ultra-violet light. This is to your advantage, because if a

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mistake is made while programming the EPROM, it is not considered fatal. The EPROM can be erased and corrected. Also, it allows you the flexibility to change programs to include improvements or modifications in the future.


EPROM is re-usable by exposing the EPROM to ultraviolet (UV) light. The EPROM has to be removed from the microprocessor board and usually requires 15 to 20 minutes of exposure. A quartz window is installed on the top side of an EPROM to facilitate access to the UV light. Electrically Erasable Programmable Read Only Memory (EEPROM) was developed to replace the difficulty of reprogramming EPROM. This type of ROM is electrically alterable and the programming circuitry is included as part of the system.




f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 6.9 An EPROM Package

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Electrically Erasable Programmable Read-Only Memory (EEPROM)

EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge. Like other types of PROM, EEPROM retains its contents even when the power is turned off. Also like other types of ROM, EEPROM is not as fast as RAM.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) Random-Access Memory (RAM)

Another kind of memory used inside computers is called random-access memory (RAM) or read/write memory. RAM memory is rather like a blackboard on which you can scribble down notes, read them, and rub them out when you are finished with them. In the computer, RAM is the working memory. Data can

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be read (retrieved) from or written (stored) into RAM just by giving the computer the address of the location where the data is stored or is to be stored. When the data is no longer needed, you can simply write over it. This allows you to use the storage again for something else. Ferrite cores, semiconductor, and bubble storage all have random access capabilities.


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Most modern RAM is of the integrated circuit (IC) or ' chip'form, containing flip flops, or MOSFETS, each flip-flop being switched to the 1 or 0 position, corresponding to a ' bit'of information.

Figure 6.10 A RAM Stick



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) Static Random Access Memory (SRAM) - SRAM' s store bits (1' s or 0' s) in memory cells that are basically flip flops. In a logic diagram, an SRAM memory cell looks like two cross connected NAND or NOR gates - that is, the output of each NAND or NOR gate is connected to one input of the other NAND or NOR gate. This is a "flip flop", which can store a 1 or a zero for as long as you apply power - but not very much power. Very little power consumption is required except during transitions - writing into the memory cell. Static power consumption (without transitions) is very low because the flip

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flop is based on CMOS NAND or NOR gates where either a P-channel or an N-channel transistor conducts, but not both - and the P and N channel transistors are connected in such a way that there is no conduction path from the power supply to ground so power is not consumed in the static state

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(except through leakage). The advantages of SRAM' s are their very high speed and very low power consumption – but SRAM memory cells generally require 6 transistors.


Static Random Access Memories are the fastest type of memory.

Dynamic Random Access Memory (DRAM) - DRAM' s store bits in memory cells that are basically capacitors under transistors. The transistor is used as a relay to the capacitor, and the capacitor stores a bit (a "1" or a "0") as either storing charge on the capacitor or not storing charge. However, the charge on a capacitor gradually leaks out over time - so, periodically (generally several thousand times per second), the contents of a DRAM need to read out and then rewritten back in - this is called a "refresh cycle", or a "refresh". As you might expect, the refresh consumes a lot of power. DRAM' s consume more power, and are slower than SRAM' s - but because a DRAM cell uses just one transistor (with a capacitor) compared to 6 transistors for an SRAM cell, a DRAM memory is much cheaper than an SRAM memory.

SDRAM - Short for Synchronous DRAM, a type of DRAM that can run at much higher clock speeds than conventional memory. SDRAM actually synchronizes itself with the CPU' s bus. SDRAM is the new memory standard for modern PCs.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) DDR SDRAM - Double Data Rate - Short for Double Data Rate-Synchronous DRAM, a type of SDRAM that supports data transfers on both edges of each clock cycle (the rising and falling edges), effectively doubling the memory chip' s data throughput. DDR-SDRAM also consumes less power, which makes it well-suited to notebook computers. DDR-SDRAM is also called SDRAM II and DDRAM. DDR-SDRAM and RDRAM are the two technologies expected to replace SDRAM.

Bubble Memory (Old RAM)

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This is a magnetic solid state memory in which data is stored as microscopic cylindrical regions (the bubbles), that can drift in a thin film of garnet. These


bubbles are generated by pulses through an aluminium loop over an insulating film on top of the garnet. Presence of a bubble is logic 1, no bubble is logic 0. These ‘bubbles’ can move around in the material in a controlled manner to form a shift register type memory. This is a non-volatile memory.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) 5.6.5 Storage Capacity An eight bit word is called a BYTE. The symbol K (capital K) is used to represent 1024 bits (210). The number of addressable locations in a memory is dependant on its number of input/output lines. A memory with 8 input lines has 28 or 256 locations; one with 16 input lines has 216 or 65,536 locations. The memory capacity in the second case assuming 8 bits (byte) per location is then a 64K memory (64 kilobytes). 5.6.6 Highway Structure

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Figure 6.11 shows the highway structure, there are three buses, the address bus, the data bus and the control bus. As we have already seen the memory consists of a number of locations, each individually identified by an ADDRESS. The address bus is therefore used to specify the memory location or


input/output port involved in the transfer. It is a one way bus and may have anything from 4 to 32 lines depending on the number of memory addresses 8

there are, 8 lines give 2 =256 addresses. The data bus is a bi-directional bus and is used to carry the data being transferred to and from the memory or input and output transfer. The control bus comprises input and output lines which synchronise the microprocessor' s operation with that of the external circuitry e.g., read/write controls, timing signals, input/output selection. This is also a bi-directional bus.

Figure 6.11 HIGHWAY STRUCTURE



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS BASIC COMPUTER STRUCTURE (DCAM 5.6 L2) 5.5.7 Instruction Words

Each microprocessor has its own unique set of instructions. These instruction words normally consist of one, two or three bytes. The first byte is referred to as the operation code (opcode), this tells the microprocessor the type of operation to be performed. The remaining bytes can be data or an address indicating where the data is stored, this is known as the operand.

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An example of a single byte instruction would be to move the contents of register B to register A so the instruction would be an eight bit word which would indicate to the microprocessor that this is required ie, the opcode. A two-byte instruction will have an opcode (8 bits) and an operand, telling the


microprocessor what the data is, or the address in the memory where it can be found. So this instruction will tell the microprocessor to do something with the data e.g., move it into a particular register.

A three-byte instruction would again make byte 1 the opcode and bytes 2 and 3 the operands, the second and third bytes each of 8 bits contain either data or an address of a 16 bit word. The second byte may hold the least significant bits and byte three the most significant bits. It should be noted in some microprocessors this may be the other way round. So the instruction here might be, "the content of the memory location whose address is specified by byte 2 and 3 is moved to a register".

Fetch-Execute Cycle

The microprocessor operates in a two phase mode, during the first phase, the fetch cycle, the next instruction is fetched from the memory and then in the second phase or execution cycle the microprocessor performs (executes) the action specified by the instruction (opcode). The program counter points to the next sequential instruction to be fetched and executed. Thus during a typical instruction cycle, the next instruction to be executed is read from the memory location indicated by the contents of the program counter. While this instruction is being executed, the contents of the program counter are incremented to point to the next instruction. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS MICROPROCESSORS (DCAM 5.7 L2) 5.7 MICROPROCESSORS 5.7.1 Microprocessor (CPU) Structure and Operation Figure 7.1 shows a block diagram of a microprocessor used in an aircraft system. The function of the components is as follows: •

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Arithmetic and Logic Unit (ALU) - The arithmetic-logic section performs all arithmetic operations-adding, subtracting, multiplying, and dividing.

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Through its logic capability, it tests various conditions encountered during processing and takes action based on the result, data flows between the arithmetic-logic section and the internal registers during processing. Specifically, data is transferred as needed from the internal register section to


the arithmetic-logic section, processed, and returned to the internal register section. At no time does processing take place in the memory section. Data may be transferred back and forth between these two sections several times before processing is completed. The result of a data operation is stored in either memory or an internal register. •

Registers - Perform a wide variety of functions, e.g. temporary storage information and specific programming operations. Accumulator - This is a general purpose register which contains one of the operands and the result of most ALU operations. Instruction Register - This register holds in binary form the opcode byte of the current instruction which the microprocessor is executing. Program Counter - This register/counter holds the address of the location in the memory where the next instruction in the program sequence is to be found. The contents of the counter can be incremented or decremented by special control pulses.

Buffer Register - Temporarily stores data.

Status Register - This is normally made up of single bit indicators (called flags). Typically the flags would include: ZERO flag – is the result zero?

CARRY flag - did a carry occur?

SIGN flag – is the number plus or minus?

PARITY flag - check whether the total number of bits is odd or even

Stack Pointer – Is used to indicate the next available location in the stack memory area. It serves as the effective address in stack addressing modes as well as subroutine and interrupts processing.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS MICROPROCESSORS (DCAM 5.7 L2) Index Register – A general purpose register that is frequently used as an index value for calculation of an effective address. When executing an instruction with indexed addressing, the CPU fetches the opcode and the base address, and then modifies the address by adding an Index Register contents to the address prior to performing the desired operation.


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Figure 7.1 MICROPROCESSOR BLOCK DIAGRAM



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS MICROPROCESSORS (DCAM 5.7 L2) •

Instruction Decoder - The binary word held in the instruction register is identified by the decoder, thus enabling the control unit to send out correct timing and control pulses.

•

Timing and Control - Contains the clock for timing pulses and also carries out a large number of functions e.g.:

Providing read/write pulses for registers, memories and input/output devices via a control bus. Incrementing and decrementing the program counter.


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CPU Clock - Also called clock rate, this is the speed at which a CPU executes instructions. Every computer contains an internal clock that regulates the rate at which instructions are executed and synchronizes all the various computer components. The CPU requires a fixed number of clock ticks (or clock cycles) to execute each instruction. The faster the clock, the more instructions the CPU can execute per second. The clock cycle is the smallest operation any computer makes. Clock speeds are expressed in megahertz (MHz) or gigahertz (GHz).



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) 5.8 INTEGRATED CIRCUITS

5.8.1 Adders Adders are combinations of logic gates that combine binary values to obtain a sum. They are classified according to their ability to accept and combine the digits. In this section we will discuss quarter adders, half adders, and full adders.

Quarter Adder

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A quarter adder is a circuit that can add two binary digits but will not produce a carry. This circuit will produce the following results:

f o g y n r i r a t e e e i n r i p g o n r P SE A M 0 plus 0 =0

0 plus 1 = 1 1 plus 0 = 1

1 plus 1 = 0 (no carry)

You will notice that the output produced is the same as the output for the Truth Table of an XOR. Therefore, an X-OR gate can be used as a quarter adder. The combination of gates in Figure 8.1 will also produce the desired results. When A and Bare both LOW (0), the output of each AND gate is LOW (0); therefore, the output of the OR gate is LOW (0). When A is HIGH and B is LOW, then B is HIGH and AND gate 1 produces a HIGH output, resulting in a sum of 1 at gate 3. With A LOW and B HIGH, gate 2 output is HIGH, and the sum is 1. When both A and B are HIGH, neither AND gate has an output, and the output of gate 3 is LOW (0); no carry is produced.

Figure 8.1 Quarter Adder



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) Half Adder

A half adder is designed to combine two binary digits and produce a carry. Figure 8.2 shows two ways of constructing a half adder. An AND gate is added in parallel to the quarter adder to generate the carry. The SUM column of the Truth Table represents the output of the quarter adder, and the CARRY column represents the output of the AND gate.

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We have seen that the output of the quarter adder is HIGH when either input, but not both, is HIGH. It is only when both inputs are HIGH that the AND gate is activated and a carry is produced. The largest sum that can be obtained from a half adder is 102 (12 plus 12).

f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 8.2 Half Adders and Truth Table



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2)

Full Adder The full adder becomes necessary when a carry input must be added to the two binary digits to obtain the correct sum. A half adder has no input for carries from previous circuits.

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One method of constructing a full adder is to use two half adders and an OR gate as shown in Figure 8.3. The inputs A and B are applied to gates 1 and 2. These make up one half adder. The sum output of this half adder and the carry-from a previous circuit become the inputs to the second half adder. The

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carry from each half adder is applied to gate 5 to produce the carry-out for the circuit. Now let' s add a series of numbers and see how the circuit operates.


First, let' s add 1 and O. When either A or B is HIGH, gate 1 has an output. This output is applied to gates 3 and 4. Since the carry-in is 0, only gate 3 will produce an output. The sum of 12 and 0 is 12. Now let' s add 12 and 12. If A and B are both HIGH, the output of gate 1 is LOW. When the carry IN is 0 (LOW), the output of gate 3 is LOW. Gate 2 produces an output that is applied to gate 5, which produces the carry-out. The sum of 12 and 12 is 102, just as it was for the half adder. When A and B are both LOW and the carry-in is 1, only gate 3 has an output and produces a sum of 12 with no carry-out. Now, let' s add A or B and a carry-in. For example, let' s assume that A is HIGH, B is LOW and carry-in is HIGH. With these conditions, gate 1 will have an output. This output (HIGH) and the carry-in (HIGH) applied to gates 3 and 4 will produce a sum out of 0 and a carry of 1. This carry from gate 4 will cause gate 5 to produce a carry-out. The sum of A and a carry (12 plus 12) is 102.

When A, B, and the carry-in are all HIGH, a sum of 1 and a carry-out are produced. First, consider A and B. When both are HIGH, the output of gate 1 is LOW, and the output of gate 2 is HIGH, giving us a carry-out at gate 5. The carry-in produces a 1 output at gate 3, giving us a sum out f 1. The output of the full adder is 112. The sum of 12 plus 12 plus 12 is 112.





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Figure 8.3 Full Adder and Truth Table



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) Parallel Adders

The adders discussed in the previous section have been limited to adding single-digit binary numbers and carries. The largest sum that can be obtained using a full adder is 112. Parallel adders let us add multiple-digit numbers. If we place full adders in parallel, we can add two- or four-digit numbers or any

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other size desired. Figure 8.4 uses STANDARD SYMBOLS to show a parallel adder capable of adding two, two-digit binary numbers. In previous discussions we have depicted circuits with individual logic gates shown. Standard symbols (blocks) allow us to analyze circuits with inputs and outputs

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only. One standard symbol may actually contain many and various types of gates and circuits. The addend would be input on the A inputs (A2 = MSB, A1= LSB), and the augend input on the B inputs (B2 = MSB, B1= LSB). For this explanation we will assume there is no input to C0 (carry from a previous circuit).


Now let' s add some two-digit numbers. To add 102 (addend) and 012 (augend), assume there are numbers at the appropriate inputs. The addend inputs will be 1 on A2 and 0 on A1. The augend inputs will be 0 on B2and 1 on B1. Working from right to left, as we do in normal addition, let' s calculate the outputs of each full adder. With A1 at 0 and B1at 1, the output of adder 1 will be a sum (S1) of 1 with no carry (C1). Since A2 is 1 and B2 is 0, we have a sum (S2) of 1 with no carry (C2) from adder 1. To determine the sum, read the outputs (C2, S2, and S1) from left to right. In this case, C2 = 0, S2 = 1, and S1 = 1. The sum, then, of 102 and 012 is 0112 or 112.

To add 112 and 012, assume one number is applied to A1 and A2, and the other to B1 and B2, as shown in Figure 8.5. Adder 1 produces a sum (S1) of 0 and a carry (C1) of 1. Adder 2 gives us a sum (S2) of 0 and a carry (C2) of 1. By reading the outputs (C2, S2, and S1), we see that the sum of 112 and 012 is 1002. As you know, the highest binary number with two digits is 112. Using the parallel adder, let' s add 112 and 112. First, apply the addend and augend to the A and B inputs. Calculate the output of each full adder beginning with full adder 1. With A1and B1 at 1, S1 is 0 and C1 is 1. Since all three inputs (A2, B2, and C1) to full adder FA2 are 1, the output will be 1 at S2 and 1 at C2. The output of the circuit, as you read left to right, is 1102, the sum of 112 and 112. Parallel adders may be expanded by combining more full adders to accommodate the number of digits in the numbers to be added. There must be one full adder for each digit.




f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 8.4 Parallel Binary Adder


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Figure 8.5 Parallel Addition


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) Subtraction Subtraction is accomplished in computers by the R' s complement and add method. R' s complement subtraction allows us to use fewer circuits than would be required for separate add and subtract functions. Adding X-OR gates to full adders, as shown in Figure 8.6, enables the circuit to perform R' s complement subtraction as well as addition.

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To add two numbers using this circuit, the addend and augend are applied to the A and B inputs. The B inputs are applied to one input of the X-OR gates.

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A control signal is applied to the other input of the X-OR gates. When the control signal is LOW, the circuit will add; and when it is HIGH, the circuit will subtract. In the add mode, the outputs of the X-OR gates will be the same as the B inputs. Addition takes place in the same manner as described in


parallel addition. Before we attempt to show subtraction, let' s review R' s complement subtraction. To subtract 102 from 112, write down the minuend (112). Perform the R' s complement on the subtrahend. Now add the minuend and the complemented subtrahend.

Disregard the most significant 1, and the difference between 112 and 102 is 012. The most significant 1 will not be used in the example shown in the following paragraph. Now let' s subtract 102 from 112 using the adder/subtracter circuit. The minuend (112) is input on the A terminals, and the subtrahend (102) is input on the terminals. In the subtract mode, a 1 from the control circuit is input to each of the X-OR gates and to the Co carry input. By applying a 1 to each of the X-OR gates, you find the output will be the complement of the subtrahend input at B1and B2. Since B1 is a 0, the output of X-OR 1 will be 1. The input B2 to X-OR 2 will be inverted to a 0. The HIGH input to Co acts as a carry from a previous circuit. The combination of the X-OR gates and the HIGH at Co produces the R' s complement of the subtrahend. The full adders add the minuend and the R' s complement of the subtrahend and produce the difference. The output of C2 is not used. The outputs of S2 and S1 are 0 and 1, respectively, indicating a difference of 012. Therefore, 112 minus 102 equal 012.





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Figure 8.6 R’s Complement Adder/Substracter



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) 5.8.2 FLIP-FLOPS

Memory type circuits use ' flip-flops'as their main components. There are many types but we shall look at three, the SR or RS, D and JK flip-flops. These are so called because on the application of a suitable pulse at the input it causes it to flip'into one of it' s two stable states and stay in that state until a second input will ' flop'it into its previous state.

SR Flip-flop


The SR flip-flop has two output terminals Q and Q . The diagram shows the SR flip-flop using NAND gates.

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SR FLIP-FLOP

When S=1, R=0; Q=1, Q =0 the flip flop is SET

When S=0, R=1; Q=0, Q =1 the flip flop is RESET

When S=0, R=0 then no change occurs Q and Q will be what they were before.

When S=1 and R=1 then Q=1 and Q equals 1. The circuit is stable while S=R=1, but if they are changed simultaneously from 1 to 0 then due to different switching times of the gates we cannot predict whether Q or Q will be 1. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) The output state is said to be indeterminate so S=R=1 should not be allowed to occur. The truth table is shown below.

S

R

Q

Q

1

0

1

0

0

1

0

1

0

0

Depends on state before inputs applied

1

1

Indeterminate


So basically the flip-flop can exist in two stable states: Q = 1 ( Q = 0) or

Q = 0 ( Q = 1)


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) Clocks In sequential logic circuits where there may be a large number of flip-flops, it is important they all act at the same time, so no circuit operates out of sequence.

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This is achieved by a CLOCK pulse from a high frequency pulse generator. The circuits may be triggered when the clock pulse changes from 1 to 0 or when it changes from 0 to 1 (edge triggered) or when the level is 1 or 0. Figures 8.7 and Figure 8.8 shows a clocked SR flip-flop and it’s truth table.


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Figure 8.7 CLOCKED SR FLIP-FLOP

INPUTS

S 0 0 1 1 0 0 1 1

DURING CLOCK PULSE

OUTPUTS BEFORE CLOCK PULSE

OUTPUTS AFTER CLOCK PULSE

R

A

B

Q

Q

Q

Q

0 0 0 0 1 1 1 1

1 1 0 0 1 1 0 0

1 1 1 1 0 0 0 0

1 0 1 0 1 0 1 0

0 1 0 1 0 1 0 1

1 0 1 1 0 0 1 1

0 1 0 0 1 1 1 1

COMMENTS

NO CHANGE IN OUTPUTS FLIP-FLOP SETS WITH Q = 1 & FLIP-FLOP RESETS WITH

Q =0

Q =1&Q=0

THIS INPUT IS NOT ALLOWED

Figure 8.8 TH TABLE - CLOCKED SR FLIP-FLOP



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) D Type SR Flip-flop This is a modified SR flip-flop. The D stands for Delay. If you look at the truth table, when the clock pulse changes (rises), whatever is at D is transferred to Q, when clock pulse falls Q stays at that level. NO MATTER WHAT IS APPLIED TO D, Q will only change state at the next clock pulse. The truth table shows that the output equals the input one clock pulse earlier, i.e. the data is held back until the clock pulse = 1.

f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 8.9 CLOCKED D TYPE FLIP-FLOP

OUTPUTS

INPUT

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BEFORE OUTPUTS

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AFTER

CLOCK PULSE

CLOCK PULSE

D

S

R

Q

Q

Q

Q

0

0

1

1

0

0

1

0

0

1

0

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0

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1

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Figure 8.10 TRUTH TABLE CLOCKED D TYPE FLIP-FLOP



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) JK Flip-flop The next diagram shows the layout and truth table of the JK flip-flop using NAND gates. The two inputs are called J and K, the operation is fully described in the truth table and J=K=1 is allowed (unlike S=R=1 in a SR flip flop) and toggles (changes state) when this input is applied. These flip-flops are used in counters and shift registers and a wide variety of logic circuits.

f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 8.11 JK FLIP-FLOP

INPUTS J 0 0 1 1 0 0 1 1

OUTPUTS

BEFORE DURING

CLOCK OUTPUTS

AFTER

CLOCK PULSE

PULSE

CLOCK PULSE

K

Q

Q

A

B

Q

Q

0

1

0

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1

0

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0

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COMMENTS

NO CHANGE IN OUTPUTS

STAYS AT OR SET TO Q = 1 & Q = 0

STAYS AT OR SET TO Q = 1 & Q = 0

TOGGLES

Figure 8.12 TRUTH TABLE JK FLIP-FLOP



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) 5.8.3 Counters

A counter is simply a device that counts. Counters may be used to count operations, quantities, or periods of time. They may also be used for dividing frequencies, for addressing information in storage, or for temporary storage. Counters are a series of flip-flops wired together to perform the type of

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counting desired. They will count up or down by ones, twos, or more. The total number of counts or stable states a counter can indicate is called

MODULUS. For instance, the modulus of a four-stage counter would be 1610, (24 ) since it is capable of indicating 00002 to 11112 . The term modulo is

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used to describe the count capability of counters; that is, modulo-16 for a four-stage binary counter, modulo-11 for a decade counter, modulo-8 for a threestage binary counter, and so forth.


A counter can also be used as a frequency divider. Thus, each stage of the counter reduces the frequency of a square wave input by 2. A four-stage counter will therefore reduce the input frequency 16 times (24 ) and a 3 stage counter 8 times (23 ) etc.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) Ripple Counters

Ripple counters are so named because the count is like a chain reaction that ripples through the counter because of the time involved. This effect will become more evident with the explanation of the following circuit. Figure 8.13, view A, shows a basic four-stage, or moduIo-16, ripple counter. The inputs

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and outputs are shown in view B. The four J-K flip-flops are connected to perform a toggle function; which, you will recall, divides the input by 2. The HIGHs on the J and K inputs enable the flip-flops to toggle. The inverters on the clock inputs indicate that the flip-flops change state on the negative-going pulse.

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Assume that A, B, C, and D are lamps and that all the flip-flops are reset. The lamps will all be out, and the count indicated will be 00002. The negative-


going pulse of clock pulse 1 causes flip-flop 1 to set. This lights lamp A, and we have a count of 00012. The negative-going pulse of clock pulse 2 toggles flip-flop 1, causing it to reset. This negative-going input to flip-flop 2 causes it to set and causes B to light. The count after two clock pulses is 00102, or 210. Clock pulse 3 causes flip-flop 1 to set and lights lamp A. The setting of flip-flop 1 does not affect flip-flop 2, and lamp B stays lit. After three clock pulses, the indicated count is 00112.

Clock pulse 4 causes flip-flop 1 to reset, which causes flip-flop 2 to reset, which causes flip-flop 3 to set, giving us a count of 01002. This step shows the ripple effect. This setting and resetting of the flip-flops will continue until all the flip-flops are set and all the lamps are lit. At that time the count will be 11112 or 1510. Clock pulse 16 will cause flip-flop 1 to reset and lamp A to go out. This will cause flip-flop 2 through flip-flop 4 to reset, in order, and will extinguish lamps B, C, and D. The counter would then start at 00012 on clock pulse 17. To display a count of 1610 or 100002, we would need to add another flip-flop. The ripple counter is also called an asynchronous counter. Asynchronous means that the events (setting and resetting of flip-flops) occur one after the other rather than all at once. Because the ripple count is asynchronous, it can produce erroneous indications when the clock speed is high. A high-speed clock can cause the lower stage flip-flops to change state before the upper stages have reacted to the previous clock pulse. The errors are produced by the flip-flops inability to keep up with the clock.





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Figure 8.13 Four Stage Ripple Counter A. Logic Diagram

B. Timing Diagram



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) 5.8.4 Decoders

Introduction

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The basic function of a decoder is to detect the presence of a specified combination of bits (code) on its inputs and to indicate that presence by a specified output level. In its general form a decoder has ‘n’ input lines to handle ‘n’ bits and from one to 2n output lines to indicate the presence of one or more n-bit combinations.


The Basic Binary Decoder

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Suppose we wish to determine when a binary 1001 occurs on the inputs of a digital circuit. An AND gate can be used as the basic decoding element because it produces a HIGH output only when all of its inputs are HIGH. Therefore we must make sure that all of the inputs to the AND gate are HIGH when the binary number 1001 occurs; this can be done by inverting the two middle bits (the Os).

The logic equation for the decoder of Figure 8.14 (a) is developed as illustrated in Figure 8.14 (b). Ao is the LSB, and A3 is the MSB. In the representation of a binary number or other weighted code, the LSB is always the right-most bit in a horizontal arrangement and the topmost bit in a vertical arrangement, unless specified otherwise.

If a NAND gate is used in place of the AND gate, as shown in Figure 8.15, a LOW output will indicate the presence of the proper binary code.




f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 8.14 A Basic Binary Decoder

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Figure 8.15 A Basic Binary Decoder



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) The Four-Bit Binary Decoder In order to decode all possible combination of four bits, sixteen decoding gates are required (24 = 16). This type of decoder is commonly called a 4-line-to16-line decoder because there are four inputs and sixteen outputs. A list of sixteen binary code words and their corresponding decoding functions is shown

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below. Rather than reproducing the complex logic diagram for the decoder each time it is required in a schematic, a simpler representation is normally used. A logic symbol for a 4-line-to-16-line decoder is shown in Figure 8.16. The BIN/DEC label indicates that a binary input makes the corresponding decimal output active. The input labels 1, 2, 4, and 8 represent the weights of the input bits.


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Decoding Functions and Truth Table for a 4-line to 16 line Decoder





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Figure 8.16 4-line to 16-line Decoder



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) The BCD to Decimal Decoder

The BCD-to-Decimal decoder converts each BCD code word (8421) into one of ten possible digit indications. It is frequently referred to as a 4-line-to-10line decoder, although other types of decoder also fall into this category (such as Excess 3 decoder). The method of implementation is essentially the same

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as for the 4-line-to-16-line decoder previously discussed, except that only 10 decoding gates are required because the BCD code represents only the ten decimal digits 0 through 9. A list of the ten BCD code words and their corresponding decoding functions is given in the table below. The logic is identical to that of the first ten decoding gates in the 4-line-to-16-line decoder.


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BCD to Decimal Converter Truth Table



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) The BCD-to-Seven-Segment Decoder/Driver

This type of decoder accepts the BCD code on its inputs and provides outputs to energize seven-segment display devices to produce a decimal readout. Before proceeding with a discussion of this decoder, let us examine the basics of a seven-segment display device. Figure 8.17 shows a common display

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format composed of seven elements or segments. Energizing certain combinations of these segments can cause each of the ten decimal digits to be produced. Figure 8.18 illustrates this method of digital display for each of the ten digits by using a darker segment to represent one that is energized. To

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produce a 1, segments b and c are energized; to produce a 2, segments a, b, g, e, and d are used; and so on.

f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 8.17 7 Segment LED DIsplay

Figure 8.18 10 Digits – 7 Segment Display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) LED Displays

One common type of seven-segment display consists of light-emitting diodes (LEDs) arranged as shown in Figure 8.19. Each segment is an LED that emits light when there is current through it. In Figure 8.19 (a) the common-anode arrangement requires the driving circuit to provide a LOW level voltage in

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order to activate a given segment. When a LOW is applied to a segment input, the LED is turned on, and there is current through it. In Figure 8.19(b) the common-cathode arrangement requires the driver to provide a HIGH level voltage to activate a segment. When a HIGH is applied to a segment input, the

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LED is turned on and there is current through it. The activated segments for each of the ten decimal digits are listed in the table overleaf. We will now examine the decoding logic required to produce the format for a seven-segment display with a BCD input. Notice that segment la'is activated for digits 0, 2,


3, 5,6, 7, 8, and 9; segment ‘b'is activated for digits 0, 1, 2, 3,4,7, 8, and 9; and so on. If we let the BCD inputs to the decoder be represented by the general form A3 A2 A1 Ao, a Boolean expression can be found for each segment in the display, and this will tell us the logic circuitry required to drive or activate each segment. For example, the equation for segment la'is as follows:

This equation says that segment ' a'is activated, or "true," if the BCD code is "0 OR 2 OR 3 OR 5 OR 6 OR 7 OR 8 OR 9." The table 8.1 below lists the logic function for each of the seven segments.




f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 8.19 7 Segment LED Display

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Table 8.1 LED Controller Segment

Activation



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) From the expressions in the table below, the logic for the BCD-to-seven-segment decoder can be implemented. Each of the ten BCD codes is decoded, and then the decoding gates are ORed as dictated by the logic expression for each segment. For instance, segment ' a'requires that the decoded BCD digits 0, 2, 3, 5, 6, 7, 8, and 9 be ORed.


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BCD-to-7 Segment Decoder Logic



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) NOTES


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) 5.8.5 Encoders

An encoder is a combinational logic circuit that essentially performs a "reverse" decoder function. An encoder accepts an active level on one of its inputs representing a digit, such as a decimal or octal digit, and converts it to a coded output, such as binary or BCD. Encoders can also be devised to encode

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various symbols and alphabetic characters. The process of converting from familiar symbols or numbers to a coded format is called encoding.

The Decimal-to-BCD Encoder


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This type of encoder has ten inputs - one for each decimal digit - and four outputs corresponding to the BCD code, as shown in Figure 8.20. This is a basic 10-line-to-4-line encoder. For instance, the most significant bit of the BCD code, A3, is a 1 for decimal digit 8 or 9. The OR expression for bit A3 in terms of the decimal digits can therefore be written.

Bit A2 is a 1 for decimal digit 4, 5, 6, or 7 and can be expressed as an OR functions as follows: A2 = 4 + 5 + 6 + 7

Bit A1 is a 1 for decimal digit 2, 3, 6, or 7 and can be expressed as A1 = 2 + 3 + 6 + 7

Finally, Ao is a 1 for digit 1, 3, 5, 7, or 9. The expression for A0 is

Ao= 1 +3+5+7+9




f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 8.20 DEC to BCD Encoder


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BCD Truth Table


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) Now we can implement the logic circuitry required for encoding each decimal digit to a BCD code by using the logic expressions just developed. It is simply a matter of ORing the appropriate decimal digit input lines to form each BCD output. The basic encoder logic resulting from these expressions is shown in Figure 8.21. Basic logic diagram of a decimal-to-BCD encoder. A O-digit input is not needed because the BCD outputs are all LOW when there are no HIGH inputs.

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The basic operation is as follows: When a HIGH appears on one of the decimal digit input lines, the appropriate levels occur on the four BCD output lines.

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For instance, if input line 9 is HIGH (assuming all other input lines are LOW), this condition will produce a HIGH on outputs A0 and A3 and LOWs on outputs A1 and A2, which is the BCD code (1001) for decimal 9.






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Figure 8.21 Decimal to BCD Encoder



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS INTEGRATED CIRCUITS (DCAM 5.8 L2) An Encoder Application

A classic application example is a keyboard encoder. The ten decimal digits on the keyboard of a calculator, for example, must be encoded for processing by the logic circuitry. When one of the keypads is pressed, the decimal digit is encoded to the corresponding BCD code. Figure 8.22 shows a simple keyboard encoder arrangement using a 74147 priority encoder.

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Notice that there are ten push-button switches, each with a pull-up resistor to + V. The pull-up resistor ensures that the line is HIGH when a switch is not depressed. When a switch is depressed, the line is connected to ground, and a LOW is applied to the corresponding encoder input. The zero key is not


connected because the BCD output represents zero when none of the other keys are depressed.

The BCD output of the encoder goes into a storage device, and each successive BCD code is stored until the entire number has been entered.





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Figure 8.22 Keyboard Encoder



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS MULTIPLEXING (DCAM 5.9 L2) 5.9 MULTIPLEXING 5.9.1 Multiplexers

Introduction

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The idea-of multiplexing/ de-multiplexing is to send data from several sources down the one data transmission line - thus saving weight. A multiplexer is a device which selects data from one of many inputs and connects this data to a common single output. Some practical examples are in Flight Data


Recording (FDR) where the Digital Flight Data Acquisition Unit (DFDAU) receives a large number of inputs from the various aircraft parameters which have to be sampled and eventually fed to the FDR.

Another example is in passenger entertainment systems where the passenger can select one audio/visual channel from amongst many channels supplied to the seat. These channels (inputs) are "multiplexed" in that each input is sampled and passed in serial fashion along a data line. When the passenger selects the required service e.g. the film channel, then only the information on that line relevant to the film channel is selected by a ' de-multiplexer'and fed to the passenger.

A multiplexer/ de-multiplixer system can be likened to a train service with several trains waiting at the platforms of say a major London station. They are all destined to go to the same town - say Manchester - with the same number of platforms, but only one main line between the two cities. Each train leaves it' s platform it turn and is caused to go onto the main line by a set of points (multiplexer). It travels on the main line until it reaches its destination station where it is caused to go to it' s own platform by another set of points (de-multiplexer).



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS MULTIPLEXING (DCAM 5.9 L2) Each-train is sent down the same main line in this fashion. Data can be transmitted like this, but at much greater speeds of course. The multiplexer unit can be likened to a mechanical rotary switch (see drawing below), with separate units of data waiting in temporary store to be transmitted down the data output line. In reality it is electronic with no moving parts and is very fast.


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SIMPLE REPRESENTATION OF A MULTIPLEXER



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS MULTIPLEXING (DCAM 5.9 L2) The next diagram shows the basic principle of a multiplexer showing the data to be transmitted. Just check, first of all you are satisfied that with the inputs on X and Y = 0. These are the ' command signals' .

So with command input of X=0 Y=0 logic 1 goes out through the OR gate. Now work through the multiplexer action when X = 1 Y = 0. You should see the

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AND gate 2 gives logic 1. Now try X=0 Y=1 and you will see the AND gate 3 gives an output of 1 and finally when X=1 Y=1 then AND gate 4 will give an output of 1. So the OR gate will have passed the input 1111 down to data highway.

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This data has to be transferred along a data highway usually by Time Division Multiplexing (TDM). In TDM the data channels each occupy the same


frequency band but divide the channel into time slots into which bits of signal are transmitted.

MULTIPLEXER LOGIC CIRCUIT



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS MULTIPLEXING (DCAM 5.9 L2) The next diagram shows four data channels operating at 200bits/sec. The buffer store is a holding store until access to the data highway is signalled. The duration of each bit is 1/200s or 5ms (5 milli seconds) so an 8 bit word occupies 40ms. The common line is operated at input channel speed times the number of channels i.e., 4 x 200 = 800 bits/sec. So each bit will have a time slot of 1.25mS. Data from the systems connected to Channels 1, 2, 3 and 4 are fed into a Buffer Store, until each store is signalled by the clock pulse to output its data onto the common line in sequence.

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As you can see from the diagram there must be some way of converting these signals to the appropriate receiving channel, i.e. channel 1 input signal data

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to be picked up by channel 1 receiving channel. This will be by a de-multiplexer, a device with a single input but with multiple outputs. There obviously has to be synchronisation between the input and output channels to ensure each data go to the correct channel. Thus all the system works on the command of


the clock (electronic).

TIME DIVISION MULTIPLEXER



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS MULTIPLEXING (DCAM 5.9 L2) 5.9.2 De-multiplexer These are similar to a multiplexer but work "the other way round". They take sequential input data from a common data line and output each piece of data to its appropriate channel. The drawing below shows the mechanical equivalent of the electronic device.


SIMPLE REPRESENTATION OF A DE-MULTIPLEXER

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The next diagram shows a 1 line to 8 line de-multiplexer. It has 8 AND gates, three NOT gates, and input line and 8 output lines. As the name implies it has 1 input line and 8 output lines, with the single data input line is connected to all eight gates. Each gate will be enabled by the signals on the select lines S2 S1 S0.

Assuming 000 input on the select lines only GATE 0 will open and the data will appear at its output. The truth table shows the logic states of the de-multiplexer.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS MULTIPLEXING (DCAM 5.9 L2)

f o g y n r i r a t e e e i n r i p g o n r P SE A M ONE LINE TO 8 MULTIPLEXER


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TRUTH TABLE FOR THE ONE LINE TO 8 MULTIPLEXER


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS MULTIPLEXING (DCAM 5.9 L2) The next diagram shows the basic principles of a passenger entertainment system.

The multiplexer selects all the inputs in turn, (12 channels of recorded music, passenger address messages and tape signals) digitises them (analogue to digital converter) into one serial data signal stream. This stream of data is fed to the seats via the sidewall disconnects. The data is received by the

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electronic boxes. Depending on the selection made by the passenger, the signal is de-multiplexed and decoded (digital to analogue) to output the required signal.


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A PASSENGER ENTERTAINMENT SYSTEM - GENERAL ARRANGEMENT



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS MULTIPLEXING (DCAM 5.9 L2) Some Definition

Simplex

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Data in Simplex channel is always one way. Simplex channels are not often used because it is not possible to send back error or control signals to the transmit end.

Half Duplex


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A half-duplex channel can send and receive, but not at the same time. It’s like one-lane bridge where two way traffic must give way in order to cross. Only one end transmits at a time, the other end receives.

Full Duplex

Data can travel in both directions simultaneously. There is no need to switch from transmit to receive mode like in half duplex. It is like a two lane bridge on a two lane highway.

The term “Full Duplex” is somewhat redundant, as “Duplex” alone will suffice. However, “Full Duplex” is still sometimes used to differentiate it from Half Duplex.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.FIBRE OPTICS

5.10.1 Definition of Fibre Optics

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In the other modules, you learn the basic concepts used in electrical systems. Electrical systems include telephone, radio, cable television (CATV), radar, and satellite links. In the past 30 years, researchers have developed a new technology that offers greater data rates over longer distances at costs lower than copper wire systems. This new technology is fibre optics.


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Fibre optics uses light to send information (data). More formally, fibre optics is the branch of optical technology concerned with the transmission of radiant power (light energy) through fibres.

5.10.2 Fibre Optic Data Links

A fibre optic data link sends input data through fibre optic components and provides this data as output information. It has the following three basic functions: •

To convert an electrical input signal to an optical signal

•

To send the optical signal over an optical fibre

•

To convert the optical signal back to an electrical signal

A fibre optic data link consists of three parts - transmitter, optical fibre, and receiver. Figure 10.1 is an illustration of a fibre optic data-link connection. The transmitter, optical fibre, and receiver perform the basic functions of the fibre optic data link. Each part of the data link is responsible for the successful transfer of the data signal.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) A fibre optic data link needs a transmitter that can effectively convert an electrical input signal to an optical signal and launch the data-containing light down the optical fibre. A fibre optic data link also needs a receiver that can effectively transform this optical signal back into its original form. This means that the electrical signal provided as data output should exactly match the electrical signal provided as data input.

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The transmitter converts the input signal to an optical signal suitable for transmission. The transmitter consists of two parts, an interface circuit and a source drive circuit. The transmitter' s drive circuit converts the electrical signals to an optical signal. It does this by varying the current flow through the light

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source. The two types of optical sources are light-emitting diodes (LEDs) and laser diodes. The optical source launches the optical signal into the fibre. The optical signal will become progressively weakened and distorted because of scattering, absorption, and dispersion mechanisms in the fibre waveguides.


The receiver converts the optical signal exiting the fibre back into an electrical signal. The receiver consists of two parts, the optical detector and the signalconditioning circuits. An optical detector detects the optical signal. The signal-conditioning circuit conditions the detector output so that the receiver output matches the original input to the transmitter. The receiver should amplify and process the optical signal without introducing noise or signal distortion. Noise is any disturbance that obscures or reduces the quality of the signal. Noise effects and limitations of the signal-conditioning circuits cause the distortion of the receiver' s electrical output signal.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.3 Fibre Optic Systems

System design has centred on long-haul communications and the subscriber-loop plant. The subscriber-loop plant is the part of a system that connects a subscriber to the nearest switching centre. Cable television is an example. Limited work has also been done on short-distance applications and some aircraft systems.

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Initially, central office trunking required multimode optical fibres with moderate to good performance. Fibre performance depends on the amount of loss and signal distortion introduced by the fibre when it is operating at a specific wavelength. Long-haul systems require single mode optical fibres with very high


performance. Single mode fibres tend to have lower loss and produce less signal distortion. In contrast, short-distance and aircraft systems tend to use only multimode technology. Examples of short-distance systems include process control and local area networks (LANs).

Short-distance and aircraft systems have many connections. The larger fibre core and higher fibre numerical aperture (NA) of multimode fibres reduce losses at these connections. In aircraft and subscriber-loop applications, system design and parts selection are related.

Designers consider trade-offs in the following areas: •

Fibre properties

•

Types of connections

•

Optical sources

•

Detector types

Designers develop systems to meet stringent working requirements, while trying to maintain economic performance. It is quite difficult to identify a standard system design approach. This module identifies the types of components chosen by the aircraft manufacturers for aircraft applications. Future system design improvements depend on continued research. Researchers expect fibre optic product improvements to upgrade performance and lower costs for short-distance applications.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) NOTES


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.4 Advantages and Disadvantages of Fibre Optics

Fibre optic systems have many attractive features that are superior to electrical systems. These include improved system performance, immunity to electrical noise, signal security, and improved safety and electrical isolation. Other advantages include reduced size and weight, environmental protection, and overall system economy. The following list details the main advantages of fibre optic systems.

Advantages of Fibre Optics


•

System Performance

•

Greatly increased bandwidth and capacity

•

Lower signal attenuation (loss)

•

Immunity to Electrical Noise

•

Immune to noise (electromagnetic interference [EMI] and radio-frequency interference [RFI])

•

No crosstalk

•

Lower bit error rates

•

Signal Security

•

difficult to tap

•

Nonconductive (does not radiate signals) - Electrical Isolation

•

No common ground required

•

Freedom from short circuit and sparks

•

Reduced size and weight cables

•

Environmental Protection


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) •

Resistant to radiation and corrosion

•

Resistant to temperature variations

•

Improved ruggedness and flexibility

•

Less restrictive in harsh environments

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Silica is the principal, abundant, and inexpensive material (source is sand) Despite the many advantages of fibre optic systems, there are some

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disadvantages. Because of the relative newness of the technology, fibre optic components are expensive. Fibre optic transmitters and receivers are still relatively expensive compared to electrical interfaces.


The lack of standardization in the industry has also limited the acceptance of fibre optics. Many industries are more comfortable with the use of electrical systems and are reluctant to switch to fibre optics. However, industry researchers are eliminating these disadvantages. Standards committees are addressing fibre optic part and test standardization.

The cost to install fibre optic systems is falling because of an increase in the use of fibre optic technology. Published articles, conferences, and lectures on fibre optics have begun to educate managers and technicians. As the technology matures, the use of fibre optics will increase because of its many advantages over electrical systems.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.5 Frequency and Bandwidth

Bandwidth is defined as the amount of information that can be transmitted at one time. In the early days of radio transmission when the information transmitted was mostly restricted to Morse code and speech, low frequencies were (long waves) were used. The range of frequencies available to be transmitted (which determines the bandwidth) was very low. This inevitably restricted us to low speed data transmission.

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As time went by, we required a wider bandwidth to send more complex information and to improve the speed of transmission. To do this, we had to increase the frequency of the radio signal used. The usable bandwidth is limited by the frequency used - the higher the frequency, the greater the


bandwidth.

When television was developed we again had the requirement of a wider bandwidth and we responded in the same way - by increasing the frequency. And so it went on. More bandwidth needed? Use higher frequency. For something like sixty years this became an established response - we had found the answer! Until fibre optics blew it all away.

The early experiments showed that visible light transmission was possible and we explored the visible spectrum for the best light frequency to use. The promise of fibre optics was the possibility of increased transmission rates. The old solution pointed to the use of the highest frequency but here we met a real problem. We found that the transmission losses were increasing very quickly. In fact the losses increased by the fourth power. This means that if the light frequency doubled, the losses would increase by a factor of 24 or 16 times.

We quickly appreciated that it was not worth pursuing higher and higher frequencies in order to obtain higher bandwidths if it meant that we could only transmit the data over a very short distance. The bandwidth of a light based system was so high that a relatively low frequency could be tolerated in order to get lower losses and hence more transmission range. So we explored the lower frequency or the red end of the visible spectrum and then even further



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) down into the infrared. And that is where we are at the present time. Infrared light covers a fairly wide range of wavelengths and is generally used for all fibre optic communications. Visible light is normally used for very short range transmissions using plastic fibre.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.6 Fibre Optic Concepts

Fibre Optic Light Transmission

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Fibre optics deals with the transmission of light energy through transparent fibres. How an optical fibre guides light depends on the nature of the light and the structure of the optical fibre.A light wave is a form of energy that is moved by wave motion. Wave motion can be defined as a recurring disturbance

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advancing through space with or without the use of a physical medium. In fibre optics, wave motion is the movement of light energy through an optical fibre.


Velocity of light

For all practical purposes the velocity of light through free space is 3 × 108m/s (186,000 miles per second). The velocity of light changes as it passes from one medium to another. When light travels through these other mediums its velocity is reduced. Because of this slowing down, the light ray bends at the surface of the new medium. In optics a medium is any substance that transmits light.

Propagation of Light

The exact nature of light is not fully understood, although people have been studying the subject for many centuries. In the 1700s and before, experiments seemed to indicate that light was composed of particles. In the early 1800s, a physicist Thomas Young showed that light exhibited wave characteristics. Further experiments by other physicists culminated in James Clerk (pronounced Clark) Maxwell collecting the four fundamental equations that completely describe the behaviour of the electromagnetic fields. James Maxwell deduced that light was simply a component of the electromagnetic spectrum. This seems to firmly establish that light is a wave. Yet, in the early 1900s, the interaction of light with semiconductor materials, called the photoelectric effect, could not be explained with electromagnetic-wave theory. The advent of quantum physics successfully explained the photoelectric effect in terms of fundamental particles of energy called quanta. Quanta are known as photons when referring to light energy. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) The electromagnetic energy of light is a form of electromagnetic radiation. Light and similar forms of radiation are made up of moving electric and magnetic forces. A simple example of motion similar to these radiation waves can be made by dropping a pebble into a pool of water. In this example, the water is not actually being moved by the outward motion of the wave, but rather by the up-and-down motion of the water. The up-and-down motion is transverse, or at right angles, to the outward motion of the waves. This type of wave motion is called transverse-wave motion. The transverse waves spread out in

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expanding circles until they reach the edge of the pool, in much the same manner as the transverse waves of light spread from the sun. However, the waves in the pool are very slow and clumsy in comparison with light, which travels approximately 186,000 miles per second (3 x 108 m/s).

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Light radiates from its source in all directions until it is absorbed or diverted by some substance (fig. 10-3). The lines drawn from the light source (a light


bulb in this instance) to any point on one of the transverse waves indicate the direction that the wavefronts are moving. These lines are called light rays. Although single rays of light typically do not exist, light rays shown in illustrations are a convenient method used to show the direction in which light is traveling at any point. A ray of light can be illustrated as a straight line.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) Properties of Light

When light waves, which travel in straight lines, encounter any substance, they are either reflected, absorbed, transmitted, or refracted. This is illustrated in Figure 10.4. Those substances that transmit almost all the light waves falling upon them are said to be transparent. A transparent substance is one

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through which you can see clearly. Clear glass is transparent because it transmits light rays without diffusing them (view A of Figure 10.5). There is no substance known that is perfectly transparent, but many substances are nearly so. Substances through which some light rays can pass, but through which

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objects cannot be seen clearly because the rays are diffused, are called translucent (view B of Figure 10.5). The frosted glass of a light bulb and a piece of oiled paper are examples of translucent materials. Those substances that are unable to transmit any light rays are called opaque (view C of Figure


10.5). Opaque substances either reflect or absorb all the light rays that fall upon them.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2)


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.7 Transmission of Light Through Optical Fibres

The transmission of light along optical fibres depends not only on the nature of light, but also on the structure of the optical fibre. Two methods are used to describe how light is transmitted along the optical fibre. The first method, ray theory, uses the concepts of light reflection and refraction. The second

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method, mode theory, treats light as electromagnetic waves. You must first understand the basic optical properties of the materials used to make optical fibres. These properties affect how light is transmitted through the fibre.

Basic Optical-Material Properties


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The basic optical property of a material, relevant to optical fibres, is the index of refraction. The index of refraction (n) measures the speed of light in an optical medium. The index of refraction of a material is the ratio of the speed of light in a vacuum to the speed of light in the material itself. The speed of 8

light (c) in free space (vacuum) is 3 X 10 meters per second (m/s). The speed of light is the frequency (f) of light multiplied by the wavelength of light (A). When light enters the fibre material (an optically dense medium), the light travels slower at a speed (v). Light will always travel slower in the fibre material than in air. The index of refraction is given by:

A light ray is reflected and refracted when it encounters the boundary between two different transparent mediums. For example, Figure 10.9 shows what happens to the light ray when it encounters the interface between glass and air. The index of refraction for glass (n1) is 1.50. The index of refraction for air (n2) is almost 1.00.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) Let' s assume the light ray or incident ray is traveling through the glass. When the light ray encounters the glass-air boundary, there are two results. The first result is that part of the ray is reflected back into the glass. The second result is that part of the ray is refracted (bent) as it enters the air. The bending of the light at the glass-air interface is the result of the difference between the index of refractions. Since n1 is greater than n2, the angle of refraction (θ2) will be s law of refraction is used to describe the relationship between the incident and the refracted rays at the greater than the angle of incidence (θ1). Snell' boundary. Snell' s Law is given by:


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As the angle of incidence (θ1) becomes larger, the angle of refraction (θ2) approaches 90 degrees. At this point, no refraction is possible. The light ray is totally reflected back into the glass medium. No light escapes into the air. This condition is called total internal reflection. The angle at which total internal reflection occurs is called the critical angle of incidence. The critical angle of incidence (θc) is shown in Figure 10.10. At any angle of incidence (θ1) greater than the critical angle, light is totally reflected back into the glass medium. The critical angle of incidence is determined by using Snell' s Law. The critical angle is given by:

The condition of total internal reflection is an ideal situation. However, in reality, there is always some light energy that penetrates the boundary. This situation is explained by the mode theory, or the electromagnetic wave theory, of light.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) Total Internal Reflection As already stated, on refraction at a denser medium, a beam of light is bent towards the normal and, vice versa. In the diagram below, the ray APB is

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refracted away from the normal. For any rarer medium the angle of refraction is always greater than the angle of incidence. By increasing the angle of incidence, the angle of refraction will eventually become 90°, as in the case of the ray AP’D. A further increase in the angle of incidence should give an

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angle of refraction greater than 90°, but this is impossible and the ray is reflection at the boundary, remaining within the denser medium, this is ‘total internal reflection’, with none of the light passing through the boundary.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) Dispersion Although it has not been stated it has been assumed that the light ray consisted of only one wavelength. Such light is called monochromatic, and is not

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naturally encountered. Most light beams are complex waves that contain a mixture of wavelengths and are thus called polychromatic. As shown in the diagram below, white light can be separated into individual wavelengths by a glass prism through the process of ‘dispersion’. Dispersion is based on the fact that different wavelengths of light travel at different velocities in the same medium. Because different wavelengths have different indexes of refraction, some will be refracted more than others.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.8 Basic Structure of an Optical Fibre

The basic structure of an optical fibre consists of three parts; the core, the cladding, and the coating or buffer. The basic structure of an optical fibre is shown in Figure 10.11. The core is a cylindrical rod of dielectric material. Dielectric material conducts no electricity. Light propagates mainly along the core

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of the fibre. The core is generally made of glass. The core is described as having a radius of (a) and an index of refraction n1. The core is surrounded by a layer of material called the cladding. Even though light will propagate along the fibre core without the layer of cladding material, the cladding does perform some necessary functions.


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The cladding layer is made of a dielectric material with an index of refraction n2. The index of refraction of the cladding material is less than that of the core material. The cladding is generally made of glass or plastic. The cladding performs the following functions: •

Reduces loss of light from the core into the surrounding air

•

Reduces scattering loss at the surface of the core

•

Protects the fibre from absorbing surface contaminants

•

Adds mechanical strength

For extra protection, the cladding is enclosed in an additional layer called the coating or buffer. The coating or buffer is a layer of material used to protect an optical fibre from physical damage. The material used for a buffer is a type of plastic. The buffer is elastic in nature and prevents abrasions. The buffer also prevents the optical fibre from scattering losses caused by microbends. Microbends occur when an optical fibre is placed on a rough and distorted surface. Microbends are discussed later in this chapter.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.9 Propagation of Light along a Fibre

The concept of light propagation, the transmission of light along an optical fibre, can be described by two theories. According to the first theory, light is described as a simple ray. This theory is the ray theory, or geometrical optics, approach. The advantage of the ray approach is that you get a clearer

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picture of the propagation of light along a fibre. The ray theory is used to approximate the light acceptance and guiding properties of optical fibres. According to the second theory, light is described as an electromagnetic wave. This theory is the mode theory, or wave representation, approach. The

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mode theory describes the behaviour of light within an optical fibre. The mode theory is useful in describing the optical fibre properties of absorption, attenuation, and dispersion. These fibre properties are discussed later in this chapter.


5.10.9.1 Ray Theory

Two types of rays can propagate along an optical fibre. The first type is called meridional rays. Meridional rays are rays that pass through the axis of the optical fibre. Meridional rays are used to illustrate the basic transmission properties of optical fibres. The second type is called skew rays. Skew rays are rays that travel through an optical fibre without passing through its axis.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) Meridional Rays

Meridional rays can be classified as bound or unbound rays. Bound rays remain in the core and propagate along the axis of the fibre. Bound rays propagate through the fibre by total internal reflection. Unbound rays are refracted out of the fibre core. The core of the step-index fibre has an index of

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refraction n1. The cladding of a step-index has an index of refraction n2 that is lower than n1. Figure 10.12 assumes the core-cladding interface is perfect. However, imperfections at the core-cladding interface will cause part of the bound rays to be refracted out of the core into the cladding. The light rays

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refracted into the cladding will eventually escape from the fibre. In general, meridional rays follow the laws of reflection and refraction


It is known that bound rays propagate in fibres due to total internal reflection, but how do these light rays enter the fibre? Rays that enter the fibre must intersect the core-cladding interface at an angle greater than the critical angle (θc). Only those rays that enter the fibre and strike the interface at these angles will propagate along the fibre.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) How a light ray is launched into a fibre is shown in Figure 10.13. The incident ray I1 enters the fibre at the angle θa. I1 is refracted upon entering the fibre and is transmitted to the corecladding interface. The ray then strikes the core-cladding interface at the critical angle (θc). I1 is totally reflected back into the core and continues to propagate along the fibre. The incident ray I2 enters the fibre at an angle greater than θa. Again, I2 is refracted upon entering the fibre and is transmitted to the core-cladding interface. I2 strikes the core-cladding interface at an angle less than the critical angle (θc). I2 is refracted into the

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cladding and is eventually lost. The light ray incident on the fibre core must be within the cone of acceptance defined by the angle θa shown in figure 10.13.

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Angle θa is defined as the acceptance angle. The acceptance angle (θa) is the maximum angle to the axis of the fibre that light entering the fibre is propagated. The value of the angle of acceptance (θa) depends on fibre properties and transmission conditions.


The acceptance angle is related to the refractive indices of the core, cladding, and medium surrounding the fibre. This relationship is called the numerical aperture of the fibre. The numerical aperture (NA) is a measurement of the ability of an optical fibre to capture light. The NA is also used to define the cone of acceptance of an optical fibre. Figure 10.14 illustrates the relationship between the acceptance angle and the refractive indices. The index of refraction of the fibre core is n1. The index of refraction of the fibre cladding is n2. The index of refraction of the surrounding medium is no. By using Snell' s law and basic trigonometric relationships, the NA of the fibre is given by:

Since the medium next to the fibre at the launching point is normally air, no is equal to 1.00. The NA is then simply equal to sin θa. The NA is a convenient way to measure the light-gathering ability of an optical fibre. It is used to measure source-to-fibre power-coupling efficiencies. A high NA indicates a high source-to-fibre coupling efficiency. Typical values of NA range from 0.20 to 0.29 for glass fibres. Plastic fibres generally have a higher NA. An NA for plastic fibres can be higher than 0.50. In addition, the NA is commonly used to specify multimode fibres.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) However, for small core diameters, such as in single mode fibres, the ray theory breaks down. Ray theory describes only the direction a plane wave takes in a fibre. Ray theory eliminates any properties of the plane wave that interfere with the transmission of light along a fibre. In reality, plane waves interfere with each other. Therefore, only certain types of rays are able to propagate in an optical fibre. Optical fibres can support only a specific number of guided modes. In small core fibres, the number of modes supported is one or only a few modes. Mode theory is used to describe the types of plane waves able to propagate along an optical fibre.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) Skew Rays

A possible path of propagation of skew rays is shown in Figure 10.15, view A, provides an angled view and view B provides a front view. Skew rays propagate without passing through the centre axis of the fibre. The acceptance angle for skew rays is larger than the acceptance angle of meridional rays.

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This condition explains why skew rays outnumber meridional rays. Skew rays are often used in the calculation of light acceptance in an optical fibre. The addition of skew rays increases the amount of light capacity of a fibre. In large NA fibres, the increase may be significant.

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The addition of skew rays also increases the amount of loss in a fibre. Skew rays tend to propagate near the edge of the fibre core. Large portions of the


number of skew rays that are trapped in the fibre core are considered to be leaky rays. Leaky rays are predicted to be totally reflected at the core-cladding boundary. However, these rays are partially refracted because of the curved nature of the fibre boundary. Mode theory is also used to describe this type of leaky ray loss.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.9.2 Mode Theory

The mode theory, along with the ray theory, is used to describe the propagation of light along an optical fibre. The mode theory is used to describe the properties of light that ray theory is unable to explain. The mode theory uses electromagnetic wave behaviour to describe the propagation of light along a fibre. A set of guided electromagnetic waves is called the modes of the fibre.

Plane Waves


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The mode theory suggests that a light wave can be represented as a plane wave. A plane wave is described by its direction, amplitude, and wavelength of propagation. A plane wave is a wave whose surfaces of constant phase are infinite parallel planes normal to the direction of propagation. The planes having the same phase are called the wavefronts. The wavelength ( ) of the plane wave is given by:

where c is the speed of light in a vacuum, f is the frequency of the light, and n is the index of refraction of the plane-wave medium.

Figure 10.16 shows the direction and wavefronts of plane-wave propagation. Plane waves, or wavefronts, propagate along the fibre similar to light rays. However, not all wavefronts incident on the fibre at angles less than or equal to the critical angle of light acceptance propagate along the fibre. Wavefronts may undergo a change in phase that prevents the successful transfer of light along the fibre.

Wavefronts are required to remain in phase for light to be transmitted along the fibre. Consider the wavefronts incident on the core of an optical fibre as shown in Figure 10.16. Only those wavefronts incident on the fibre at angles less than or equal to the critical angle may propagate along the fibre.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) The wavefront undergoes a gradual phase change as it travels down the fibre. Phase changes also occur when the wavefront is reflected. The wavefront must remain in phase after the wavefront transverses the fibre twice and is reflected twice. The distance transversed is shown between point A and point B on Figure 10.17. The reflected waves at point A and point B are in phase if the total amount of phase collected is an integer multiple of 2

radian. If

propagating wavefronts are not in phase, they eventually disappear. Wavefronts disappear because of destructive interference. The wavefronts that are

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in phase interfere with the wavefronts that are out of phase. This interference is the reason why only a finite number of modes can propagate along the fibre.

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The plane waves repeat as they travel along the fibre axis. The direction the plane waves travel is assumed to be the z direction as shown in Figure 10.17.


The plane waves repeat at a distance equal to

/ sin θ. Plane waves also repeat at a periodic frequency:

=2 sin θ /

The quantity

defined as the propagation constant along the fibre axis. As the wavelength ( ) changes, the value of the propagation constant must also

change. For a given mode, a change in wavelength can prevent the mode from propagating along the fibre. The mode is no longer bound to the fibre. The mode is said to be cut off. Modes that are bound at one wavelength may not exist at longer wavelengths. The wavelength at which a mode ceases to be bound is called the cut-off wavelength for that mode. However, an optical fibre is always able to propagate at least one mode. This mode is referred to as the fundamental mode of the fibre. The fundamental mode can never be cut off.

The wavelength that prevents the next higher mode from propagating is called the cut-off wavelength of the fibre. An optical fibre that operates above the cut-off wavelength (at a longer wavelength) is called a single mode fibre. An optical fibre that operates below the cut-off wavelength is called a multimode fibre. Single mode and multimode optical fibres are discussed later in this chapter. In a fibre, the propagation constant of a plane wave is a function of the wave' s wavelength and mode. The change in the propagation constant for different waves is called dispersion. The change in the propagation constant for different wavelengths is called chromatic dispersion. The change in propagation constant for different modes is called modal dispersion.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) These dispersions cause the light pulse to spread as it goes down the fibre (fig. 10-18). Some dispersion occurs in all types of fibres. Dispersion is discussed later in this chapter.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.10 Fibre Types There are two main fibre types: •

Step index (multimode, single mode)

•

Graded index (multimode)


5.10.10.1 Step Index Fibre

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Step index fibre is so called because the refractive index of the fibre ‘steps’ up as we move from the cladding to the core of the fibre. Within the cladding the refractive index is constant, and within the core of the refractive index is constant.

Figure 19



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) Single Mode Fibres

The core size of single mode fibres is small. The core size (diameter) is typically around 8 to 10 micrometers (µm). A fibre core of this size allows only the fundamental or lowest order mode to propagate around a 1300 nanometer (nm) wavelength. Single mode fibres propagate only one mode, because the

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core size approaches the operational wavelength ( ). The value of the normalized frequency parameter (V) relates core size with mode propagation.

In single mode fibres, V is less than or equal to 2.405. When V < 2.405, single mode fibres propagate the fundamental mode down the fibre core, while

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high-order modes are lost in the cladding. For low V values (<1.0), most of the power is propagated in the cladding material. Power transmitted by the cladding is easily lost at fibre bends. The value of V should remain near the 2.405 level.


Single mode fibres have a lower signal loss and a higher information capacity (bandwidth) than multimode fibres. Single mode fibres are capable of transferring higher amounts of data due to low fibre dispersion. Basically, dispersion is the spreading of light as light propagates along a fibre. Dispersion mechanisms in single mode fibres are discussed in more detail later in this chapter.

Signal loss depends on the operational wavelength ( ). In single mode fibres, the wavelength can increase or decrease the losses caused by fibre bending. Single mode fibres operating at wavelengths larger than the cut-off wavelength lose more power at fibre bends. They lose power because light radiates into the cladding, which is lost at fibre bends. In general, single mode fibres are considered to be low-loss fibres, which increase system bandwidth and length.





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Figure 20 Single Mode Fibres



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) Multimode Fibres

As their name implies, multimode fibres propagate more than one mode. Multimode fibres can propagate over 100 modes. The number of modes propagated depends on the core size and numerical aperture (NA). As the core size and NA increase, the number of modes increases. Typical values of

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fibre core size and NA are 50 to 100 µm and 0.20 to 0.29, respectively. A large core size and a higher NA have several advantages. Light is launched into a multimode fibre with more ease. The higher NA and the larger core size make it easier to make fibre connections. During fibre splicing, core-to-core

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alignment becomes less critical. Another advantage is that multimode fibres permit the use of light-emitting diodes (LEDs). Single mode fibres typically must use laser diodes. LEDs are cheaper, less complex, and last longer. LEDs are preferred for most applications.


Multimode fibres also have some disadvantages. As the number of modes increases, the effect of modal dispersion increases. Modal dispersion (intermodal dispersion) means that modes arrive at the fibre end at slightly different times. This time difference causes the light pulse to spread. Modal dispersion affects system bandwidth. Fibre manufacturers adjust the core diameter, NA, and index profile properties of multimode fibres to maximize system bandwidth.





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Multimode Fibres



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.10.2 Graded index Fibre Graded Index Fibre has a different core structure from single mode and multimode fibre. Whereas in a step-index fibre the refractive index of the core is constant throughout the core, in a graded index fibre the value of the refractive index changes from the centre of the core onwards. In fact it has what we

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call a Quadratic Profile. This means that the refractive index of the core is proportional to the square of the distance from the centre of the fibre.

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Graded index fibre is actually a multimode fibre because is can support more than one fibre mode. But when we refer to ‘multimode’ fibre we normally mean ‘step index multimode’.






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Graded index Fibre



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.11 Properties of Optical Fibre Transmission

The principles behind the transfer of light along an optical fibre were discussed earlier in this section. You learned that propagation of light depended on the nature of light and the structure of the optical fibre. However, our discussion did not describe how optical fibres affect system performance. In this case, system performance deals with signal loss and bandwidth.

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Signal loss and system bandwidth describe the amount of data transmitted over a specified length of fibre. Many optical fibre properties increase signal loss and reduce system bandwidth. The most important properties that affect system performance are fibre attenuation and dispersion. Attenuation reduces


the amount of optical power transmitted by the fibre. Attenuation controls the distance an optical signal (pulse) can travel as shown in Figure 10.21.

Once the power of an optical pulse is reduced to a point where the receiver is unable to detect the pulse, an error occurs. Attenuation is mainly a result of light absorption, scattering, and bending losses. Dispersion spreads the optical pulse as it travels along the fibre. This spreading of the signal pulse reduces the system bandwidth or the information-carrying capacity of the fibre. Dispersion limits how fast information is transferred as shown in Figure 10.21. An error occurs when the receiver is unable to distinguish between input pulses caused by the spreading of each pulse. The effects of attenuation and dispersion increase as the pulse travels the length of the fibre as shown in Figure 10.22.

In addition to fibre attenuation and dispersion, other optical fibre properties affect system performance. Fibre properties, such as modal noise, pulse broadening, and polarization, can reduce system performance. Modal noise, pulse broadening, and polarization are too complex to discuss as introductory level material. However, you should be aware that attenuation and dispersion are not the only fibre properties that affect performance.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.11.1 Attenuation

Attenuation in an optical fibre is caused by absorption, scattering, and bending losses. Attenuation is the loss of optical power as light travels along the fibre. Signal attenuation is defined as the ratio of optical input power (Pi) to the optical output power (Po). Optical input power is the power injected into the

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fibre from an optical source. Optical output power is the power received at the fibre end or optical detector. The following equation defines signal attenuation as a unit of length:


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Signal attenuation is a log relationship. Length (L) is expressed in kilometres. Therefore, the unit of attenuation is decibels/kilometre (dB/km). As previously stated, attenuation is caused by absorption, scattering, and bending losses. Each mechanism of loss is influenced by material- material properties and fibre structure. However, loss is also present at fibre connections. Fibre connector, splice, and coupler losses are discussed later. The present discussion remains relative to optical fibre attenuation properties.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.11.2 Absorption

Absorption is a major cause of signal loss in an optical fibre. Absorption is defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. Absorption in optical fibres is explained by three factors: •

Imperfections in the atomic structure of the fibre material

•

The intrinsic or basic material-material properties

•

The extrinsic (presence of impurities) material-material properties

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Imperfections in the atomic structure induce absorption by the presence of missing molecules or oxygen defects. Absorption is also induced by the diffusion


of hydrogen molecules into the glass fibre. Since intrinsic and extrinsic material properties are the main cause of absorption, they are discussed further.

Intrinsic Absorption

Intrinsic absorption is caused by basic material-material properties. If an optical fibre were absolutely pure, with no imperfections or impurities, then all absorption would be intrinsic. Intrinsic absorption sets the minimal level of absorption. In fibre optics, silica (pure glass) fibres are used predominately. Silica fibres are used because of their low intrinsic material absorption at the wavelengths of operation. In silica glass, the wavelengths of operation range from 700 nanometers (nm) to 1600 nm. Figure 10.23 shows the level of attenuation at the wavelengths of operation. This wavelength of operation is between two intrinsic absorption regions. The first region is the ultraviolet region (below 400-nm wavelength). The second region is the infrared region (above 2000-nm wavelength).

Intrinsic absorption in the ultraviolet region is caused by electronic absorption bands. Basically, absorption occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level. The tail of the ultraviolet absorption band is shown in Figure 10.23.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) The main cause of intrinsic absorption in the infrared region is the characteristic vibration frequency of atomic bonds. In silica glass, absorption is caused by the vibration of siliconoxygen (Si-O) bonds. The interaction between the vibrating bond and the electromagnetic field of the optical signal causes intrinsic absorption. Light energy is transferred from the electromagnetic field to the bond. The tail of the infrared absorption band is shown in Figure 10.23.

Extrinsic Absorption

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Extrinsic absorption is caused by impurities introduced into the fibre material. Trace metal impurities, such as iron, nickel, and chromium, are introduced into the fibre during fabrication. Extrinsic absorption is caused by the electronic transition of these metal ions from one energy level to another. Extrinsic


absorption also occurs when hydroxyl ions (OH-) are introduced into the fibre. Water in silica glass forms a silicon-hydroxyl (Si-OH) bond. This bond has a fundamental absorption at 2700 nm. However, the harmonics or overtones of the fundamental absorption occur in the region of operation. These harmonics increase extrinsic absorption at 1383 nm, 1250 nm, and 950 nm. Figure 10.23 shows the presence of the three OH- harmonics. The level of the OHharmonic absorption is also indicated.

These absorption peaks define three regions or windows of preferred operation. The first window is centred at 850 nm. The second window is centred at 1300 nm. The third window is centred at 1550 nm. Fibre optic systems operate at wavelengths defined by one of these windows. Visible light has a wavelength between 400 and 750 nm. Therefore all three of these wavelengths used in fibreoptic data transmission are within the infrared range. The amount of water (OH-) impurities present in a fibre should be less than a few parts per billion. Fibre attenuation caused by extrinsic absorption is affected by the level of impurities (OH-) present in the fibre. If the amount of impurities in a fibre is reduced, then fibre attenuation is reduced.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.11.3 Scattering

Basically, scattering losses are caused by the interaction of light with density fluctuations within a fibre. Density changes are produced when optical fibres are manufactured. During manufacturing, regions of higher and lower molecular density areas, relative to the average density of the fibre, are created. Light

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traveling through the fibre interacts with the density areas as shown in Figure 10.24. Light is then partially scattered in all directions.

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In commercial fibres operating between 700-nm and 1600-nm wavelength, the main source of loss is called Rayleigh scattering. Rayleigh scattering is the main loss mechanism between the ultraviolet and infrared regions as shown in Figure 10.23. Rayleigh scattering occurs when the size of the density


fluctuation (fibre defect) is less than one-tenth of the operating wavelength of light. Loss caused by Rayleigh scattering is proportional to the fourth power of the wavelength (1/

4).

As the wavelength increases, the loss caused by Rayleigh scattering decreases. If the size of the defect is greater than one-tenth of

the wavelength of light, the scattering mechanism is called Mie scattering. Mie scattering, caused by these large defects in the fibre core, scatters light out of the fibre core. However, in commercial fibres, the effects of Mie scattering are insignificant. Optical fibres are manufactured with very few large defects.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.11.4 Bending Loss - Bending the fibre also causes attenuation. Bending loss is classified according to the bend radius of curvature: microbend loss or macrobend loss.

Microbends are small microscopic bends of the fibre axis that occur mainly when a fibre is cabled.

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Macrobends are bends having a large radius of curvature relative to the fibre diameter. Microbend and macrobend losses are very important loss

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mechanisms. Fibre loss caused by microbending can still occur even if the fibre is cabled correctly. During installation, if fibres are bent too sharply, macrobend losses will occur.


Microbend losses are caused by small discontinuities or imperfections in the fibre. Uneven coating applications and improper cabling procedures increase microbend loss. External forces are also a source of microbends. An external force deforms the cabled jacket surrounding the fibre but causes only a small bend in the fibre. Microbends change the path that propagating modes take, as shown in Figure 10.25. Microbend loss increases attenuation because loworder modes become coupled with high-order modes that are naturally leaky.

Macrobend losses are observed when a fibre bend' s radius of curvature is large compared to the fibre diameter. These bends become a great source of loss when the radius of curvature is less than several centimetres. Light propagating at the inner side of the bend travels a shorter distance than that on the outer side. To maintain the phase of the light wave, the mode phase velocity must increase. When the fibre bend is less than some critical radius, the mode phase velocity must increase to a speed greater than the speed of light. However, it is impossible to exceed the speed of light. This condition causes some of the light within the fibre to be converted to highorder modes. These high-order modes are then lost or radiated out of the fibre.

Fibre sensitivity to bending losses can be reduced. If the refractive index of the core is increased, then fibre sensitivity decreases. Sensitivity also decreases as the diarneter of the overall fibre increases. However, increases in the fibre core diameter increase fibre sensitivity. Fibres with larger core size propagate more modes. These additional modes tend to be more leaky. For Training Purposes Only




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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.11.5 Dispersion

There are two different types of dispersion in optical fibres. The types are: 1. intramodal dispersion 2. intermodal dispersion

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Intramodal, or chromatic, dispersion occurs in all types of fibres. Intermodal, or modal, dispersion occurs only in multimode fibres. Each type of dispersion

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mechanism leads to pulse spreading. As a pulse spreads, energy is overlapped. This condition is shown in Figure 10.26. The spreading of the optical pulse as it travels along the fibre limits the information capacity of the fibre.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 1) Intramodal Dispersion

Intramodal, or chromatic, dispersion depends primarily on fibre materials. There are two types of intramodal dispersion. a. Material dispersion. b. Waveguide dispersion.

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Intramodal dispersion occurs because different colours of light travel through different materials and different waveguide structures at different speeds.


a. Material Dispersion

Material dispersion occurs because the spreading of a light pulse is dependent on the wavelengths'interaction with the refractive index of the fibre core. Different wavelengths travel at different speeds in the fibre material. Different wavelengths of a light pulse that enter a fibre at one time exit the fibre at different times. Material dispersion is a function of the source spectral width. The spectral width specifies the range of wavelengths that can propagate in the fibre. Material dispersion is less at longer wavelengths.

b. Waveguide Dispersion

Waveguide dispersion occurs because the mode propagation constant (B) is a function of the size of the fibre' s core relative to the wavelength of operation. Waveguide dispersion also occurs because light propagates differently in the core than in the cladding. In multimode fibres, waveguide dispersion and material dispersion are basically separate properties. Multimode waveguide dispersion is generally small compared to material dispersion. Waveguide dispersion is usually neglected. However, in single mode fibres, material and waveguide dispersion are interrelated. The total dispersion present in single mode fibres may be minimized by trading material and waveguide properties depending on the wavelength of operation.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 2) Intermodal Dispersion

Intermodal or modal dispersion causes the input light pulse to spread. The input light pulse is made up of a group of modes. As the modes propagate along the fibre, light energy distributed among the modes is delayed by different amounts. The pulse spreads because each mode propagates along the fibre at

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different speeds. Since modes travel in different directions, some modes travel longer distances. Modal dispersion occurs because each mode travels a different distance over the same time span, as shown in Figure 10.27. The modes of a light pulse that enter the fibre at one time exit the fibre a different

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times. This condition causes the light pulse to spread. As the length of the fibre increases, modal dispersion increases.


Modal dispersion is the dominant source of dispersion in multimode fibres. Modal dispersion does not exist in single mode fibres. Single mode fibres propagate only the fundamental mode. Therefore, single mode fibres exhibit the lowest amount of total dispersion. Single mode fibres also exhibit the highest possible bandwidth.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.12 Choice of Fibres Multimode Fibre Multimode fibre is suitable for local area networks (LAN’s) because it can carry enough energy to support all the subscribers to the network. In a LAN the distances involved, however, are small. Little pulse spreading can take place and so the effects of dispersion are unimportant.

Single Mode Fibre


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Using Single Mode Fibre eliminates Multimode Dispersion. The core is so narrow that only one mode can travel. So the amount of pulse spreading in a single mode fibre is greatly reduced from that of a multimode fibre. Chromatic dispersion however remains even in a single mode fibre. Thus even in single mode fibre pulse spreading can occur. But chromatic dispersion can be reduced by careful design of the chemical composition of the glass. The energy carried by a single mode fibre, however, is much less than that carried by a multimode fibre. For this reason single mode fibre is made from extremely low loss, very pure, glass. Single mode low absorption fibre is ideal for telecommunications because pulse spreading is small.

Graded Index Fibre

In graded index fibre rays of light follow sinusoidal paths. This means that low order modes, i.e. oblique rays, stay close to the centre of the fibre, high order modes spend more time near the edge of core. Low order modes travel in the high index part of the core and so travel slowly, whereas high order modes spend predominantly more time in the low index part of the core and so travel faster. This way, although the paths are different lengths, all the modes travel the length of the fibre in tandem, i.e. they all reach the end of the fibre at the same time. This eliminates multimode dispersion and reduces pulse spreading. Graded Index fibre has the advantage that it can carry the same amount of energy as multimode fibre. The disadvantage is that this effect takes place at only one wavelength, so the light source has to be a laser diode which has a narrow linewidth.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.13 Joining Optical Fibres Optical fibres have to be joined together to make longer lengths of fibre or existing fibres lengths, which have been broken, have to be repaired. Also the ends of the fibre have to be fitted with convenient connectors (terminations) to allow them to be easily plugged into equipment such as power meters, data

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transmitters, etc. Unlike electrical cables where all that is needed to solder lengths of cable together, the process of joining two fibres or terminating the end of a fibre is more complex and requires special equipment. Splicing is the process of joining the two bare ends of fibre together. There are two main types of splicing: • •

Fusion Splicing and

f o g y n r i r a t e e e i n r i p g o n r P SE A M Mechanical Splicing


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5.10.13.1 Fusion Splicing In fusion splicing the ends of the fibre are aligned either manually using micro-manipulators and a microscope system for viewing the splice, or automatically either using cameras or by measuring the light transmitted through the splice and adjusting the positions of the fibres to optimise the

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transmission. The ends of the fibres are then melted together using a gas flame or more commonly an electric arc. Near perfect splices can be obtained with losses as low as 0.02 dB (best mechanical splice 0.2dB).

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One of the systems in top of the range fusion splicers is called a profile Alignment System (PAS). This system uses a TV camera to view the splice before


is it fused. The image is sent to a microcomputer inside the splicer, which is programmed to recognise when the cores of the two fibres form a continuous straight line. An adjustment is made to bring the fibres into alignment in that plane. The camera then moves to a new position to view the splice in an orthogonal plane. The same process aligns the fibres in this plane too. The camera then goes back to the original view and starts to make fine adjustments in that plane. It goes to the second plane and makes fine adjustments in that plane too. This goes on until the alignment is as close as possible. At this point the arc is fired and the heat from the arc melts the fibres together locally.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.13.2 Mechanical Splicing

In mechanical splicing the two fibre ends are held together in a splice. This consists of some device usually made of glass which by its internal design automatically brings the two fibres into alignment. The openings at each end of the device are usually fluted to allow the fibres to be guided into the

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capillary where the alignment takes place. The splice is first filled with optical cement whose refractive index is the same as that of the core of the fibre. After the fibres have been entered into the splice they are adjusted to give the optimum transmission of light. At this point they are clamped in position and the whole assembly is exposed to ultra-violet light that cures the cement.

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Mechanical splices are best used for multimode fibre. Some splices now exist which are suitable for single mode fibre, but have a loss of 0.1dB. This is


five times the loss of the best fusion splice.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.13.3 Possible Alignment Errors During Splicing The ends of the fibres must be precisely lined up with each other; otherwise the light will not be able to pass from one fibre across the gap to the other fibre. There are four main alignment errors and any splicing technique is designed to deal with ends of these errors. The four alignment errors in splicing optical fibres are: •

Lateral

•

Axial

•

Angular

•

f o g y n r i r a t e e e i n r i p g o n r P SE A M Poor End Finish

These are illustrated in the diagrams below.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.14 Aircraft Fibre Optic Networks At the time of producing these notes the only information available on aircraft fibre optic networks, was that related to the optical networks fitted to the Boeing 777, these systems therefore form the basis of these notes. This section is only intended to give the reader an insight into the use of fibre optic networks on aircraft, and does not give details about the systems in which they are employed. Fibre optic networks have advantages over wire type networks in that they: •

can carry more data than wire buses.

•

weigh less than wire buses.

•

are immune to electromagnetic radiation.


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They do however require special, high tolerance connectors and the switching of optical signals is slightly more involved.

Given concerns about

electromagnetic compatibility, fibre optic networks are likely to replace current wire type networks such as ARINC 429 and even 629 in the not too distant future.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) Construction of the Cable

Each fibre optic strand is .0055 inches in diameter this is covered with several layers of material, cladding (to keep light in) primary buffer (protects glass

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fibre during manufacture), secondary buffers are coloured to permit identification of each fibre optic strand. The strand is now .035 inches in diameter.

The cable itself can have three or five fibre optic strands, a number of black filler strands may be used to make up the cable these are also .035 inches in

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diameter. A polyester tape covers the strands, it makes the cable more flexible. A woven aramid yam protects the optical fibres from damage. The outer jacket is a purple thermoplastic.

f o g y n r i r a t e e e i n r i p g o n r P SE A M Fibre Optic Cable



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) Connectors Type A Connector

These are used at production breaks that are not regularly connected and disconnected. This is a multi-channel, in line (butt type} connector which has

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very low light loss between the fibre optic components. The connector has alignment keys on the plug and alignment grooves on the receptacle to accurately align the optical components, guide pins in the plug fit into cavities in the receptacle; these pins touch the bottom of the cavities so you cannot over-tighten the connector.


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The coupling nut on the plug barrel has a yellow band; the receptacle barrel has a red and yellow band. When the red band on the receptacle is at least 50% covered by the coupling nut the connection is correct. Three start threads on the plug make sure of a straight start on first joining. The plug and receptacle have ceramic contacts that touch when connected. The light signal goes through the holes in the end of the ceramic contacts when they are in direct physical contact with each other.

Connectors



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) Type B Connector

This type of connector is used to connect to LRU' s and is therefore more frequently connected and disconnected. It is a multi-channel, expanded beam (ball lens) connector; light loss is low but not as good as the Type A connector. It has the similar alignment keys and guide pins as the Type A.

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The connector has a miniature ball lens at the end of each fibre behind a protective window. This lens, expands and focuses the light through the

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protective windows of the plug and receptacle to another ball lens which narrow the light and sends it into a fibre.


Some maintenance points with these connectors are:

1. Before examining, ensure equipment is switch off, light from the optical fibre could damage your eyes. 2. Only use approved procedures to clean the connectors and lenses.





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Type B Connector




5.10.15 Applications in Aircraft Systems

The Boeing 777 uses a fibre optic communications network called OLAN (Onboard Local Area Network) this is divided into two parts: 1. Avionics local area network (LAN) 2. Cabin LAN

The Avionics LAN connects the following LRU' s: a) Left and right AIMS (Aircraft Information Management System).


b) MAT (Maintenance Access Terminal}. c) First Officers Side Display. d) Captains Side Display. e) Brouter.

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The Brouter receives and sends signals to LRU' s and connects signals to the P MATS (portable maintenance access terminals).

The cabin LAN connects the following LRU' s:

1. ZNTU 1, 2, 3 (Zone Network Controller/Telephone Distribution Network) 2. CFS (Cabin File Server)

ARINC 636 is a fibre optic data bus, one strand of fibre optic per bus. The two strands provide a primary (PRI) and secondary (SEC) bus. Fibre optics is also used in other areas on aircraft, for example, in a radiation pyrometer system.





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OLAN – General Layout



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) The system is used to provide a signal of actual turbine blade temperature for engine temperature and exhaust gas measurement. The pyrometer head and its sighting tube are mounted as shown to measure the temperature of the blades by radiation (radiation emitted by a body, at any wavelength is a function of temperature). This radiated signal is fed via the fibre optic link to a photo-cell in the detector where it is then fed to an amplifier and then to the indicator, or engine monitoring system.


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Fibre Optic Link – Radiation Pyrometer



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS FIBRE OPTICS (DCAM 5.10 L2) 5.10.16 Fibre Optic Testing

The simplest way to check a fibre optic cable for continuity is to point a commercially available flashlight (torch) at one end and check that light emits from

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the other (the torch may have to be moved slightly to see the light at the other end). Of course, disconnect the cable at both ends first, and fit dust caps to the equipment connections.

If light is not seen, change the cable and retest system.


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On the Boeing 777 systems there are 5 optical fibres in each fibre stub cable, but only 4 are used. If one fibre only is dark, then the connector can be changed so that the dark fibre corresponds to position E in the connector.

This procedure is stated in the B777 manual.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) 5.11 ELECTRONIC DISPLAYS 5.11.1 Light Emitting Diodes (LEDs)

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An LED consists of a junction diode made from the semiconductor compound gallium arsenide phosphide. It emits light when forward biased, the colour of the light emitted is in direct proportion to the current flow. Light emission in the red, orange, green and yellow regions of the spectrum is obtained depending on the composition and impurity content of the compound.

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When a P-N junction is forward biased, electrons move across the junction from the n-type side to the p-type side where they recombine with holes near


the junction. The same occurs with holes going across the junction from the p-type side. Every recombination results in the release of a certain amount of energy, causing, in most semiconductors, a temperature rise. In gallium arsenide phosphide some of the energy is emitted as light gets out of the LED because the junction is formed very close to the surface of the material.

Figure 11.1 Light Emitting Diode

In applying this to aircraft displays either the 7 segment or dot-matrix configurations may be used. In the 7 segment display for numerical indication as shown in figure 11.2 and figure 11.3, each segment is an LED mounted within a reflective cavity with a plastic overlay.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) In a dot matrix display each dot making up the numbers is an individual LED. The size of the matrix determines how many LED' s are used for instance in figure 11.4 a matrix of 5 columns, 9 rows, and uses 45 individual LED' s.


Seven Segment LED Display (Typical Pin Connection)

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Figure 11.3

Standby Engine Indicator LED Display

Figure 11.4 LED Display (Dot Matrix)



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) 5.11.2 Liquid Crystal Displays (LCDs) Liquid crystals are organic (carbon) compounds which exhibit both solid and liquid properties. A ' cell'with transparent metallic conductors called electrodes, on opposite faces, containing a liquid crystal (esters and biphenyls), and on which light falls, goes ' dark'when a voltage is applied across the electrodes, this is caused by molecular arrangement within the liquid crystal (see figure 11.5).

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The pattern of the conducting electrodes on a seven segment LCD for producing the numbers 0 to 9 is shown in Figure 11.6. Only the liquid crystal under

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those electrodes to which the voltage is applied goes ' dark' . The display has a silvered background which reflects back incident-light and it is against this continuously visible background (except in darkness when it has to be illuminated) that the numbers show up the dark segment.


This whole unit would have to be hermetically sealed with a special thermoplastic material to prevent contamination of the liquid crystal compound by water vapour and oxygen.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2)

f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 11.5 Principle of LCD

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Figure 11.6 Structure of an LCD

Figure 11.7 Liquid Crystal Display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) Flat Panel Liquid Crystal Displays (LCD) Displays

The CRT has a high voltage and power requirement. It is heavy and bulky and the temperature gets high thus requiring cooling. Also, if it fails all data is lost. To overcome all the disadvantages listed above the LCD was developed. It is small (an inch or two deep - 25 to 50mm), light with a low power requirement and is used on flight decks and passenger entertainment systems.

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A disadvantage of the LCD over the CRT is that it has a narrow viewing angle and has to be back-lit in low ambient light.


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The Boeing 777 has six 8" x 8" (203mm x 203mm) identical flat panel LCD displays as its main instrument panels. The principle of operation is known as the "active matrix" display. The principle is shown in figure 11.8. Thousands of tiny LCD' s are used to form the picture, these are arranged in a matrix of cells that represent pixels (picture elements), each colour being turned on and off by a transistor.

Note. The term ' active display'means a display which produces light when electrically activated - for example, LED and flat panel LED displays. The term "passive display'is a display which either transmits light from an auxiliary light source after modulation by the display device, or which produces a pattern viewed by reflective light – e.g., an LCD display.

Figure 11.9 shows the clarity of an LCD standby attitude indicator.




f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 11.8 LCD Principle


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Figure 11.9 LCD Display of Attitude Indicator


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) 5.11.3 Cathode Ray Tube (CRT)

Before we look at the colour generation on a CRT it would be useful to look at the basic principles of an ordinary CRT. It is an evacuated glass tube that is

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designed in such a way that electrons are caused move along the tube and deflected so as to write across a fluorescent screen, similar to a pencil drawing lines one below the other down a piece of paper. Once the last line is drawn at the bottom of the screen, the electron beam starts again at the top. The

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markings (lines) it has made on the screen will last for a short while as the inner part of the screen is fluorescent.


The beam writes lines very quickly, too quick for the human eye to see, and by adjusting the density of the electron flow as the beam moves across the screen different density lines can be imaged and a picture can be produced. A moving picture is possible as "re-write" times are fast. Figure 11.10 shows a working cross section of a CRT with electrostatic deflection and a voltage divider which provides appropriate voltages from a voltage supply.

The heater (H), is an electrically heated tungsten wire inside the cathode (C). It is insulated from the hollow cylindrical nickel cathode, and when a current is passed through the heater, it raises the temperature of the cathode to a point where the electrons become agitated and the cathode emits electrons (thermionic emission). The negatively charged electrons are attracted to and accelerated towards the anodes A1 and A2 (these are usually cylinders with holes at either end and are positive with respect to the cathode; A2 more so than A1). The electrons are attracted to the anodes, and because of their acceleration the electrons move through the centre of the anodes to impinge on the screen.

The grid G, also a hollow cylinder, has a negative voltage which can be varied by R1. Varying R1 can make the voltage on the grid more or less negative to the cathode thus controlling the amount of electrons and thus the brilliance of the display. Focusing is achieved by altering R2, thus altering the voltage between A1 and A2. When the electrons strike the screen S, which is coated with a phosphor coating, it causes the phosphor to luminescent and give a spot of light on the screen.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) There is a return path for the electrons from the screen to the cathode, otherwise unwanted negative charge would build up on the screen. This does not happen because when struck by electrons, the screen emits secondary electrons, these are attracted to and collected by a conductive coating (graphite) on the inside of the tube and returned to the cathode via the power supply.

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To trace out a display it is necessary for the spot to be deflected horizontally and vertically. Figure 11.10 shows an electrostatic deflection system where the two sets of plates Y1 and Y2 and X1 and X2 deflect the beam vertically and horizontally. The Y plates deflect the beam vertically and the X plates deflect the beam horizontally, by making one plate of the pair more positive than the other.


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Figure 11.10 The Working of a CRT



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) Figure 11.11 shows the location of the electro-magnetic deflection coils.

The coils are placed around the neck of the tube, the ones on the top and bottom when energised will deflect the beam horizontally as shown, reversal of polarity causes movement in the opposite direction.

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For horizontal deflection the coils on the top and bottom of the neck, when polarised, will deflect the beam horizontally, and those fitted to either side of

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the neck, when polarised, will deflect the beam vertically. By combining the two effects the beam can be made to move to any position by controlling the polarities of the two sets of coils.




f o g y n r i r a t e e e i n r i p g o n r P SE A M Deflection Coils

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Horizontal Deflection

Vertical Deflection Figure 11.11



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) Scanning

To produce a picture on the screen it is scanned from left to right and from top to bottom. Similar to the way you read a book. The left to right sweep is controlled by a timebase which consists of an amplifier and relaxation oscillator. The timebase applies a varying signal to the X plates (electrostatic

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deflection system) or coils at the top and bottom (electro magnetic deflection system) and the signal to be screened is applied to the Y plates (electrostatic) and coils on the side (electromagnetic).


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Figure 11.12 Electrostatic Deflection



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) Time-base Deflection in an Electrostatic CRT

The time-base generator' s function is to make the spot sweep across the screen at a steady speed to the right hand side and then made to fly back'to the left hand side and start out again across the screen. The time-base generator produces a sawtooth or ramp waveform (see figure 11.13). The voltage on

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the X plates is made to rise evenly and then fall rapidly back to its starting value (flyback). This will trace out a straight line as shown in figure 11.14(A).

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If a signal, such as a sine wave, is applied to the Y plates and is synchronised with the time-base generator output then the spot will trace out the waveform applied to the Y plates as it sweeps across the screen as shown in figure 11.14(B).


In its basic form the time-base generator is an RC circuit which means that the ramp of the waveform is not linear. But provided the time constant of the RC circuit is very much greater that the time required for the beam to scan across the screen the ramp is almost linear.

Figure 11.13

Timebase Waveform


Figure 14 (A)

Trace Form Timebase Voltage

Figure 14 (B) A Synchronised Sin Wave Trace


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) Time-base Deflection for a Magnetic CRT

As mentioned earlier, in a magnetic CRT the spot is deflected by magnetic fields which are produced by currents flowing through deflection coils. To produce a horizontal trace the time base is applied to the X coils (top and bottom).

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The current through the coils must be made to vary in a linear manner. If we were to apply a sawtooth voltage to the coils the inductance of the coils would oppose the change in the current and this would give a non-linear change in current.


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So a voltage waveform has to be applied to the coils to produce a current through the coils that rises linearly. This waveform turns out to be a form of a trapezoid waveform as shown in figure 11.15.

Electromagnetic Deflection

Wave Form

Figure 11.15



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) Circular Time-base

We have seen how a horizontal time-base trace may be produced in electrostatic and magnetic CTRs. There are occasions when other forms of time-base traces are required - for example a circular time-base. If a sinewave voltage of the required frequency is applied to R and C connected in series, and VC is

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applied to the X plates and VR is applied to the Y plates. VC and VR are 90° out of phase, their frequency is the same and if XC equals R their amplitude is the same, a circular time base results.

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A similar arrangement may be used for magnetic CRTs. With this time-base the spot moves at a constant speed so that its motion represents a true time


base against which radar ranges may be measured.

Circuit For Circular Time-Base

Waveforms and Display – Circular Time-Base

Figure 11.16



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTRONIC DISPLAYS (DCAM 5.11 L2) 5.11.4 Principle of the Colour CRT

These are widely used in aircraft cockpit displays - particularly for the larger aircraft though some feeder type aircraft use a ' green screen'flight deck. Unlike the black and white CRT the colour CRT has three electron guns. The CRT has 3 electron guns each dedicated to a colour - red, green or blue.

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With reference to figure 11.17, the inside of the screen is coated with many thousands of tiny dots of red, green and blue phosphors, arranged in small

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areas, each area contains a phosphor of each type. The beam from a particular gun must only be able to strike the designated phosphor, i.e. the ‘red'gun electrons strike only the red dots, the ' green'gun the green dots and the ‘blue'gun the blue dots. To achieve this, a perforated steel sheet called a shadow


mask is accurately positioned adjacent to the screen.

This shadow mask is very accurately manufactured with small holes through which the beam paths pass through. As the three electron guns scan the screen under the influence of the deflection coils, the shadow mask ensures that each beam strikes only its designated phosphor. By independent control of the three guns and their beam currents other colours may be generated. If only the red, green, or blue colours is required then, as the scanning is so fast, you would see a completely red, blue or green screen.

If all three guns are operating with an equal mix of red, green and blue this would give a white trace, if only red and blue were emitting the mix would be a mauve trace. If the green gun electron beam and red gun electron beam current was higher than the blue then a yellowish trace would appear. So by controlling the intensity of the three electron beams various colours may be obtained.




f o g y n r i r a t e e e i n r i p g o n r P SE A M Principle of The Colour CRT

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Principle of Operation of The Mask

CRT – General View

Figure 11.17



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2) 5.12 ELECTROSTATIC SENSITIVE DEVICES

In many of the LRU' s of the modern aircraft fault diagnosis will involve changing printed circuit boards. These boards will contain many components which are extremely vulnerable to static discharge.

5.12.1 What is an Electrostatic Discharge?


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Static electricity (an electrical charge at rest) is stored on non-conductive where an electrical discharge will take place. The static charge is generated whenever any two materials are rubbed together, or pulled apart.

The most common conception of static electricity and it accompanying discharge, is the miniature lightning shock we receive when we walk across a nylon carpet and touch a door handle or light switch.

We find from experience that if we first touch the metal fittings with a key, the discharge can be seen but not felt. This phenomenon is especially acute in modern office blocks and hotels that are air conditioned.

The discharge occurs because different materials receive different levels of charge and the accompanying list (known as the Triboelectric Series) shows the different level of charge with respect to cotton, the reference material.

The higher up, or lower down the table, the greater is the charge and hence the greater discharge when the two materials are bought together. Page 306 shows typical electrostatic voltage that can occur. (Note the importance of humidity).



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2) AIR HUMAN HANDS ASBESTOS RABBIT FUR GLASS MICA HUMAN HAIR NYLON WOOL LEAD SILK ALUMINIUM PAPER COTTON STEEL WOOD AMBER SEALING WAX HARD RUBBER NICKEL COPPER BRASS SILVER GOLD PLATINUM SULFUR ACETATE RAYON POLYESTER CELLULOID ORLON SARAN POLYURETHANE POLYPROPYLENE PVC (VINYL) KEL-F (CTFE) SILICON TEFLON

INCREASINGLY POSITIVE


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INCREASINGLY NEGATIVE

The Triboelectic Series Materials



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2) 5.12.2 Typical Electrostatic Voltages The following table shows the sort of voltages that can occur between two surfaces/ objects:

MEANS OF STATIC GENERATION WALKING OVER CARPET WALKING OVER VYNIL FLOOR

ELECTROSTATIC VOLTAGES RELATIVE HUMIDITY 10 to 20% 65 to 90% 35,000 1,500 12,000 250


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WORKER AT BENCH

6,000

100

VINYL ENVELOPES FOR WORK INSTRUCTIONS

7,000

COMMOM POLY BAG PICKED UP FROM BENCH

20,000

1,200

WORK CHAIR PADDED WITH POLYURETHANE FOWM

18,000

1,500

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600


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2) The next table lists some static sensitive devices and the voltages that can cause damage. The damage can vary from a slight degradation of performance, giving rise to intermittent and spurious indications; to complete destruction, giving rise to total system failure. The amount of damage varies with the amount of energy that strikes them.

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The less obvious kind of damage can cause considerable and expensive maintenance headaches which may lead to lack of confidence in the equipment.


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Static Sensitive Device

Sensitivity Range where damage can occur

Field Effect Transistor (MOS/FET)

150 – 1000 volts

CMOS Bipolar Transistors

250 – 1000 4,000 – 15,000

Silicon-Controlled Rectifiers (SCR) 4,000 – 15,000 Thin-Film Resistors


150 – 1000


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2) Today, digital systems and micro-electronics have already established a foothold in the control and operation of transport aircraft. The benefits include high "Mean Time Between Failure" rates (MTBF), low maintenance costs, improved performance and reduced weight.

In order to realise these advantages, the maintenance engineer will be required to handle these devices in such a way as to avoid known and preventable causes of component failure.

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The following procedures show how to identify ESDS (Electro Static Discharge Sensitive) LRU' s and the precautions for handling the units and printed circuit boards within the units.


The diagram shows the ESDS symbols in common usage which includes commercial, government (military) and international symbols. The international symbol is the most common but other symbols may be used on some LRU' S.

Removal and Installation of ESDS Printed Circuit Boards. The equipment and supplies needed will include: •

Conductive bags - 3M 2100 series.

•

Wrist straps - 3M P/N 2064, 2066, 2067

•

100% cotton twine - commercially available.

•

ESDS Labels (JEDIC international label – multisource 3M P/N 7102)



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2)


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Figure 12.1 Typical ESDS Decals



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2) 5.12.3 ESDS Equipment and Material Handling

5.12.3.1 Dust caps

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Conductive or anti-static dust caps should be used when available. If conductive or anti-static dust caps are not available, non-conductive dust caps may be used but with caution since they do not provide complete ESDS protection during handling.


5.12.3.2 ESDS Removing and Installing Procedure

Removing PCBs with ESDS decals

1.

Check with the AMM for any special instructions required.

2.

Remove system electrical power and tag.

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WARNING: USE ONLY WRIST STRAPS WITH A GROUNDING LEAD RESISTANCE OF GREATER THAN 1 MEGOHM. INADVERTENT CONTACT BETWEEN THE WRIST STRAP AND A HIGH VOLTAGE IS A SHOCK HAZARD.

3.

Connect wrist strap assembly to a convenient ground on component containing the PCB and to skin of person removing PCB.

4.

Gain access to PCB.

5.

Inspect inside the unit checking in particular for clearance between the PCB to be removed and the equipment/PCBs either side.

6.

Disconnect any PCB free-end connectors and label if necessary.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2) 7.

Remove printed circuit board using extractors provided, taking care that it slides out smoothly. If it feels obstructed, do not pull too hard, but check for possible obstruction and clear.

8.

Immediately insert into a conductive bag and identify with an ESD label and identification label. Use an ESDS label or 100% cotton twine to close conductive bag.

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CAUTION: DO NOT USE STAPLES OR ADHESIVE TAPE TO CLOSE CONDUCTIVE BAG. DAMAGE TO BAG MAY EXPOSE PCB TO ELECTROSTATIC DISCHARGE.

9.


Close and secure aircraft unit unless replacement PCB is to be installed immediately.

10. Disconnect wrist strap from ground and operator.

11. Place bagged PCB in a rigid container to maintain integrity of conductive bag during transportation. 12. Record work done in log book and sign.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2) Installing PCB with ESDS Decals

1. Check that system electrical power is off. 2. Check AMM for details of procedure to be adopted.

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WARNING: USE ONLY WRIST STRAPS WITH A GROUNDING LEAD RESISTANCE GREATER THAN 1 MEGOHM INADVERTENT CONTACT BETWEEN A HIGH VOLTAGES IS A SHOCK HAZARD.


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3. Check documentation on "new" PCB such as DCA ARC/AAT & etc to ascertain that it is the correct one - check IPC. Check that part and serial numbers are correct and that the documents are signed and come from an approved supplier.

4. Connect wrist strap assembly to convenient ground on unit where the PCB is to be installed and to wrist of person installing PCB. 5. Gain access to receptacle that PCB is to be installed into.

6. Inspect the inside of the receptacle to make sure that the PCB slideways are clear and that fixed end connectors are clear and not damaged. Check for adequate clearance. Check all other PCBs and equipment inside unit as far as possible for security, damage, clearance and signs of contamination and burning. Renew any suspect items, investigate the reasons for the damage and test the system. 7. Remove "new" PCB from conductive bag.

CAUTION: DO NOT TOUCH CONNECTOR PINS OR OTHER EXPOSED CONDUCTORS. DAMAGE TO COMPONENTS CAN RESULT.

8. Inspect the PCB for damage and verify that it is the correct part number and make a note of the serial number. 9. Install PCB into position using extractors provided. Lock extractors. Make sure that PCB slides easily into position and visually check that there is clearance between the PCB and adjacent components. Remove extractors. 10. Fit any free-end connectors. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2) 11. Close and secure unit. 12. Disconnect wrist strap. 13. Reinstate power and carry out a functional check/BIT check in accordance with the AMM. 14. Record all work done in the log book and sign.

.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2) Removal and Installation of metal-encased ESDS LRUs. Metal-encased ESDS units can be either rack mounted, panel mounted or bolted on.

Remove Metal Encased LRU' s with ESDS Labels

1. Remove system electrical power and tag. 2. Check the AMM. 3. Remove ESDS labelled unit from rack, panel, or mounted position.

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CAUTION: DO NOT TOUCH CONNECTOR PINS OR OTHER EXPOSED CONDUCTORS. DAMAGE TO COMPONENTS MAY RESULT.


4. Install dust caps on all connectors. Do not touch electrical pins in connectors. Dust caps from unit being installed may be used on the unit being removed.

5. Transport unit carefully with dust caps installed. 6. Record all work done and sign the log book.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROSTATIC SENSITIVE DEVICE (DCAM 5.12 L2) Installing Metal Encased LRU' s With ESDS Labels

1. Check the AMM. 2. Check that system electrical power is off and tag. CAUTION:

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DO NOT TOUCH CONNECTOR PINS OR OTHER EXPOSED CONDUCTORS. DAMAGE TO COMPONENTS CAN RESULT

3. Inspect unit for any signs of damage. Check stores documentation (DCA ARC/AAT & etc). Confirm that description, part number, and serial numbers agree.

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4. Remove all dust caps from connectors of unit being installed. Do not touch electrical pins in connectors.


5. Place unit in position and secure.

6. Switch electrical power on and carry out functional/BIT check. 7. Record all work done and sign.

The bonding of all electrostatic components is also important. On equipment containing circuits carrying more than 50V (rms or DC) no contact resistance should exceed 0.05 ohms.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS SOFTWARE MANAGEMENT CONTROL (DCAM 5.13 L2) 5.13 SOFTWARE MANAGEMENT (DCA AN 57)

5.13.1 Introduction

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Extensive use is now being made in aircraft of software-based equipment and systems. Typically, software may be used in primary and secondary flight controls, engine controls, electrical generation and distribution, brakes, radio and navigation equipment, flight instruments, and automatic flight control. The

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software may have a direct influence on aircraft safety. Hence, in meeting the appropriate aircraft requirements and, for Controlled Items, the Requirements of BCAR Sections A, it is necessary to investigate the software design and to control its certification and post-certification configuration in a manner


equivalent to that for conventional safety-critical systems.

RTCA document DO-178A entitled Software Considerations in Airborne Systems and Equipment Certification, provides guidance to aircraft constructors, equipment manufacturers and aircraft operators on software practices that would support the certification of software-based equipment and systems. This document is acceptable as a basis for the certification of the software in aircraft equipment and systems.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS SOFTWARE MANAGEMENT CONTROL (DCAM 5.13 L2) 5.13.2 Requirements

5.13.2.1 General

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In addition to the normal assessment of system criticality, the responsible Design Organization (normally the aircraft constructor) shall assign a Software Criticality Category to each software-based equipment or system which shall relate to the severity of the effect of possible software errors within the equipment or system.


Note: Software Critically Categories are defined in FAR 25.1309 & JAR 25.1309.

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Where the equipment is to be the subject of an application for Accessory Approval, the level of Software Criticality Category shall be declared in the Declaration of Design and Performance as defined by BCAR Section A, Chapter A4-8 or, an equivalent procedure acceptable to DCA.

Details of equipment and system Software Criticality Categories should be provided to the aircraft operators and DCA to assist in the evaluation of postcertification modifications.

Initial Certification in respect of a software-based equipment or system, the responsible design organization shall provide evidence to the DCA that the software has been designed tested and integrated with the hardware in a manner which ensures compliance with BCAR Section A.

Post-Certification Modifications In respect of equipment and systems with software in the Major, Hazardous or Catastrophic Criticality Categories, a modification which affects software shall not be embodied unless it has been approved by the responsible Design Organization.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS SOFTWARE MANAGEMENT CONTROL (DCAM 5.13 L2) 5.13.2.2 Interpretation of Requirements

Initial Certification

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An applicant for the approval of a software-based equipment or system may use the guidance material given in document DO-178A (or an agreed equivalent standard) as a means of securing DCA approval of the associated software. A software Configuration Management Plan, e.g. as defined in Part

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7 of document 00-178A, will be required as a means of software identification and change control to be effective throughout the life of the equipment. The plan will need to be managed by the responsible design organization.


Post-Certification Modifications

Modifications to software will be subject to the same approval procedures as are applied to hardware modifications (see BCAR Section A, Chapters A2-5, A5-6, A6-6 or A6-7 as appropriate). Modified software will need to be identified and controlled in accordance with the procedures stated in the Software Configuration Management Plan. The guidance material given in document DO-178A (or an agreed equivalent standard) may be used as a basis for the approval of software modifications.

NOTE: The term ‘maintenance’ is often used by software specialists when referring to modifications to software. The DCA will require the design and investigation of a modification, including those proposed by the aircraft operator, to involve the support service provided by the design organization responsible for the equipment or system. It is unlikely that an aircraft operator could justify the establishment of its own software design capability. Therefore, where the Software Criticality Category is Major, Hazardous or Catastrophic, the aircraft constructor would normally be the appropriate Design Organization, although tasks may be delegated to equipment manufacturers or software organizations. Aircraft operators will need to ensure that their normal procedures will report software problems to the responsible design organization.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS SOFTWARE MANAGEMENT CONTROL (DCAM 5.13 L2)


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) 5.14 ELECTROMAGNETIC ENVIRONMENT

5.14.1 General

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Gross navigational errors have been observed in omega navigation instruments of a passenger aircraft which was on a flight from Newark to Saint Maarten. The readings of the instruments disagreed with each other and were inconsistent in time and heading with the last known position of the aircraft.

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Subsequent investigation pointed to the source of error-causing EMI as a portable television set being watched by a passenger. In yet another incident, the operation of a laptop computer by a passenger was found to interfere seriously with the navigational equipment of the aircraft during take-off and landing.


The Electro-Magnetic (EM) environment exists due to the transmission of electrical energy into free space. This energy is radiated from radar, radio, television, and other sources. These transmitters are ground-based, shipborne or airborne. Electricity and EM waves are interrelated. An EM field consists of an electric field and a magnetic field. These fields exist with all electric circuits since any current-carrying conductor creates a magnetic field around the conductor, and any two points in the circuit with a potential difference (voltage) between them creates an electric field. These two fields contain energy, but in circuits this field energy is usually returned to the circuit when the field collapses. If the field does not fully return its energy to the circuit, it means the wave has been partially radiated, or set free, from the circuit. This radiated energy is undesired, as it may cause interference with other electronic equipment in the vicinity. If it is undesired radiation from a transmitter it is termed Radio Frequency Interference (RFI), if it is specifically from a ground based transmitter it is termed High Intensity Radiated Field (HIRF) and if from another source, it is termed Electromagnetic Interference (EMI) or, more simply, noise.

Similarly, any magnetic field that cuts a conductor and which is at right angles to that conductor will create an electric field about that conductor, and any two points on that conductor with a potential difference (voltage) will cause a current to flow. If it is unwanted it is EMI. The increased use of electronics in safety-critical areas of modern aircraft, coupled with the growing severity of the High Intensity Radiated Fields (HIRF) environment, has led to international aviation authorities demanding evidence that the aircraft are immune to a specified electromagnetic environment. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2)


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) 5.14.2 Lightning versus Static Discharge Airplane flight crews and passengers occasionally experience the sensation of a lightning strike to the airplane. The bright flash and thunderous noise accompanying many strokes can be frightening, but there is virtually no danger to the airplane occupants. In some cases a bright flash may illuminate the

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cabin yet no thunder clap is heard; likewise, in some instances, a loud boom may be heard in the absence of a flash of light. .Published accounts of these lightning strokes to an airplane often erroneously attribute the flash and thunder clap to a static discharge. Airplane static discharges cannot produce a

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bright flash or noise audible in the cabin. In fact, static discharges from the airplane occur almost continuously in flight without anyone being aware of them. Static dischargers installed near the wing and fin tips control the point of discharge and reduce the amount of radio noise which static discharges are


capable of producing. An airplane continuously acquires an electrical charge from collisions with airborne dust and precipitation particles. The airplane voltage may rise as high as 100,000 volts, but the airplane charging capacity is not sufficient for it to acquire a damaging (or flash and thunder producing) discharge.

The airplane static charge is similar to the electrical charge a person gathers while walking across a carpeted room. Almost everyone has had the unpleasant sensation of a mild shock when reaching for a door knob after walking on a carpet. The discharge to the door knob may release a potential of 10,000 to 20,000 volts, but like an airplane, the charge storing capacity of the body is low, so that both the finger and the door knob survive the static discharge without burns or physical damage. An airplane continues to acquire a charge as it impacts with airborne particles. As the charge increases the airplane potential rises, sometimes to as high as 100,000 volts, until the breakdown threshold of the static discharger is reached. When the breakdown threshold is reached, static discharge begins. This discharge may be likened to a water reservoir filled to the brim. As more water is added, it begins to overflow. The overflow begins at the lowest points at the edge of the reservoir; likewise, the static discharge from the airplane begins from the points of lowest threshold, the static dischargers.

The flash and thunder accompanying a lightning stroke is due to the rapid release of charge from a cloud charge center. The energy produced is dependent upon the speed at which the electric charge is transferred. If the wall of the reservoir burst, the resultant cascade of water wreaks more havoc For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) than a mere trickle of overflow. Likewise, the rapid breakdown of the air path by a lightning stroke releases considerable energy, some of which produces the characteristic flash and ear splitting boom. Static discharges and lightning strokes both involve the release of an electrical charge from a charge center. The vast difference in the effects associated with each is attributable to the difference in discharge time and the amount of charge released. The charge storing capacity of even the largest jet airplane is one millionth that of an average lightning charge.

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While airplane static discharge generally occurs continuously in flight, even an instantaneous discharge could not produce the startling effects of a lightning

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stroke. If the maximum airplane charge were instantaneously discharged, approximately 0.3 kilowatts of power would be produced; enough for a single flash of a light bulb. The power released by a single average lightning stroke is in the order of 3750 million kilowatts, more than the combined peak capacity


of every electrical power plant in the United States.

Static discharges do produce undesirable effects. Radio frequency noise resulting from corona discharge of static electricity can disrupt radio communications. This effect is, however, reduced or eliminated by bonding isolated parts and by installing static dischargers which control the place of discharge of frictional electric charge. While effective in reducing “ the undesirable effects of static discharge, static dischargers have little or no influence on lightning strokes.

Although a lightning stroke to the airplane produces a bright flash and deafening boom, which usually causes concern among the occupants, the design of modern airplanes provides them with immunity to serious lightning damage from strokes of the intensity within known lightning strike phenomena. On occasion some airplane damage may be sustained, but there is very slight hazard to life and limb from lightning strikes on the airplane. In no case will static discharges produce -visible damage, a flash or a report.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) 5.14.3 Electromagnetic Interference (HIRF) Electromagnetic Interference may come from inside or from outside the aircraft. The internal sources include computers, headsets, radios and navigation aids as well as the electrical power distribution system of the aircraft. Although they may only radiate tiny fields, these are very close to the aircraft’s aerials and possibly sensitive equipment’s.

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External sources include ground transmitters such as radio, radar, television and telephone. Some of these have very high power levels and even though

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the signals travel large distances, they can still have serious effects on the aircraft. Ground and airborne installations are sometimes referred to as fixed and intermittent transmitters. Because a ground transmitter is fixed, its location is marked on most aviation maps and can be avoided.


External signals may also come from airborne transmitters such as high powered radar or radio on military aircraft. If a military aircraft were to transmit in close proximity to a civil aircraft there would be no possibility of avoidance.

The increased use of digital equipment in aircraft has meant the aviation industry has had to increase the amount of attention paid to the problem of High Intensity Radiated Fields (HIRF’s), and has had to increase the level of shielding on aircraft.

Effects of HIRF

Electromagnetic interface can jam equipment, burn out electrical circuits, cause false readings on instruments, cause a false command in a flight control system (especially fly-by-wire), detonate a squib, or even cause an explosion of fuel vapour.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) 5.14.4 Lightning Strikes The earth acts as a large spherical capacitor plate with the other plate being the ionosphere. It has a capacitance of approximately 1⋅7 farad. The potential difference between the plates is in the order of 100 Mvolts. At any one time there are about 1500 localised holes in the atmosphere dielectric that lets the

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charge leak from one plate to the other. These we call thunderstorms, and the leakage current that takes the form of an electrical spark, we call lightning. With this current of about 25,000 amps, a lightning strike can generate something like 2⋅5 Tera watts of power. Correct bonding of the aircraft means that

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only about 625 kwatts of energy has to be dissipated in the airframe if struck by lightning. The current only lasts a few milliseconds it is not a problem to the structure, unless composite structures are used.


The effect of this short burst of electrical energy on the electronic equipment inside the aircraft can be dramatic. When lightning occurs it creates radio noise. It is not unknown for Automatic Direction Finding (ADF) needles to point at nearby storms rather than the selected transmitter.

During a lightning strike, voltages can be superimposed on signals that have no protection. These can increase the signal levels 500 times. The currents produced by this phenomenon can be 300,000 times higher than under normal conditions.

To protect against the indirect effects of lightning strike, certain design features are implemented. Protected components are segregated, wire lanes are separated and system components are protected by filters, which only allow spike free currents to access them. The filters have to neutralise power with a magnitude of 500 kwatts.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) 5.14.5 Protection The following methods of protecting against lightning strikes and other forms of electromagnetic interference are in use in the aircraft industry: • Proper grounding and shielding with inductances kept to as low a value as possible. • Guided shields for external computer cables that run right up to the main grounding point. • Symmetrical or separated ground connections for all critical signals. • The separation of critical electronic circuits from interference prone areas. • The eradication of voltage surges.


5.14.6 Maintenance

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The initial protection against voltage transients caused by lightning strikes and HIRF’s is provided by the aircraft manufacturer. These should last for the life of the aircraft. However, for systems to be fully protected, it is up to the maintenance engineer to adhere to good maintenance practices at all times. To ensure the continuing safety of the aircraft:

• Bonding checks must be correctly carried out.

• Bonding terminals must be correctly torque loaded.

• Crimping compounds must be used where specified. • Screens must be correctly terminated and earthed.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) 5.14.7 Static The effects of static electricity are of considerable importance in the design, operation and maintenance of aircraft. Static electricity will cause noise interference in radio communication equipment as well as other electronic systems. During flight, an aircraft picks up static charges because of contact with

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particles such as rain, snow, ice and dust. The charge results mainly from the high speed impact or frictional passage of these airborne particles and the charge rate is particularly high when for example, ice crystals precipitate out from a cold, moist atmosphere. Hence the expression ‘precipitation static’.

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Precipitation is termed ‘hard’ or ‘soft’ to distinguish between the different types of particles. Hard precipitation refers to the relatively dry particles such as


snow, ice, hail and sand. Soft precipitation refers to wet particles such as rain and sleet.

Aircraft are not shaped like a sphere, therefore the surface charge they experience will not be even. It has in fact numerous protuberances. These protuberances cause a redistribution of the electric charge field. The field is concentrated at the tip of a protuberance, with the consequent higher field intensity in the atmosphere immediately at the tip. As a result, this portion of the atmosphere could reach such excessive voltage gradients that charge leakage could start, and after ionisation a complete breakdown could occur.

When an aircraft is struck by hard precipitation, the particles carry away a charge and the aircraft is left with a charge of opposite polarity with respect to the surrounding atmosphere. During this charging time, the smaller exposed radii of the aircraft extremities and protuberances will reach corona starting potentials and will begin to discharge. If the aircraft is large and fast, and the precipitation is dense and fairly dry, the charging will continue. If the charging rate exceeds the discharge rate, the larger radii and /or the less exposed protuberances will reach their corona starting potentials. The discharge currents involved begin as fractions of a micro amp but in some conditions they may reach the order of milliamps. The charging mechanisms result in a discharge of pulsed radio frequency energy by the corona, which for example can be heard in the earphones of an ADF receiver as a slow ‘popping’ noise, rising to a crescendo of screaming or crying, as the aircraft speed increases. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) The energy released can also be observed in light form. Although visible corona appear as a continuous light, in fact a release of electrical energy from the corona is in pulse form. The energy is spread over the radio spectrum, but is in the main contained in the lower frequencies. If the charging mechanism could be removed, the problems of interference from static would be relieved, but this is not possible. However it is possible to bring about a reduction of the charge and to provide means of discharging the aircraft static in a regulated and electrically quiet manner.

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The device used to perform this task is called the Static Discharger. Static discharges are intended to prevent or reduce the radio noise experienced. This is achieved since the discharger provides the means of shifting the discharge point aft of the trailing edge of the extremity where the RF coupling is at


a minimum.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) Static Dischargers The dischargers are designed so that the charge travels along the resistive coatings over the glass fibre material to the discharger tip positioned some way from the wing or tail surface. Since the RF coupling is at a minimum here the dischargers acts as low impedance discharge points. This means that the

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voltage required to cause and sustain discharging is minimised. Dischargers need to be fitted in sufficient numbers to ensure that their total discharge current holds the aircraft potential below the threshold of direct discharge at the higher charge accumulation rates anticipated.


Types of Static Dischargers

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Figure 14.2 shows an older type of static discharger comprising a stranded cotton wick, chemically treated with metallic silver, covered with a protective plastic sheath leaving a short tail exposed. An aluminium anchor plate is fitted to the sheath for attachment to the aircraft. During service the wick will erode reducing the discharge efficiency. To restore efficiency the plastic sheath is progressively trimmed exposing fresh wick fibres. Normally the sheath is marked with a limit of trim which when reached means the replacement of the whole discharger.

Figure 14.2



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) Rod Discharger Many modern dischargers, as shown below, consists of tapered glass fibre rods which give mechanical support. This glass fibre is rendered conductive by a coating of material having high electrical resistivity to provide a path back from the discharge tip assembly. The conductive coating is protected by bakeon synthetic finish and in some types is further protected by a heat-shrunk sheath of Skydrol - resistant plastics. Three types of discharger tips are normally used: • a minute brush of extremely fine 80/20 nichrome wires • solid carbon which is machined to a 90 degree point


• tungsten needles

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The glass fibre rod is terminated at its thicker end by one of several attachment fittings, assembled together with conducting cement.

Figure 14.3



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) The diagram below shows a special type of lightning divertor fitted at the apex of nose radomes on some aircraft. The radome apex is normally particularly prone to noisy discharge. To minimise the noise from this source a discharger assembly can be screwed into the lightning divertor spike. This discharger comprises a 4 wire brush discharger mounted at the forward end of the rigid dielectric support, critically angled back so that each wire brush remains well exposed to high electric field density, while remaining protected by its polythene shroud against bunching of wires under air pressure. A high resistance spiral track inside the radome provides the current path back to the airframe.

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Light aircraft discharges are normally of the flexible type. Similar to the plastic sheath discharger the plastic sheath is replaced by a flexible nylon cord.

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The dischargers found on the very smallest aircraft are merely braided wire extending the tip beyond the trailing edges of the wings and tailplane.




PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS ELECTROMAGNETIC ENVIRONMENT (DCAM 5.14 L2) Sitting of Static Dischargers

It is important that sufficient numbers of static dischargers are fitted and sited in positions where they can efficiently discharge the static with the minimum of interference being induced into radio aerials. Optimum sitting of dischargers can only be determined by a thorough investigation of the characteristics of

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the particular aircraft type. In general, the wing, tail and fin tips, particularly at the trailing edges, are the locations of the greatest potential gradient. Dischargers positioned at these points, with additional units at spacing of 9 inches (24 cm) around these regions will normally give satisfactory results. Dischargers should not be located near to radio aerials.


Maintenance of Static Dischargers

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The dischargers should be checked in accordance with procedures detailed in the relevant maintenance manual and at the periods prescribed therein. In addition the following points should be observed.

• Periodically a general check should be made to ensure that all dischargers are securely mounted, are not broken or missing. • The efficient operation of the dischargers is dependent on good electrical contact between the base and the aircraft. This resistance should in general not exceed 0⋅⋅05 ohms. However, provided there is no static interference with the radio systems, a resistance not exceeding 0⋅1 ohms may be acceptable. If the discharger exceeds the acceptable limit, the dischargers must be removed and the contact surfaces cleaned. • Where dischargers of the tapered glass-fibre rod are fitted, the condition of the resistive coating between the base and tip, and of the Skydrolresistant plastics, should be checked for physical continuity, particularly at the base and tip joints. The electrical resistance between the base and tip should be within the limits specified by the manufacturer for the type of discharger. Typically, values between 8 and 100 Mohms are acceptable for trailing edge dischargers and 5 to 60 Mohms for tip-mounted dischargers.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) NOTES


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15 TYPICAL ELECTRONIC/DIGITAL AIRCRAFT SYSTEMS

5.15.1 On Board Maintenance and BITE

Acquisition

The acquisition of aircraft system data is performed by four major electronic systems:

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• the EICAS/ECAM system which monitors the operational data in order to display warnings and system information,


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• the Flight Data Recording System (FDRS) which is mandatory and records aircraft operational parameters for incident investigation purpose, • the Central Maintenance System (CMS) which monitors the BITE data in order to record the system failures,

• the Aircraft Condition Monitoring System (ACMS) which records significant operational parameters in order to monitor the engines, the aircraft performance and to analyze specific aircraft problems.

Consolidation

In normal operation, the ECAM permanently displays normal aircraft parameters and the ACMS and FDRS permanently record aircraft system parameters. When an anomaly is detected by an aircraft system, the ECAM displays the abnormal parameter or function and its associated warning and the CMS records the failure information detected by the system BITE.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Retrieval

All the information can be retrieved through: • the cockpit Multipurpose Control Display Unit (MCDU), • the EICAS/ECAM displays, • the cockpit printer, • the down loading system, • a ground station via ACARS,


• the FDR.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Analysis

Maintenance operations can be divided into three groups: • minor trouble shooting which is performed with the help of the EICAS/ECAM and the CMS through the MCDUs and the printer or ACARS down linked reports.

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• in-depth trouble shooting which is performed with the help of the CMS and the ACMS through the MCDUs and printed reports.

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• long term maintenance which is performed with the help of the ACMS and the FDRs through printer, ACARS down-linked and down-loaded reports or


recorded tapes.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2)


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Figure 15.1 On Board Maintenance Facilities



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Built In Test Equipment (BITE)

General

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A system is composed of Line Replaceable Units (LRUs) which can be: computers, sensors, actuators, probes, etc. Most of these LRUs are controlled by digital computers. For safety reasons, these LRUs are permanently monitored, they can be tested and trouble shooting can be performed. In each system,

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a part of a computer is dedicated to these functions: it is called Built In Test Equipment. In some multi- computer systems, one computer is used to concentrate the BITE (Built In Test Equipment) data of the system. During normal operation, the system is permanently monitors: internal monitoring,


inputs/outputs monitoring, link monitoring between LRUs within the system.

Fault Detection

If a failure occurs, it can be permanent (consolidated) or intermittent.

Isolation

After failure detection, the BITE is able to identify the possible failed LRUs and can give a snapshot of the system environment when the failure occurred.

Memorization

All the information necessary for maintenance and trouble shooting is memorized in a Non Volatile Memory.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Concept The BITE information stored in the system BITE memories is sent to a centralized maintenance device. The manual tests (SYSTEM TEST and SPECIFIC TESTS) can be initiated via this centralized maintenance device. Its main advantages are:

Test

•

single interface location (cockpit).

•

easy fault identification.

•

reduction of the trouble shooting duration.

•

simplification of the technical documentation.

•

standardization of the equipment.


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The test function can be divided into four groups: 1) Power Up Test

The power up test is first a safety test. The purpose of a safety test is to ensure compliance with the safety objectives. It is executed only on ground after long power cuts (more than 200ms). Its duration is function of the system which is not operational during the power up test. If the aircraft is airborne, the power up test is limited to a few items to enable a quick return to operation of the system. The typical tasks of a power up test are:

• test of microprocessor, • test of memories,

• test of ARINC 429

• various I/O circuits, • configuration test.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 2) Cyclic Tests These tests are carried out permanently. They do not disturb system operation. The typical tasks of a cyclic test (also called IN OPERATION TEST) are: Watchdog test (a watchdog is a device capable of restarting the microprocessor if the software fails), RAM test. Permanent monitoring is performed by the operational program (e. g. ARINC 429 messages validity).

3) System Test

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The purpose of this test is to offer to the maintenance staff the possibility to test the system for trouble shooting purposes. This test can be


performed after the replacement of a LRU in order to check the integrity of the system or sub-system. It is similar to the POWER UP TEST but it is more complete. It is performed with all peripherals supplied.

4) Specific Tests

For some systems, specific tests are available. The purpose of these tests is to generate stimuli to various command devices such as actuators or valves. They can have a major effect on the aircraft (automatic moving of slats or flaps, engine dry cranking).





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Figure 15.2 BITE Philosophy



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.2 ARINC Communication Addressing and Reporting System (ACARS)

This system enhances air-ground communications and reduces flight crew workload through the use of high speed, digital data link messages. These messages down link from the aircraft to the operations centre by way of VHF radio, landlines and the ARINC data link control station. Messages can up-

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link to an aircraft through the same network. Voice communication between the aircraft and ground telephone circuits is also possible. The network covers the USA, Europe and many other countries around the world. Figure 15.3 shows the data link between the aircraft and operations.

The ACARS Management Unit (MU) provides the following:


a) Collection of data from the aircraft systems automatically or on command. b) Sends data to a VHF transceiver.

c) Sends and receives data between the aircraft and ground stations.

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d) Alerts the flight crew via modularised avionics and warning assembly (MAWEA) with a chime that a voice call is ready.

The ACARS Switch Module is used to connect the centre or right VHF transceiver with either of the ACARS MU.The Control Display Units (CDUs) on the flight deck are used for:

a) Control and down link data input to the MU. b) Voice or data mode selection. c) Up-link message display.

d) Centre or right VHF transceiver selection.

The EFIS/EICAS interface units (EIUs) inputs to the ACARS MU giving chocks off, take off, landing and chocks in information plus aircraft identification. The airborne data loader (ADL) loads ACARS software. The central maintenance (CMC) sends system fault data to the ACARS MU for down link and



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) receives ACARS fault data. The Data Management Unit (DMU) sends aircraft condition monitoring system (ACMS) report data to the ACARS MU for down link. The audio management unit (AMU) turns on the VHF call lights on the audio control panels (ACPs) when a voice call is ready.

ACARS can send data to the flight deck printer. The ACARS MU select switch controls which ACARS MU is active. The VHF Transceiver, down links data

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from the aircraft to ground station. Receives up-link data from the ground to the aircraft. The radio communication panels (RCPs) are used for voice or data mode selection. Outputs from the active ACARS MU go through the ACARS/VHF switching module.

f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 15.3 ARINC Communication System Data Link


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Figure 15.4 ACARS Overview


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.3 Electronic Centralised Aircraft Monitors (ECAM)

ECAM is fitted to Airbus aircraft and is designed to provide operational assistance for both normal and abnormal operations. The display enables the crew to determine aircraft system status and alert them of abnormal configurations.

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The system calls the attention to the seriousness of a situation, helps the crews in their diagnosis and advises on their corrective actions. Components include:


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2 ECAM display units installed on the centre instrument panel (Status/Warning/Corrective Action and System Synoptics).

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1 ECAM control panel located on the centre pedestal.

•

Control/ brightness potentiometers for the ECAM DU' s located on the ECAM control panel.

•

1 set of transfer selector switches installed on the centre pedestal. They provide EIS reconfiguration controls in case of DMC or ECAM DU failure.

•

2 visual attention getters (MASTER WARN/red and MASTER CAUT/amber) installed in front of each pilot.

•

2 speakers installed just in front of each side stick controller.

•

1 audio cancel switch.

•

2 speaker controls.

•

2 FWCs (Flight Warning Computers) located in avionics compartment which acquire all data to process warning, status, memo, and generate radio automatic call-out.

•

2 SDACs (System Data Analogue Converters) located in avionics compartment which acquire all data for display of system information through animated synoptic diagrams.

•

3 DMCs (Display Management Computers) located in avionics compartment common to EFIS and ECAM. They acquire and process signals from A/C sensors and computers in order to generate proper code of graphic instructions corresponding to the images to be displayed.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Display Units

These units are mounted side-by-side; the left-hand unit is dedicated to information on the status of systems, warnings and corrective action in a sequenced check-list format, while the right-hand unit is dedicated to associated information in pictorial or synoptic format.


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Figure 15.5 ECAM Display Units



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Display Modes There are four display modes, three of which are automatically selected and referred to as flight phase-related, advisory (memo and status), and failurerelated modes. The fourth mode is manual and permits the selection of diagrams related to anyone of 12 of the aircraft' s systems for routine checking, and also the selection of status messages provided no warnings have been ' triggered'for display. The selections are made by means of illuminated push-button switches on the system control panel.

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In normal operation the automatic flight phase-related mode is used, and in this case the displays are appropriate to the current phase of aircraft operation, i.e. pre-flight, take-off, climb, cruise, descent, approach, and after landing. An example of a pre-flight phase is shown in figure below; the left-hand display


unit displays an advisory memo mode, and the right-hand unit displays a diagram of the aircraft' s fuselage, doors, and arming of the escape slides deployment system.

Figure 15.6 Pre-Flight Phase-Related Mode Display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The failure-related mode takes precedence over the other two modes and the manual mode. An example of a display associated with this mode is shown in figure 15.7. In this case, while taxiing out for take-off, the temperature of the brake unit on the rear right wheel of the left main landing gear bogie has become excessive. A diagram of the wheel brake system is immediately displayed on the right-hand display unit, and simultaneously the left-hand unit displays corrective action to be taken by the flight crew. In addition, an aural warning is sounded, and a light (placarded ' LG WHEEL' ) on a central warning

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light display panel is illuminated. As the corrective action is carried out, the instructions on the left-hand display are replaced by a message in white confirming the result of the action. The diagram on the right-hand display unit is appropriately ' redrawn' .


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Figure 15.7 Failure Related Mode Display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) In the example considered, the warning relates to a single system, and by convention such warnings are signified by underlining the system title displayed. In cases where a failure can affect other subsystems, the title of the sub-system is shown ' boxed' , as for instance in the display shown in figure 15.8. Warnings and the associated lights are cleared by means of ' CLEAR'push-button switches on either the ECAM control panel or a central warning light display panel.


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Figure 15.8 Display of Failure Affecting a Sub-System



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Status messages, which are also displayed on the left-hand display unit, provide the flight crew with an operational summary of the aircraft' s condition, possible downgrading of autoland capability, and as far as possible, and indications of the aircraft status following all failures except those that do not affect the flight. The contents of an example display are shown in figure 15.9.


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Figure 15.9 Example of Status Display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Control Panel

The layout of this panel is shown in figure 15.10; all switches, with the exception of those for display control, are of the push-button, illuminated caption type. 1.

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SGU selector switches control the respective symbol generator units, and the lights are off in normal operation of the system. The ' FAULT' caption is illuminated amber if a failure is detected by an SGU' s internal self-test circuit. Releasing a switch isolates the corresponding SGU, and causes the ' FAULT'caption to extinguish, and the ' OFF'caption to illuminate white.

2.

f o g y n r i r a t e e e i n r i p g o n r P SE A M pressed. A display is automatically cancelled whenever a warning or advisory occurs.

3.

CLR switch Light illuminated white whenever a warning or status message is displayed on the left-hand display unit. Pressed to clear messages.

4.

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Synoptic display switches permit individual selection of synoptic diagrams corresponding to each of 12 systems, and illuminate white when

STS switch permits manual selection of an aircraft status message if no warning is displayed; illuminated white. Pressing the switch also causes the CLR switch to illuminate. A status message is suppressed if a warning occurs or if the CLR switch is pressed.

5.

RCL switch Enables previously cleared warning messages to be recalled provided the failure conditions which initiated them still exist. Pressing the switch also causes the CLR switch light to illuminate. If a failure no longer exists the message ' NO WARNING PRESENT'is displayed on the left-hand display unit.





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Figure 15.10 Control Panel



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) System Testing

Each flight warning computer of the system is equipped with a monitoring module which automatically checks data acquisition and processing modules, memories, and the internal power supplies as soon as the aircraft' s main power supply is applied to the system. A power on test routine is also carried out for correct operation of the symbol generator units. During this test the display units remain blank.

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In the event of failure of the data acquisition and processing modules, or of the central warning light display panel, a ' failure warning system'light on the panel is illuminated. Failure of a computer causes a corresponding annunciator light on the maintenance panel, captioned ' FWC FAULT' , to illuminate. A


symbol generator unit failure causes a ' FAULT'caption on the appropriate pushbutton switch on the system control panel to illuminate.

Manual self-test checks for inputs and displays are carried out from a maintenance panel shown in figure 15.11. When the ' INPUTS'switch is pressed, a ' TEST'caption is illuminated white, and most of the inputs to each computer are checked for continuity. Any incorrect inputs appear in coded form on the left-hand display unit. The right-hand display unit presents a list of defective parameters at the system' s data analogue converter. The diagrams of systems appear on the right-hand display unit with the caption ' TEST'beside the system title, as each corresponding push-button switch is pressed. Calibrated outputs from the data analogue converter are also displayed. Any defective parameters are identified by a flag display.

A' DISPLAYS'push-button switch is provided on the maintenance panel and when pressed it initiates a check for correct operation of the symbol generator units, and the optical qualities of the display units by means of a test pattern display. The ' LOAD'caption is illuminated each time a failure is memorized in the relevant test circuits of the SGUs.

The annunciator lights on the maintenance panel illuminate white simultaneously with a failure warning system light on the central warning light display panel when a corresponding computer fails. The ' INHIB OVRD'switch enables inhibited warnings to be displayed.





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Figure 15.11 Maintenance Panel



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.4 Electronic Flight Instrument Systems (EFIS)

The EFIS consists of data being sent to symbol generators that convert the data to symbols for display on the CRTs. Figure 15.12 shows the general arrangement.

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The display units are the EADI (Electronic Altitude Director Indicator) and EHSI (Electronic Horizontal Situation Indicator). The symbol generators process

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data from the EFIS control panel and interfacing systems to provide video signals to the EADI and EHSI CRTs.

The left symbol generator feeds the Captains EADI and EHSI, the right symbol generator feeds the First Officers EADI and EHSI. The centre symbol


generator acts as a back-up too be switched in should the left or right generator fail.

In order to ensure that the light intensity of the EADI and EHSI CRT displays is compatible with ambient light conditions in the flight deck, light sensors are provided to automatically adjust the intensity. The sensors operate on the photo-diode principle, there are two mounted on the glareshield to measure light coining in through the flight deck windshields, and one each on the front of the EADI and EHSI to detect light within the flight deck compartment area.

The control panel controls the displays on the EADI and EHSI and the control switches are grouped accordingly. A control panel is provided for each pilot.





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Figure 15.12 EFIS – General Arrangement



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Display Units Each display unit consists of the sub-units shown in figure 15.13. The power supply units provide the requisite levels of AC and DC power necessary for overall operation; the supplies are automatically regulated and monitored for undervoltage and overvoltage conditions. The video/monitor card contains a video control microprocessor, video amplifiers and monitoring logic for the display unit. The main tasks of the processor and associated ROM and RAM

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memories are to calculate gain factors for the three video amplifiers (red, blue and green), and perform input and sensor and display unit monitor functions. The input/output interface functions for the processor are provided by analogue multiplexers, an A/D converter and a D/A converter. The function of the

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convergence card is to take X and Y deflection signals and to develop drive signals for the three radial convergence coils (red, blue and green) and the one lateral convergence coil (blue) of the CRT. Voltage compensators monitor the deflection signals in order to establish on which part of the CRT screen the


beams are located (right or left for the X comparator, and top or bottom for the Y comparator).

Signals for the X and Y beam deflections for stroke and raster scanning are provided by the deflection amplifier card. The amplifiers for both beams each consist of a two-stage preamplifier and a power amplifier. Both amplifiers use two supply inputs, 15 V DC and 28 V DC; the former is used for effecting most of the stroke scanning or writing, while the latter is used for repositioning and raster scanning. The interconnect card serves as the interface between the external connector of a display unit and the various cards. Digital line receivers for the signals supplied by the SGs are also located on this card. In a typical system, six colours are assigned for the display of the many symbols, failure annunciators, messages and other alphanumeric information, and are as follows: White Green Magenta Cyan Yellow Red

Display of present situation information

Display of present situation information where contrast with white symbols is required, or for data having lower priority than white symbols. All ' fly to'information such as flight director commands, deviation pointers, active flight path lines. Sky shading on an EADI and for low-priority information such as non-active flight plan map data.

Ground shading on an EADI, caution information display such as failure warning flags, limit and alert annunciators and fault messages. For display of heaviest precipitation levels as detected by the weather radar.





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Figure 15.13 Display Unit



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Symbol Generators (SGs)

These provide the analogue, discrete and digital signal interfaces between an aircraft' s systems, the display units and the control panel, and they perform symbol generation, system monitoring, power control and the main control functions of the EFIS overall. The interfacing between the card modules of an SG is shown in figure 15.14, and card functions are given in Table 15.1.


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Figure 15.14 Symbol Generator and Card Interfacing





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Table 15.1



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Control Panel

A control panel is provided for each system, and, as shown in figure. 15.15, the switches are grouped for the purpose of controlling the displays of their respective EADI and EHSI units as listed in Table 15.2.


Figure 15.15 Control Panel


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Table 15.2


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Remote Light Sensor

This is a photodiode device which responds to flight deck ambient light conditions and automatically adjusts the brightness of the CRT displays to a compatible level.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Display Presentations

The EADI displays traditional pitch and roll attitude indications against a raster-scanned background, and as may be seen from the example illustrated in figure 15.16, the upper half is in cyan and the lower half in yellow. Attitude data is provided by a gyro system. Also displayed are flight director commands,

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localizer and glide slope deviation, selected airspeed, ground speed, AFCS and autothrottle system modes, radio altitude and decision height.

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Figure 15.16 illustrates a display representative of an automatically-controlled approach to land situation together with the colours of the symbols and alphanumeric data produced via the EFIS control panel and SGs. The autoland status, pitch, roll-armed and engaged modes are selected on the AFCS


control panel, and the decision height is selected on the EFIS control panels. Radio altitude is digitally displayed during an approach, and when the aircraft is between 2,500 and 1,000 ft above ground level. Below 1,000 ft the display automatically changes to a white circular scale calibrated in increments of 100 ft, and the selected decision height is then displayed as a magenta-coloured marker on the outer scale.

The radio altitude also appears within the scale as a digital readout. As the aircraft descends, segments of the altitude scale are simultaneously erased so that the scale continuously diminishes in length in an anticlockwise direction. At the selected decision height plus 50 ft, an aural alert chime sounds at an increasing rate until the decision height is reached. At the decision height, the circular scale changes from white to amber and the marker changes from magenta to amber; both the scale and marker also flash for several seconds. A reset button is provided on the control panel and when pressed it stops the flashing and causes the scale and marker to change from amber back to their normal colour. (Note: Radio altitude is not display above 2500 feet).

If during the approach the aircraft deviates beyond the normal ILS glide slope and/or localizer limits (and when below 500 ft above ground level), the flight crew are alerted by the respective deviation pointers changing colour from white to amber; the pointers also start flashing. This alert condition ceases when the deviations return to within their normal limits.





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Figure 15.16 EADI Display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The EHSI presents a selectable, dynamic colour display of flight progress and plan view orientation. Four principal display modes may be selected on the control panel: MAP, PLAN, ILS and VOR. Figure 15.17 illustrates the normally-used MAP mode display which, in conjunction with the flight plan data programmed into a flight management computer, displays information against a moving map background with all elements positioned to a common scale.

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The symbol representing the aircraft is at the lower part of the display and an arc of the compass scale, or rose, covering 300 on either side of the instantaneous track is at the upper part of the display. Heading information is supplied by the appropriate IRS, and the compass rose is automatically

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referenced to magnetic north (via a crew-operated ' MAG / TRUE'selector switch) when between latitudes 730 N and 650 S, and to true north when above these latitudes. When the selector switch is set at ' TRUE' , the compass rose is referenced to true north regardless of latitude.


Tuned VOR/DME stations, airports and their identification letters, and the flight plan entered into the flight management system computer are all correctly oriented with respect to the positions and track of the aircraft, and to the range scale (nm/in) selected on the EFIS control panel (figure 15.15). Weather radar ' returns'may also be selected and displayed when required, at the same scale and orientation as the map. Indications of other data such as wind speed and direction, lateral and vertical deviations from the selected flight profile, distance to waypoint, etc., are also displayed.

The map display also provides two types of predictive information. One combines current ground speed and lateral acceleration into a prediction of the path over the ground to be followed over the next 30, 60 and 90 seconds. This is displayed by a curved track vector, and since a time cue is included the flight crew is able to judge distances in terms of time. The second prediction, which is displayed by a range to altitude arc, shows where the aircraft will be when a selected target altitude is reached.





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Figure 15.17 EHSI in MAP Mode



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) In the PLAN mode, a static map background with active route data oriented to true north is displayed in the lower part of the EHSI display, together with the display of track and heading information as shown in figure 15.18. Any changes to the route are selected at the keyboard of the flight management system display unit, and appear on the EHSI display so that they can be checked by the flight crew before they are entered into the flight management computer.


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Figure 15.18 EHSI in ‘PLAN’ Mode (The display is a north up display)



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The VOR and ILS modes present a compass rose (either expanded or full) with heading orientation display as shown in figure 15.19 (A&B). Selected range, wind information and system source annunciation are also displayed. If selected on the EFIS control panel, weather radar returns may also be displayed, though only when the mode selected presents an expanded compass rose.


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Figure 15.19 (A) VOR Mode Display





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Figure 15.19 (B) ILS Mode Display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Failure Annunciation

Failure of data signals from such systems as the ILS and radio altimeter are displayed on each EADI and EHSI in the form of yellow flags ' painted'at specific matrix locations on their CRT screens. In addition, fault messages may also be displayed: for example, if the associated flight management

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computer and weather radar range disagree with the control panel range data, the discrepancy message ' WXR/MAP RANGE DISAGREE'appears on the EHSI.

Data Source Selection


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In the type of system described, means are provided whereby the pilots can, and independently of each other, connect their respective display units to alternate sources of input data, e.g. left or right ADCs, flight management computers, flight control computers, and standby IRS.

Each pilot has a panel of selector switches arranged as shown in figure 15.20. The upper rotary type of switch connects either the left, centre or right flight control computer to the EADI as the source of attitude data. The other switches are of the illuminated push type and are guarded to prevent accidental switching. In the normal operating configuration of systems they remain blank, and when activated they are illuminated white.





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Figure 15.20 Source Selector Switch Panel



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.5 Engine Indicating and Crew Alerting System (EICAS)

EICAS offers improved flight operations by reducing crew workload through automatic monitoring of engine parameters and an alerting system which works continuously from power-up through to after flight maintenance. Only the parameters required to set and monitor engine thrust are displayed full time.

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EICAS monitors the remaining parameters and automatically displays any out-of-tolerance parameter in the appropriate colour. The alerting system' s coloured messages are designed to communicate both the failure and the urgency for responding to the failure, so that crew attention is only diverted to the

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extent appropriate to the malfunction. By utilising CRT displays, EICAS provides improved flight deck geometry by reducing the panel space required for engine instrument displays, as well as status and maintenance indications.


The EICAS system includes two colour high resolution shadow mask CRTs, two computers, and two control panels. These components, together with two display-switching modules, the cancel/recall switches, and the captain' s and first officer' s master caution lights, jointly perform the various EICAS functions. The EICAS computer processes and formats for display all engine and aircraft system information required by the crew. Only one computer at a time is used for displaying the data on both display units. Computer selection is done on the display select panel.

Primary engine parameters and crew alerting messages are displayed on the upper display unit, and secondary engine parameters are displayed on the lower display unit. System status and status messages, or maintenance data and maintenance messages, can also be displayed on the lower display unit (DU) on demand, by using the appropriate display switches on the display select panel or maintenance panel.

The master CAUTION lights provide backup indication for the caution messages displayed, and caution and advisory message review and control is done by using the cancel/recall switches.





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Figure 15.21 EICAS- General Layout



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Display Categories

EICAS is designed to categorise alerts and displays according to function and usage. This results in three modes of display: (1) The Operational Mode (2) The Status Mode (3) The Maintenance Mode

Operational Mode


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The operational mode displays those engine parameters required on the ground and in flight, and provides continuous monitoring of aircraft engine subsystems. The upper display unit is dedicated full-time to primary engine parameters and alert messages for monitoring by the crew. The lower display unit shows secondary engine parameters, those that do not require constant crew monitoring. At power up, all engine parameters are displayed automatically, with the primary engine display on the upper CRT, and secondary engine display on the lower CRT.

Status Mode

The status mode display provides subsystems parameters as will as status messages on the lower CRT. These are needed by the crew to determine the readiness of the aircraft for dispatch, and are closely associated with the Minimum Equipment List (MEL). Primary engine parameters remain displayed on the upper CRT.

Maintenance Mode

The maintenance mode displays provide aircraft subsystem parameter and maintenance messages on the lower DU. These data, together with test display, are needed by the ground crew for post-flight maintenance and troubleshooting action. Primary and secondary engine parameters are displayed together on the upper DU in the compact-full format whenever a maintenance page is displayed on the lower DU.





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Primary Engine Parameters

The primary engine parameters N1 and EGT are displayed on the upper DU at all times. The parameters shown are only those required to set and monitor engine thrust. The primary thrust setting parameter for the GE engine is N1. N1is displayed in a digital readout and also by an analogue pointer. EGT is a thrust monitoring parameter.

Secondary Engine Parameters


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The secondary parameters consist of N2, fuel flow, oil pressure, oil temperature, oil quantity, and vibration. They are displayed for each engine automatically at power up or, when they are manually selected for display on the lower DU.

Display Unit (DU) Features White Red Green

-

General colour used for all scales, normal operating range of pointers, digital readouts and digital readout boxes.

-

Used for warning messages, redline limit on scales, exceedance condition for pointers, digital readouts, and digital readout boxes.

-

Used for thrust mode and reference readout, N1 target cursor, TA1 readout and cursor, selected temperature readout, and thrust

reverser messages.

Blue Yellow

-

Not used for any EICAS displays (displayed during EICAS Test only).

-

Used for caution and advisory messages, yellow band on scales, yellow band condition for pointer, digital readouts and digital readout boxes, TA1 readout and cursor, maximum limit marker, and thrust reversal readout.

Magenta (pink)-

Used for in-flight start envelope, fuel-on command, cross-bleed messages and N1-FMC target cursor.

Cyan (Light blue)-

Used for all parameter names, status cue and secondary engine date cue.

Black

Background colour.

-



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Displays during Exceedance Conditions

If any parameter reaches an exceedance condition, EICAS automatically displays both engine parameters in the exceedance colour for fast crew attention and action. These automatic features are described later in more detail.


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Figure 15.22 Primary & Secondary Engine Displays



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Status Mode Display

The status mode display provides data for the crew to determine the aircraft' s readiness for dispatch. This display is associated with the MEL. It can be manually selected by pressing the STATUS switch on the display select panel. In addition to status messages, analogue pointers are used to indicate the positions of the rudder, elevator and aileron control surfaces. Also, digital readouts show selected subsystem parameters.

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The status mode can be useful in-flight for anticipating possible ground maintenance actions that may be required. For this function, a STATUS cue is provided in the upper left hand corner of the lower DU whenever a change in status occurs whilst the status page is not being displayed. The status


messages are automatically displayed on the upper right-hand side of the DU.

NOTE: When a DU fails in flight, the status cue and status displays are inhibited. However, they are not inhibited on the ground.

Pressing the STATUS switch once causes the status mode display to appear on the lower DU. Pressing the STATUS switch once again, causes the lower DU to blank. Since the status message field is limited to eleven messages per page, there are situations where more than one page of status messages can exist. In this case, each actuation of the STATUS switch causes the next page of messages to be displayed. After all have been displayed, a final actuation of the STATUS switch causes the lower DU to blank.

Operational and Status Mode Switching Initial Displays

After power is first applied to the aircraft' s left and right 115V ac buses, the primary engine parameters are displayed on the upper DU, and the secondary engine parameters on the lower DU. Alternate displays can be obtained by utilised the display select panel' s ENGINE or STATUS switches, as described below.





Figure 15.23 Status Mode Display


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Figure 15.24 Mode Switching


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Secondary Engine Parameters

Pressing the ENGINE switch once causes the lower DU display to change from a status mode display, or blank display, to the secondary engine parameter display. Pressing this switch again causes the lower DU to blank. Figure 15.25 shows the function of all controls on the select panel.


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Figure 15.25 Display Select Panel



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Maintenance Mode Displays

There are six maintenance formats within the EICAS maintenance modes. They are designed to provide a flight deck display of maintenance information to aid the ground crew in troubleshooting and verification testing of the major subsystems. They are also used to record system parameters on the ground, or

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at the time of an in-flight fault for later readout on the ground. These formats are not available to the pilot' s in flight (unless optional EICAS Flight Test switch installed in the main equipment centre).

Maintenance Display Formats


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The six maintenance formats can be displayed on the lower DU, These formats are controlled using the EICAS maintenance panel. 1) Environmental Control System/Messages (ECS/MSG)

The ECS/MSG format provides parameters of the air cycle cooling pack and the zone temperature control system. It also provides a display of maintenance messages.These messages are not directly related to the dispatch capability of the aircraft, but direct the maintenance crews to individual component failures. These messages, in white, are in most cases a repeat of the status messages displayed on the status page format.

2) Electrical/Hydraulic (ELEC/HYD)

The electrical portion of the ELLEC/HYD format provides the system parameters of both the ac and dc systems. The hydraulic portion provides data for each of the aircraft' s three hydraulic systems.

3) Performance/Auxiliary Power Unit (PERF/APU)

In addition to the APU parameters, the PERF/APU format displays engine data and aircraft environmental data. This format is useful for determining the environment existing at the moment that an engine data recording is taken. The engine and aircraft environmental data are used for engine analysis. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 4) Engine Exceedance (ENG/EXCD) This format shows the maximum exceedance level and accumulated time of exceedance for selected engine parameters as well as EGT profiles for start or operational exceedances. This data is valuable in the determination of engine overboost severity and the subsequent maintenance action required.

5) Config/Maintenance Control and Display Panel (COMF/MCDP)

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The Configuration portion of the CONF/MCDP format provides maintenance personnel with a convenient central location to verify that all enginesensitive computers are programmed to correspond with the installed engine. The EICAS contains Built-in-Test Equipment (BITE) to assist


maintenance personnel. The Maintenance Control Panel (MCDP) located in the main equipment centre is an interactive computer designed as an onboard maintenance centre for the Plight Management System (FMS). The MCDP provides messages for two major functions, namely in-flight fault recording and ground test.

The maintenance messages from the MCDP can be displayed on EICAS. Test sequencing is controlled at the MCDP or remotely, with a portable control panel from the flight deck area. With the remote panel, the ground crew can troubleshoot and retest the flight management system avionics without having to access the main equipment centre.

6) Electronic Propulsion Control System (EPCS)

This format provides real time and recorded data for specific engine parameters (control positions, temperatures and pressures).





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Figure 15.26 EICAS Display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) General Operation Maintenance Formats – Switching

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To display any one of the maintenance formats, press the appropriate display select switch. Pressing the same switch once again reverts the display back to the primary engine display on the upper DU, and to the secondary engine display on the lower DU. An exception is the case of the ECS/MSG display

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format, where multiple message page capability exists. In this case, the ECS/MSG switch is pressed once for each message page before the format is removed.


Figure 15.27 below shows the function of the remaining controls on the maintenance panel. elec/hyd (electrical and hydraulic systems) switch.

Figure 15.27 EICAS Maintenance Panel



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) ALERT messages

There are three levels of alert messages.

1. Level A Warning (RED)

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Any time a level A warning is generated, one of the EICAS DU' s displays a red warning message, Also, warning system signals

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simultaneously cause the master WARNING lights to illuminate and the firebell or siren aural to sound.


Pressing either master warning light resets the warning system' s circuitry for the master warning lights and the associated level A aurals. This action causes the WARNING light to extinguish and the warning aural to cease.

Figure 15.28 Level A Warning



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 2. Level B Caution (Yellow) Any time a level B caution is generated, the EICAS displays a yellow message, it illuminates both master caution lights, and requests a beeper tone from the warning system,

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NOTE: The level B beeper sound is generated by the warning system for a duration of 0.8 seconds. Pressing either master caution light resets the control circuits for both master caution lights, and causes both lights to extinguish.


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Figure 15.29 Level B Caution



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 3. Level C Advisory (Yellow) Any time a level C advisory is generated, the EICAS displays a yellow message indented one space to the right below the last level B caution message.

NOTE: No aural tones or master caution lights are activated.


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Figure 15.30 Level C Advisory



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The level A, B and C messages are categorised and positioned as follows: Level A:

Warnings - red messages positioned at the top of the display.

Level B:

Cautions - yellow messages positioned just below the Level A

Level C:

Advisories - yellow messages - indented one space to the right just below the level B messages.

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NOTE: This sequence applies when all three levels are displayed. In the case of a single message, it appears at the top of the display. The most recent

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(newest) message generated is always positioned at the top of its category, level A, B or C, as appropriate.

f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 15.31 Alert Messages



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Cancel/Recall

Warning messages cannot be cancelled, but caution and advisory messages can be removed from the display and recalled by using the EICAS CANCL/RECALL switches.

Messages Cancel Feature

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In the example shown, pushing the CANCEL switch removes only the caution and advisory messages from the display, whilst the red message remains.


Messages - Recall Feature

Cancelled messages are recalled by pressing the RECALL switch. When pressed, the message RECALL momentarily appears on the bottom message line, as shown. The previously cancelled caution and advisory messages then reappear on the display, as follow;

The level B cautions are positioned below any level A messages, and are followed by any level C advisories just below the cautions, but indented one space to the right.

Figure 15.32 Cancel/Recall For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Paging

Message Overflow Feature

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The alert message display feature incorporates a multiple paging capability. Up to eleven messages can be displayed at one time. If a display requirement for more than eleven messages exist, only ten are actually displayed. The bottom message will be replaced by a "PAGE 1" notation. The additional

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messages will then be displayed on subsequent pages which can be accessed by utilising a paging feature.


Message Paging Feature

Alert messages (caution and advisories) are cycle using the CANCEL and RECALL switches. Pressing the CANCEL switch removes all level B and level C messages presently displayed and causes the overflow level B and C messages to be displayed. Also, the page number is incremented. The level A messages are unaffected.

Pressing the CANCEL switch after the last page of messages is shown removes the caution and advisory messages, as well as the page number.

NOTE: The warning messages are unaffected.





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Figure 15.33



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Display Unit Failure and Parameter Exceedance

The examples shown are for a failure of the lower DU combined with a parameter exceedance. In these examples the advisory message, "EICAS DISPLAY" appears.

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On the left, the right engine' s oil pressure value has dropped into the yellow band which results in a yellow display readout. A further decrease beyond the red-line limit would cause the readout to become red in colour.


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The low oil pressure exceedance would also cause all oil parameters to appear automatically on the upper display in a digital readout, in a compact-partial format. The oil parameters are displayed for both engines to assist in problem evaluations.

On the right, the right engine' s N2 exceedance also caused the upper DU to switch to compact-partial format to permit the N2 exceedance value to be displayed in red. Again, both left and right N2 engine parameters are displayed.





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Figure 15.34 DU Failure & Parameters Exceedance



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) STANDBY ENGINE INDICATOR

The SEI provides primary engine information to the flight crew in the event of an EICAS failure which would result in a loss of the normal primary engine parameter displays- When operating, the SEI presents digital LCD displays of the primary engine parameters. These displays include Nl rotor speed,

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Exhaust Gas Temperature (EGT), and N2 rotor speed. The values shown on the right and left of the SEI displays are safe-limit decals.

Power Supplies and Internal Circuits

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Two independent power supplies are provided, each with a separate power source. The SEI retains full functional capability with either power supply failed.


The SEI also contains four input signal conditioning circuits, two cold junction compensation circuits, a microprocessor, two memories and BITE circuits.

Display Control Switch

This switch has two positions, "ON and AUTO". In the ON position, the unit is completely functional with the LCD displays permanently on. In the AUTO position, the unit is not functional except for the EICAS validity detection circuits. When both EICAS validity discretes are detected as invalid, (open) indicating a double EICAS DU failure, double EICAS computer failure or an EICAS Test, the SEI comes on and displays the primary engine parameters.

Self Test Switch

This is a three position rotary switch which is spring loaded to the centre position. Utilising a small screwdriver, it can be rotated to the right or left during a self test. In the right position, power is removed from the number one power supply. The number two power supply provides the power for the unit to perform the self test.

In the left position, power is removed from the number two power supply, and the number one power supply provides the self-test power. The self test can be performed with the display control switch in AUTO or ON position.





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Figure 15.35 Standby Engine Indicator



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.6 Fly-By-Wire

A considerable weight saving is achieved by replacing all the mechanical linkages (cables, rods, pulleys etc) by electrical wires and computers.

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The A320 has a fly-by-wire system where the pilot' s inputs are via a side stick positioned on his left side (the right-hand side for the second officer) and rudder pedals. The B777 is also fly-by-wire (with some fly-by-light) but has conventional flight deck controls (control wheels and rudder pedals). With both

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aircraft, when the pilot puts an input into the flying control system, a signal from the control (in or near the flight deck) provides an electrical signal to the flying control computers. These provide electrical analogue signals to electrically controlled Power Flying Control Units (PFCU’s) near the flying control


surface. Control position and aircraft attitude is feed back to the computer so it can monitor the aircraft' s movement.

The main advantages of fly-by-wire are: 1. Saving in weight.

2. Requires less maintenance. 3. More responsive.

4. Increased fuel economy

5. Built in protection systems.

A disadvantage is that the system is all electrical/electronic and lightning strikes could be a problem. The system has back-up facilities such a duplication and triplication of hardware and software. To overcome the possibility of complete electrical failure there is a mechanical standby mode. It gives limited flying control authority but if all else fails it can be used.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The A320 Fly-By-Wire System

The A320 aircraft flight controls consist of primary and secondary systems. The primary systems - ailerons, roll spoilers, elevators, trimmable horizontal stabiliser (tailplane) and rudder- control pitch, yaw and roll flight attitudes.

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The secondary systems comprise leading edge slats and trailing edge flaps for low speed flight handling, airbrakes/load alleviation spoilers for deceleration/load alleviation at all flight speeds and lift dumpers for deceleration after landing.


The A320 flight controls are hydraulically actuated and electrically or mechanically controlled.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Input signals for the various controls are:

Pitch control: Elevator

-

Electrical

Stabiliser

-

Electrical for normal or alternate control. Mechanical for manual trim control

-

Electrical

Roll control: Ailerons


Spoilers

-

Electrical

-

Mechanical (electrical for yaw damping, turn co-ordination and trim)

Yaw control: Rudder

Load alleviation spoilers:

-

Electrical

Slats/flaps control:

-

Electrical

Speed brakes control:

-

Electrical

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All surfaces are hydraulically actuated. Controls in the cockpit consist of two side sticks, conventional rudder pedals and pedestal mounted controls and indicators.





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Figure 15.36 A320 – Flying Controls



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Electrical control is achieved by three types of computer: •

ELAC (Elevator and Aileron Computers) – 2 These two computers achieve:

•

Aileron control.

-

Normal elevator and stabiliser control.

SEC (Spoilers and Elevator Computers) - 3 These three computers achieve:

•

-

-

Upper wing surfaces control.

-

Standby elevator and stabiliser control.

FAC (Flight Augmentation Computers) – 2


These two computers achieve rudder control.

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In addition the Flight Control Data Concentrator acquires data from the ELAC' s and SEC' s and sends this to ECAM (Electronic Centralised Aircraft Monitor -the flight deck screen displays and CFDS.)

The Electrical Flight Control System (EFCS) includes the ELACs, SECs, FCDCs and vertical accelerometers.





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Figure 15.37 A320 Fly By Wire – General Arrangement





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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The EFCS is designed according to the following principles:

a) Redundancy and dissimilarity. The EFCS includes two ELACs, three SECs, two FCDCs and four accelerometers. The ELACs and SECs are both able to achieve roll and pitch control

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of the aircraft. These two types of computer differ by their internal architecture, hardware, type of microprocessor, software, and are built by different manufacturers. For each computer type, the control and monitoring software are also different. By having the computers different in terms of hardware

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and software, but both designed to satisfy the same parameters, the possibility is remote that IF there is a fault in one the chances are almost zero that the same fault is in the other.


b) Monitoring

The monitoring of each computer is achieved as follows: •

Monitoring channel. Each computer consists of two physically and electrically separated channels, one being dedicated to the control functions, the other to the monitoring functions. These two channels perform the actuator command signal computation using different digital processes. The monitoring channel permanently compares the results of these computations and inhibits the signal to the actuator, should a discrepancy occur.

•

Self-monitoring capacity. Each channel is able to detect the failure of the critical signals it receives or it emits and to detect internal failures by testing of the processor and monitoring of its internal power supply.

•

Cross-talk. Each control and associated monitoring channel permanently exchange information via digital buses, therefore consolidating and validating information received from different sensors.

•

Automatic power-on and pressure-on safety tests performed without movement of the surfaces.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Pitch Control Pitch control is achieved by two elevators and the trimmable horizontal stabiliser (THS).

Elevators Two electrically controlled hydraulic servo jacks (PFCUs) are fitted on each elevator. Each servo jack has 3 control modes. •

Active

=

jack position is electrically controlled.

•

Damping

=

jack follows control surface movement.

•

Centring

=

jack is hydraulically maintained in neutral position.


In normal operation: -

one jack is in active mode,

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-

the other one is in damping mode.

-

Some manoeuvres (high load factors) may cause the second jack to become active.

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In case of failure of the active servo jack the damped one becomes active and the failed jack is automatically switched to the damping mode. If the servo jacks are not electrically supplied, they are automatically switched to the centering mode.

Stabiliser (tailplane)

Actuated by a screw jack driven by two hydraulic motors. These two hydraulic motors are in turn driven by one of three electric motors or the mechanical trim wheel. Mechanical control from the pitch trim hand wheel has priority over electrical control.





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Figure 15.38 The Elevator / Stabilizer System



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Roll Control Roll control is achieved by one aileron and four spoilers on each wing.

Aileron

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Two electrically controlled hydraulic servo jacks are fitted to each aileron. Each servo jack has two control modes, active and damping. Damping mode is automatically selected in case of hydraulic low pressure or electrical failure. When aileron demands come from the Load Alleviation Function (LAP)

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computer, the two jacks of each aileron become active. The ailerons are normally controlled from ELAC 1 in which case the left and right ailerons are driven by the blue and green hydraulic systems servo jacks respectively.


If a failure occurs in ELAC 1, the aileron control is automatically transferred to ELAC 2. In the event of blue or green hydraulic system low pressure, ELAC 2 takes over. Mechanical control from the pitch trim hand wheel has priority over electrical control of the affected aileron. In case of double ELAC failure, or blue and green hydraulic low pressure, all aileron servo jacks revert to damping mode (i.e. a zero aerodynamic hinge moment deflection).

Spoilers

Each surface is controlled by one servo control supplied from the green, yellow or blue hydraulic system, which is controlled from SEC 1, 2 or 3. Surfaces are automatically retracted to zero position when a fault is detected by the corresponding SEC, or when not electrically supplied. In case of loss of hydraulic pressure the surface remains at the existing position unless it is pushed down by the airflow.

Yaw Control

The rudder is powered by three independent hydraulic jacks which operate in parallel and are controlled from the flight deck via a conventional cable flying control system. Yaw mechanical control is by conventional manual rudder controls from the pilots rudder pedals and is always available.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Yaw Damping and Turn Co-ordination In flight, yaw damping and turn co-ordination functions are automatic. In normal operation the three hydraulic servo jacks are driven by a green hydraulic servo actuator controlled by FAC 1. A yellow servo actuator controlled by FAC 2 remains synchronised and will take over in case of failure. The yaw commands for turn co-ordination and yaw damping are computed by the ELACs and transmitted to the FACs. There is no feedback to the rudder pedals from yaw damping and turn co-ordination functions.

Yaw Alternate Law

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The alternate yaw damper becomes active if roll normal law fails. Only the yaw damping function is available and damper authority is limited to ±5° of rudder.


Rudder Trim

Rudder trim is achieved by two electric motors acting at the artificial feel unit. In normal operation FAC 1/MOTOR 1 are driving with FAC 2/MOTOR 2 remaining synchronised as back up.

Figure 15.39 The Rudder System



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Manual Flight The pilot can apply rudder trim at 1°/sec from the RUD TRIM rotary switch.

Automatic Flight Autopilot (A/P) commands (yaw auto trim) are applied.

Manual Control

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In manual control (roll normal law) asymmetry compensation computed by the ELACs and transmitted through the FACs is applied for fine trimming, after


the rudder has been approximately trimmed by the pilot. When the A/P is engaged, rudder trim orders are computed by the FAC and FMGC.

Mechanical Control

The two pairs of pedals are interconnected and drive cable system to the artificial feel unit which is linked via a differential to the hydraulic rudder actuator.

A/P Control

The trim actuator and yaw damper servo actuators are used to introduce the A/P signals. The trim actuator drives the mechanical control (and pedals) through the artificial feel. In this mode the artificial feel break-out force is increased by a solenoid actuated device (100N - about 10 kg - although kg is strictly a mass and not a force).

The ELAC and SEC also feed two computers, the FCDC. These monitor and analyse ELAC and SEC maintenance messages at power on (on ground), inflight and after touch down. It stores the data and delivers failure indications, e.g. stored failures, failure history, trouble shooting as well as failed LRUs to the Centralised Fault Display Interface Unit for onward transmission to the Maintenance Control Display Unit.





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Figure 15.40 Flight Control System – General Arrangement



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The B777 Fly-By-Wire System

The Boeing 777, like the A320, has a highly integrated flying control system. Unlike the A320 it uses conventional flight-deck controls. Signalling is via ARINC 629 data buses, and various computers and control units. The control surfaces are hydraulically powered via PCUs.

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The flight deck controls consist of a control column for control of the elevators with a hand wheel for control of the ailerons, flaperons, and roll spoilers. The

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rudder bar controls the rudder. These controls are provided with artificial feel and back-drive motors to move them in the correct sense when the system is in auto-pilot mode. An aileron trim actuator is also fitted in the system.


The ARINC 629 data bus is a twisted pair of wires transmitting data in both directions to all computers/LRUs (Line Replaceable Units). Each computer/LRU is connected to the bus by untwisting the twisted pair locally and clamping on an Inductive Couple Unit (which does not cut the insulation of the bus). In operation, each computer listens to the bus and waits for a quiet period before it transmits. It then waits its turn until all the other computers/LRUs have transmitted before transmitting again (the system of listening and transmitting is called "protocol").

Transmitting and receiving is carried out on the same bus. Any computer/LRU can listen to any data on the bus and receive the data according to how its personality PROM (Programmable Read Only Memory) is programmed. In other words the computer' s permanent memory knows what information on the data bus is for its use.

Having recognised that the information on the bus at that instant is to be "read", it will take it in, put it in temporary store (RAM) and act on it according to its pre-programmed instructions.

The flight control system uses 3 buses (LEFT, CENTRE and RIGHT).





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Figure 15.41 Flight Controls ARINC 629 Data Bus System Boeing 777



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Flight deck control movement is converted into an electrical analogue signal by transducers (XDCRs) fitted to the flying control system under the flight deck floor. This signal is then sent to the Actuator Control Electronics LRU (ACE).

The pilot' s controls are connected via the Actuator Control Electronics (ACE) unit to the PCU. Other units such as the Primary Flight Computer (PFC) are

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connected into the system by the ARINC 629 bus. The drawing below shows a block schematic for the ailerons but the rudder and elevators are similar in principle. The hand-wheels are connected electronically to the ailerons and flaperons, and mechanically by cable to some of the spoilers.

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The flight deck control is connected to position and force transducers which signals the pilot' s intention by an analogue signal to the ACE. This is in two


way communication via the data bus with the PFC. An analogue command signal is sent to the PCU to move the ailerons in the desired direction. Positional feed-back is sent to the ACE which controls the range and speed of movement of the PCU - and hence the control surface.

The controls have artificial feel to simulate air loads on the control surfaces, and trimming is achieved by biasing the system neutral by a trim unit actuator. Should one side jam then the other side can be operated independently by overcoming the force limiters. Should the spoiler control cables become jammed then system operation is assured by the shear-out action of the cable pulley.

The flight control system can operate in three modes (1) Normal Mode (2) Secondary Mode- the same as Normal but some of the protection devices such as bank angle protection do not work. (3) Direct Mode where some of the computers are not used. Direct Mode is selectable by the pilot or automatically selected by the system.





Figure 15.42 Aileron/Flaperon Flight Deck Linkage - Boeing 777


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Figure 15.43 Flight Control System – Normal Operation Boeing 777


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Autopilot When auto-pilot is engaged the back-drive actuator will move the pilots controls in response to auto-pilot commands. When-ever the auto-pilot is engaged the back-drive actuators are active. This system gives the same effect as some mechanical systems, ie when auto-pilot commands a control surface movement the flight deck controls move in the same sense (correct direction).

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This simulates a conventional mechanical system - which most pilots are conversant with, thus being more reassuring to the flight crew. When auto-pilot is

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selected the PCU is controlled by the ACE, PFC and Auto-pilot Flight Director Computer (AFDC) via the bus. The AFDC will also send an analogue signal to the back-drive actuator to move the flight deck controls to correspond to control surface movement. Thus the system simulates closely the


characteristics of a conventional mechanical flying control system.

Fault Finding

Besides checking the AMM fault location section there is an on-board fault finding computer which is accessed via the Maintenance Access Terminal (MAT) on the flight deck.

The MAT comprises: •

A display screen

•

Keyboard

•

Trackball - similar to a mouse - controls the cursor

•

Selection switch

A Portable MAT (PMAT) can be used which is plugged into the system at various points on the aircraft.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) To use the MAT proceed as follows:

1. Check the log book for any recorded defects. 2. Check any warning flags and/or displays in the flight deck (electric power on). These are called Flight Deck Effects (FDEs). 3. Check for any displays on the EICAS CRTs. (EICAS = Engine Indicating and Crew Alerting System) (elect power on). 4. At the MAT terminal on the flight deck select either: •

INBOUND FDEs

•

LINE MAINTENANCE FDEs

•

FAULT HISTORY


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5. Under each FDE there will be a maintenance message. Select MAINTENANCE MESSAGE DATA and the recommended maintenance action will be displayed.

Some tests can be carried out using the MAT. These can cause the control surfaces to move as well as the flight deck controls, so it is important to ensure that they are free to move with no obstructions, and warning notices displayed.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.7 Flight Management Systems (FMS)

The FMS is a combined flight control, guidance and navigation system. It provides a single focal point (for area navigation functions and performance management functions) which enables the crew to select, activate and modify a three dimensional route structure from data stored internally and transmits

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steering and thrust commands to automatically fly the selected route and displays data for visual monitoring of current dynamic conditions referenced to the route.

The FMS is comprised of five major subsystems.

1.


Digital Flight Control System (DFCS) •

Two Flight Control Computers (FCC)

•

Mode Control Panel (MCP)

2.

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Auto-throttle (A/T) System •

Auto-throttle computer (ATC)

•

Two servo actuators

3.

Flight Management Computer System (FMCS) •

Flight Management Computer (FMC)

•

Two Control and Display Units (CDU)

•

Thrust Mode Annunciator (TMA)



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) •

Stored within the FMC is: Operational program, navigation database (used to define route selection and contains airports, procedures, waypoints, navaids etc) and performance database. This portion of the internal data is inputted and updated by a portable data base loader and connector in the flight deck area.

4.

Radio Navigation System / Inertial Reference System (IRS) / Global Positioning System (GPS)

5.

Electronic Flight Instrument System (EFIS)


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Figure 15.44 FMS Schematic



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The CDU provides the interface between the crew and the FMC, and data exchange is provided by ARINC 429 busses. Sequence of Operation (assuming flight plan entered)

Take off

Pilot pushes TO/GA button, the thrust levers automatically advance to N1. After take-off at the appropriate altitude the auto

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pilot is engaged. When stabilised on the climb, LNAV and VNAV are engaged. The flight profile, as entered in the FMC, will then be flown to automatically.

Climb


Level off

Smooth transition to level flight is accomplished automatically in accordance with the flight plan. The FMC adjusts airspeed and continually adjusts for changing conditions to optimise performance.

Cruise

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The FMC can provide complete auto-pilot and auto-throttle control during climb.

The FMC continuously computes recommended altitude, the aircraft follows the pre-set route and the FMC automatically tunes into the navigational aids as required.

Descent

Because descending too early or too late can have a significant impact on fuel economy, the FMC computes the optimum start of descent point.

Approach

The auto-pilot system provides guidance using localiser and glideslope signals to provide a system that is capable of landing in category 3 weather conditions.

Go- Around

Go-around guidance initiated by TO/GA button on throttle, if selected prior to touchdown - throttle will advance and the auto-

pilot will command heading or pitch attitude.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Transition to each control mode is made automatically by the FMC at each change in the flight phase. Figure 15.45 shows a typical flight profile and figure 15.46 shows the controls of the CDU.

f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 15.45 Typical Flight Scenario


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Figure 15.46 FMS CDU


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The following CDU Controls and Features are used in BITE. CRT

-

14 lines, 24 characters per line. Scratch pad on bottom line.

LINE SELECT KEY

-

6 on each side of CRT. Identified as 1L (No 1 LEFT), 2L, (LSK) 3R, 4R, etc.

BRT

-

CRT Brilliance Control Knob.

EXEC key

-

EXECUTE key, used to start certain test procedures

PREV PAGE key NEXT PAGE key NUMERIC

-

(such as ½, 2/2 or 1/3, 2/3, 3/3, etc.)

-

Used to select Access Codes to request engineering BITE pages

-

Used to select certain words or letters in testing routines


CHARACTER keys ALPHABETIC

CHARACTER keys

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Used as paging control for multiple page sequence displays as determined by the presence of multiple page numbers

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Select one of the INIT/REF pages (POS INIT, POS REF, etc.), which then allows LSK 6L – INDEX selection of the

INIT.REF key

-

INIT/REF 1/1 page, so that the maintenance BITE pages may be selected (LSK 6L). This also allows de-selection of any BITE procedure

CLR key

-

Push and hold to clear all data or message on the scratch pad. Single push and release clears the last characters only

FAIL Annunciator Light

-

Indicates FMC failure. A signal is sent to give a level C warning on the EICAS. This output is called a DISCREET signal Indicates an FMC generated message on the scratch pad, This signal (discreet) is set whenever an ALERT message is

MSG Annunciator Light

-

shown in the CDU scratchpad. It causes an EICAS message and turns on the FMC annunciator amber light in the flight deck.

The entire 24 character spaces of the fourteenth line of the display are used as a scratch pad block for entry of data from the keyboard.

All messages which appear on the scratch pad can be cleared by using the CLR key of the CDU. Messages are normally accompanied by illumination of the MSG annunciator at the right side of the CDU panel. The annunciator is extinguished when the message is cleared from the scratch pad. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Figures 15.47 and 15.48 shows the power up sequence and how access to BITE index is obtained.

Information to be entered by the crew is displayed in box prompts. Information displayed as default information is displayed as dash prompts.

f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 15.47 FMC Power Up & Access to BITE

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Figure 15.48 BIT Index

‘IDENT’ page display on system power-up



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.8 Global Positioning Systems (GPS)

Introduction

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In recent years, satellite navigation systems have revolutionized the concept of instrument navigation. Developed as military systems by the United States and the Soviet Union, coupled with the computer systems which have developed at the same time, they allow much more accurate and available position fixing for aircrew. ICAO refers to them as ' global navigation satellite systems'or ' GNSS' .


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The United States system, called ' Global Positioning System' , or GPS, is the most commonly available. However, the Soviet, now Russian, ' Glonass' system is also commercially available with suitable receiver equipment. Receivers have been produced which can accept signals from both systems, and other countries are considering developing new systems either on their own or in conjunction with others.

Principles of Operation

Basic Principle

Basically, all satellite navigation systems use the same principle as DME. The receivers measure the time it takes for a radio signal to travel from a transmitter in a satellite at a known point in space. Knowing the time, and the speed of propagation of the waves, a distance can be calculated by the receiver' s computer. The receiver then measures the time for a signal from another satellite at a known position to reach it and calculates that distance. The possible positions from the first satellite can be plotted as a sphere, centred on the satellite, whose radius is the calculated distance. The same applies to the distance from the second satellite, and where the two spheres touch provide the possible positions of the receiver. The two spheres touch along a circle in space, as in figure 15.49.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) A third satellite can provide a further sphere. The possible positions lie on the circle joining spheres 1 and 2, that joining 1 and 3, and also that joining 2 and 3. These three circles meet at only two points in space. One of these is so far removed from the earth that the other must be the correct position of the receiver. The computer in the receiver makes all these calculations and displays the result as a three-dimensional point in space related to fixed features on the earth, commonly the lines of latitude and longitude for horizontal, and the centre of the earth for vertical, reference.

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Even with only two satellites providing ranges, an observer at a known altitude could feed that altitude into his receiver' s computer. That altitude is another

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sphere in space, so where the two spheres from the satellites touch that altitude sphere is his position. There are of course two possible positions, but if the observer can give his receiver a rough position it will discard the obvious false one. This facility has limited use in aviation, because fortunately there are


enough satellites to provide sufficient spheres for three-dimensional position fixing over the whole of the earth for most of the time.

Figure 15.49 Position Spheres



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Detailed Principle

This simplistic view of the principles of operation has skated over the massive problems of providing the positions of the satellites and the time of transmission of the signals. In order to provide cover over the whole world, the satellites must orbit at an angle to the equator, and so must move quickly in

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relation to the earth. The transmission from every satellite includes the orbit that each satellite is following, and its position in the orbit, as an ' almanac' . The receiving computer uses that almanac to calculate where each satellite should be in its orbit, so it knows which ones will be in view and what approximate

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range to expect from each one. This allows faster acquisition of each satellite' s signal, and selection of the most appropriate satellites for accurate fixing. In fact, the ranges as received at the receivers are not exactly accurate.


They are called ' pseudo-ranges' , and more calculations must be made to measure the true range from each satellite. The satellite transmits its own exact position and path, called the ' ephemeris' , to provide the exact spot from which the ranges must be measured. To provide correct timing, each satellite carries a very accurate atomic clock. It also transmits, as part of the ephemeris, the corrections which its own clock requires to bring it into line with UTC. To reduce cost, the clocks in the receivers are less accurate. However, by using four satellite signals to compute its position; a receiver will find that the calculated positions between each group of three do not match, as in the complex diagram at figure 15.50. The error is in the timing, and the receiver computer makes changes to its clock to bring all the computed solutions together. When that happens, its clock is in agreement with those in the satellites. It can then measure true ranges from the satellites instead of pseudo-ranges.

These ranges can be plotted (or computed) to find the receiver' s position in relation to the centre of the earth, as Cartesian x, y, and z co-ordinates from that central point. However, navigators traditionally need to know their position in relation to the surface of the earth. The earth is not a sphere, in fact not even a regular spheroid, so the Cartesian positions are compared using mathematical formulae to a theoretical ellipsoid computer model which fairly closely resembles the actual surface of the earth. This allows the receiver' s three-dimensional position to be presented in latitude, longitude, and altitude relative to that ellipsoid. The satellites are supported by ground stations, which receive signals from all the satellites in turn as they pass overhead. The ground stations monitor the signals, and amend the almanac and ephemeris messages transmitted from the satellites. For Training Purposes Only




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Figure 15.50 Position Fixing



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) GPS

Space Segment

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The American Global Positioning System (GPS), also called NAVSTAR, uses 24 satellites, with three or more spares, in orbit at anyone time. The satellites ' fly'in six different orbits, 20,200km (10,900 nm) above the earth' s surface, taking 12 hours for each orbit. The orbits are at 550 to the earth' s spin axis, to

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give the best combination of good coverage, constantly varying geometry, and ease of orbital insertion. All these satellites transmit navigation information on the same frequencies consecutively, in one of 40 time segments or PRNs (pseudo-random noise numbers), but each sends a different code at the start


of its transmission. The satellites are usually referred to by the PRN which they use.

At the time of writing, there are two frequencies used, both in the ' L'radar band of the UHF band, but a third frequency is expected to be available for civilian use from 2015. Information between satellites is transmitted on a different frequency, and an S band link carries information from the ground stations (control segment). Because the satellites are moving, the Doppler Effect, changes the received frequencies. The receiver computers compensate by altering their receiver frequencies, using the satellite ephemeris to make the calculations.

The first frequency (L1), transmitted at 1575.42 MHz, carries the ' coarse acquisition'or ' CA'code, which can be received and interpreted by all receivers. Information carried by the CA code provides the ' standard positioning service' , or SPS. The frequency also carries, at 900 phase difference, a ' precision guidance' , or ' P'code, which can only be interpreted by military receivers. This ' P'code is also carried on a second frequency (L2) of 1227.6 MHz, providing the ' precise positioning service'or PPS. The ' P'code can also be ' encrypted'to prevent unauthorized receivers from using it, and when that happens it is referred to as ' Y'code. The third civilian frequency will be called L5, and will also carry the CA code. Each satellite carries four clocks, which can be compared internally with each other to provide an average time with a high probability of accuracy. Each one is capable of nanosecond (10-9 second) accuracy. However, if one clock is noticeably different from the rest, the satellite can discard that and average For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) the other three. The satellites also carry solar panels and batteries to provide power for their transmitters and receivers, and a system of spinning reaction wheels to keep them pointing at the earth.

GPS satellites have improved progressively. They must be replaced periodically, about every 10 years. Block IIR satellites can provide navigational data without uploads from the ground segment for 180 days, as against the 14 days for the original satellites. Control Segment


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The control segment consists of the ground stations. The master station does all the calculations, and the other ground stations provide communications between the satellites and the master station. The master station with its extremely accurate atomic clock compares each satellite' s average clock time with its own. It then transmits all the clock errors for retransmission by every satellite as part of its ephemeris message. The master station can also transmit necessary corrections to the satellite' s on-board computer.

If any satellite is transmitting false information, or is in the wrong orbit, the master station tells every satellite that that particular one is unserviceable. Each satellite' s transmissions contain information about its own and every other' s health, clock corrections, and ephemeris, so the unserviceablity will be retransmitted in that message. However, it may take several orbits for the unserviceable satellite to fly over the master station after its failure, and the other satellites also have to pass over the master for the information to reach them.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) User Segment (aircraft equipment)

The GPS receiver is a simple device, requiring little more than a small screened aerial mounted on the skin of the aircraft. Hand-held devices are capable of showing position to previously unimagined accuracy. The computers and their associated software are the complex parts of the aircraft equipment.

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Computers can calculate actual track and groundspeed made good, and required track and ETA to intended waypoints. Given inputs from flight instruments or an air data computer, the GPS computers can make all navigational calculations. Data loaded into the computer software can display instructions to follow procedures, and direct the aircraft clear of restricted airspace.


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A receiver which has no knowledge of the positions of the satellites must download a full set of almanac information before it can receive navigation signals. When first switched on, the receiver will search for any signals on the correct frequencies. Once the almanac message is received from any satellite, the receiver must load its own computer with the information. Once it has received the almanac from one satellite, the receiver selects one satellite whose signal it should be able to receive. It generates its own version of the pseudo-random code being transmitted by that satellite, and searches for the same code at approximately the correct time. Once it has matched the codes, it ' locks on'to that signal. It then looks for signals from the other satellites it expects to receive.

The receiver itself can be one of three categories; multi-channel, multiplex, or sequential. Sequential receivers lock on to one satellite at a time, measure the pseudo-range, then search'and lock on to the next. This is of little use in aviation, where the receiver is moving quickly. Multi-channel receivers are expensive, because they have to receive the signals from all the satellites in view independently and simultaneously, but are the preferred type for aviation. Multiplex receivers ' time share'their receiving and computing time between all the satellites in view. Because the satellites send their information at different times in the short transmission sequence (different PRNs), a multiplex receiver can store the information from each while it is receiving information from the others. Once the receiver has ' locked on'to a particular satellite, the computer can process the information from it during the time it would normally be attempting to find its signal. When another satellite' s signal is due, the computer stores the information from the first to work on it again later in the cycle. It therefore progressively builds up its navigation information. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Whatever system the receiver uses, the time between first switching it on to being able to compute an accurate position, the ' time to first fix' , is quite long. figures of between 10 and 20 mm are quoted for different equipments. This time can be reduced if the receiver knows its own position either from having been switched off there, or having the correct information put into it by the operator. It can be reduced even more if the almanac which it last used is still current, so that it can select the satellites without waiting to download the new almanac.

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Once continuous navigation signals have been received from four satellites, the receiver computer' s first task is to synchronise its own clock (actually a

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quartz oscillator), by computing fixes from each group of three and comparing them, as explained earlier. This eliminates its own clock error, and allows a final accurate position to be calculated. Several aircraft clocks are available with GPS inputs to show this accurate time to the pilot.


Pilot interface with some GPS instruments is by means of a numeric keypad, but those designed for light aircraft tend to use a menu system. A full computer or alpha-numeric keyboard controls the flight management system of airliners, and is used to interface with the GPS equipment. Functions accessible allow loading of waypoints, often in three dimensions, arranging flight profiles by listing the desired waypoints in the order required, and the ' goto'or ' direct'function, which with one keystroke gives directions to any waypoint selected. ' Go-to'is an invaluable function, giving instant directions for diversion if necessary. However, it must be used with care, as without an up-to-date database loaded, it may lead pilots through restricted airspace or controlled airspace without clearance.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Figure 15.51 shows a typical GPS control and display unit (CDU) available for light aircraft, showing the ' go-to'key marked with a D with an arrow through it, and a button marked ' NRST'to select the nearest aerodrome from the database (for ease of diversion calculations in an emergency). The information on the screen includes the destination on this leg (the mode selected), with distance to go and groundspeed. It also shows a pictorial representation of the flight path, with the boundaries of controlled or restricted airspace to the right of the track. Listed below the screen are the menus available, controlled by the knob on the right and the cursor (CRSR) switch.

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Figure 15.52 shows a more sophisticated CDU with a colour screen which has been developed for business aircraft. The database includes a map of the ground below the aircraft' s track, complete with contours and current aviation database. Although this is a stand-alone CDU primarily dedicated to GPS,


inputs from other equipment, such as lightning detection can be displayed on the same screen, and a similar ' moving map'can be displayed on an EFIS screen with inputs from an area navigation system. The control at the top right of the screen is a ' joystick'for controlling the cursor and scrolling through the menus. •

Position dilution of position (PDOP) is the total effect of all the errors in three dimensions, latitude, longitude, and altitude.

•

Horizontal dilution of precision (HDOP) is the total effect in the earth' s horizontal plane (distance)

•

Vertical dilution of precision (VDOP) is the total effect in the earth' s vertical plane (altitude)

•

Time dilution of precision (TDOP) is the effect the errors have on the accuracy of the receiver' s clock

•

User equivalent range error (UERE) is the effect the errors have on the accuracy of calculating the range to one satellite




f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 15.51 Light Aircraft GPS Unit

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Figure 15.52 Colour GPS Display



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Selective Availability The ' CA'code provides accuracy in the region of 30 m. However, the US military considered that accuracy too great for use by possibly unfriendly nations. They have the facility to place an irregular (but known to them) time delay in the transmissions. This is called ' selective availability' , and historically reduced the 95% accuracy of a GPS fix to 100m. When GPS was originally made available for civilian use, selective availability was permanently in force, but in

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May 2000 it was discontinued. The US government has, however, reserved the right to reimpose it for national security, and also to switch off or even jam the signals if it so wishes.

Satellite Position Error


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The orbit of each satellite, and its position in that orbit, is affected by gravity. The gravitational pull comes not only from the earth, but also from the moon, the sun, and the planets. All of these are predictable, and computer calculations to correct these have all been made, but other objects may also have an effect. Other satellites, space debris, and even passing asteroids will all affect the ephemeris to some small degree. Minor malfunctions in the satellites themselves may produce changes in either the orbit or the clocks. As mentioned before, there is a delay before the master ground station can correct, or broadcast the errors in, satellites'ephemeris. Receivers may therefore be calculating positions from incorrect data.

Satellite Clock Error

The accuracy of the satellite clocks has been mentioned above. However, even the tiny errors which the satellite clocks suffer, despite their regular updates from the master station, can affect the calculated position by about 1.5 metres.

NOTAMS

If a satellite is taken out of service, for example for servicing, a GPS NOTAM will be issued. That will refer to the particular satellite by its pseudo-random noise (PRN) number, then give the date and times between which it is unusable. During these times, pilots should ' de-select'the particular satellite from their navigation computers if that function is available, in order to minimise time wasted by the equipment searching for a non-existent or useless signal.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Ionospheric Refraction UHF signals are not normally regarded as being refracted by the ionosphere, but such accuracy is required that even the very small amount of refraction they suffer increases the time taken for the signal to reach the receiver as it bends through a shallow angle. This can be seen in figure 15.53. The total distance covered by the signal along the solid line in the figure is greater than the correct path shown by the dashed line. It is especially noticeable when

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the signals pass through the layers of the ionosphere at a shallow angle, and is less of a problem when the satellite is overhead the receiver. The signal is also subject to attenuation as it passes through the layer of ions, which is effectively thicker and therefore has more effect at a shallow angle.

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The combined error is called the ' ionospheric group delay'which when combined with the delay from other satellites produces a total position error in the


order of five metres. In fact, ionospheric group delay is inversely proportional to the square of the frequency, so if the receiver receives two frequencies, as the military ones do, the computer can calculate the correct ranges from the two different ones received.

Figure 15.53 Ionospheric Group Delay



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Tropospheric Refraction Refraction and attenuation also takes place in the troposphere, producing a ' tropospheric group delay' . However, this is small and regarded as an acceptable error for most applications.

Solar Disturbances

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The ionosphere and the earth' s magnetic field protect the surface, and the troposphere, from damaging radiation. However, satellites fly well above the

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earth' s surface and away from that protection. The ' solar wind'carries that radiation, which includes electromagnetic disturbances and particles, as a stream of energy which can cause ' perturbations'to the satellites'orbits, and in extreme cases cause damage to the satellites themselves. Work is being


carried out to detect changes in the solar wind before they reach the earth, by positioning warning satellites in orbit between the earth and the sun.

Receiver Measurement Errors

Like any man-made objects, the receivers have small inaccuracies in their construction which give rise to errors in the position calculated. These are very small, in the order of 0.3 metres.

Geometric Dilution of Position (GDOP)

Geometry requires that the satellites used for position calculation must be in different parts of the sky. The spheres of constant range described earlier must cut each other ' cleanly' , at an angle approximating to an ideal 60 Receiver computers will use the almanac to decide which are the most advantageous satellites, and use the signals from those only. However, at certain times it may not be possible to find enough satellites giving that clean cut, perhaps because of unserviceablity or maintenance. GDOP has been known to produce a large error for about 20 mm in anyone day.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Multi-path errors UHF signals are subject to reflection from many surfaces, and a reflected signal will arrive at the receiver later than the direct wave. Usually, the reflected signal will be weaker than the direct wave and the receiver computer can reject it, but where the direct wave is ' masked'by some object in its direct path, the reflected signal may be accepted as the true signal, giving a range error and therefore a position error. Receiver antenna design can reduce multi-path

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errors by discarding signals which arrive from unusual angles. For example, any signal received from the ground beneath the aircraft, or from the direction of the aircraft' s fin structure, should be disregarded. On average, multi-path signals produce a final position error of about 0.6 metres.

Interference


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The GPS signals, being broadcast from such a high altitude, are very weak when received. The signal to noise ratio is actually less than one, so the receiver has to identify the weak signal within the noise by its characteristics. As the electromagnetic spectrum becomes more crowded, harmonic interference from other transmissions, perhaps satellite communications transmissions from a nearby transmitter or the aircraft itself, may be relatively strong enough to cause interference. Deliberate interference can be produced in the form of ' jamming'from quite a weak transmitter close to the target receiver, and such jamming equipment has been made available for sale commercially in recent years.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) GPS Accuracy and Integrity

Accuracy The CA signal (unless subject to selective availability) is assumed to be accurate in the horizontal plane to within 30 m for 95% of the time. It is considered

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accurate to within 300m for 99.99% of the time. Because the receiver uses satellites which are high in the sky, the computed vertical position is less accurate. The 95% position should be within 156 m (500 ft) vertically, and the 99.99% position within 500 m (1600 ft).

Carrier Wave Measurement


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Very small changes in position, in the order of centimetres, can be measured by comparing the phase changes of the signal carrier waves. If the satellites were geostationary (in a fixed position above a point on the earth), the changes in phase would equate to movement on the surface of the earth, but the moving satellites require major calculations from the satellite ephemeris to compare the actual phase changes with the changes to be expected from the satellites'orbits. This method is used successfully in survey work. However, at the time of writing, carrier wave measurement is not in use for aviation purposes, although it has been suggested that it could become a source for velocity calculations in the future.

RAIM

Because of the possible errors in the system, regulating authorities demand that, before it can be used as a primary navigation aid, a GPS receiver must be able to detect when its information is unreliable. In other words, it must monitor its own ' integrity' . Using more satellites, this can be achieved. Four signals allow timing corrections, and five signals allow the receiver to detect that one of the signals is incorrect. This is known as ' remote autonomous integrity monitoring, or RAIM. Six signals allow the receiver computer to use a ' failure detection and exclusion, or FDE algorithm to determine which the incorrect signal is and discard it. A satellite system used for ' stand-alone'navigation, and especially precision approach guidance, must be capable of monitoring its signals well enough to prevent errors affecting its accuracy. It must also warn the crew of any errors which might affect that accuracy. To do that, it needs to know when the required six satellites will be ' in view'or available for reception.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The system therefore must predict that availability, and warn the crew in advance of any time when insufficient signals will be available. This is referred to as predicted RAIM (P-RAIM). It is feasible that the receiver computer can use barometric altitude information to act as one of the positioning spheres (in fact an ellipsoid) to allow RAIM with only four satellite signals, or FDE with five.


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PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.9 Inertial Reference Systems (IRS)

Principles of the Strap-down Inertial Navigation System (INS)

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Inertial navigation is the process of determining an aircraft' s location using internal sensors rather than external references (such as satellites). A

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microprocessor calculates velocity, position, and attitude from the inertial sensor' s acceleration rate and angular rate measurements. Three accelerometers and three gyros are required because an aircraft can simultaneously accelerate longitudinally, laterally and vertically, and rotate about the same three


orthogonal axes - pitching, rolling, and yawing.

In a strapdown inertial reference system the gyros and accelerometers are mounted rigidly in the system chassis which is in turn mounted to the aircraft. There are no gimbals to keep the sensors level with the surface of the earth. The accelerometers are mounted such that the axis of one accelerometer is always along the longitudinal aircraft axis, one is along ' the lateral axis, and one is along the vertical axis. Likewise, the gyros are mounted such that one gyro senses roll, one senses pitch, and the other senses yaw. The accelerometer produces an output that is proportional to the acceleration applied along the sensor' s input axis. A simple accelerometer is a mass mounted so that it is free to move along one axis, but constrained by a spring.

Newton' s second law can be written as: F = ma

where F = force in N, m = mass in kg a = acceleration

As the aircraft accelerates so the mass, because of it' s inertia, will not want to move and be deflected against the spring which is fixed. This means that the amount of deflection of the spring is a measure of the amount of acceleration of the aircraft. If the mass is constant, which it is, then F

a. In other words,

the more the aircraft accelerates the more force is produced. So we have a measure of acceleration (which we do not really want) so how can we get speed and distance (which we do want)? For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) A Mathematical Aside

If we wanted to know the speed of a car for example, then provided we knew the distance travelled (d) and the time taken (s) then we could find the velocity (v).

e.g. Distance travelled 30 miles (48km). Time taken 1 hour. Velocity 30 miles per hour (48 km/h).

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If the car was accelerating and we knew the velocity (V1) at a particular time (T1) and recorded the new velocity (V2) at time (T2) then we can work out the


acceleration.

That' s all very well but what good does that do us here? Well, if we can work out the acceleration just knowing the time and the distance then we should be able to do the calculations in reverse and work out time and distance from acceleration. If we know the distance and we can tell the computer where we started from (latitude and longitude) and then we will know where we are anywhere around the globe. The process of calculating distance from acceleration is called INTEGRATION. If acceleration is integrated once the result is velocity, if it is integrated a second time the result is distance.

The process can be done by hand using pen and paper but a computer can do the calculation many thousands of times a second giving an accurate continuous readout of aircraft speed (in mph). and distance travelled, and latitude and longitude readout. The microprocessor integrates the acceleration signal to calculate a velocity. Integration is a function that can be viewed as a multiplication by time. For example, a vehicle accelerating at three feed per second squared (3ft/s2) (0.9m/s2) would be travelling at a velocity of 30 feet per second (30ft/s) (9.1m/s) after 10 seconds if it started from rest.

Although it is used to calculate velocity and position, acceleration is meaningless to the system without additional information. For example, consider an accelerometer strapped down to the longitudinal axis of the aircraft and measuring a forward acceleration. Is the aircraft accelerating north, south, east, west, up or down? In order to navigate around the world, the system must know how this aircraft acceleration is related to the earth' s surface. For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Because accelerations are measured by accelerometers that are mounted to the lateral, longitudinal, and vertical axes of the aircraft, the IRS must know the relationship of each of these axes to the surface of the earth. The laser gyros in a strapdown system make the measurements necessary to describe this relationship in terms of pitch, roll and heading angles.

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These angles are calculated from the angular rates measured by gyros though an integration - similar to the manner in which velocity is calculated from acceleration. Given the knowledge of pitch, roll, and heading that the gyros provide, the microprocessor resolves the acceleration signals into earth-related accelerations, and then performs the horizontal and vertical navigation calculations.


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Under normal flight conditions, all six sensors sense motion simultaneously and continuously, thereby entailing calculations that are substantially complex. A powerful, high-speed microprocessor is required in the IRS in order to rapidly and accurately handle this additional complexity.

Gravity

Vertical velocity and altitude are calculated using the acceleration that is measured perpendicular to the earth' s surface. However, an inertia! accelerometer cannot distinguish between gravitational force and actual aircraft acceleration.

Consequently, any accelerometer that is not perfectly parallel to the earth' s surface will measure a component of the earth' s gravity in addition to the true aircraft acceleration. Therefore, the IRS microprocessor must subtract the estimated local gravity from the measured vertical acceleration signal. This prevents the system from interpreting gravitational force as upward aircraft acceleration.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Earth' s Rotation

As discussed previously, the purpose of the gyros is to measure rotational motion of the aircraft with respect to the earth. However, the laser gyro in a strapdown configuration inherently measures movement of the aircraft with respect to inertial space (a sort of fixed place in space).

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Another way of looking at this is that the gyros measure the motion of the aircraft with respect to the earth, plus the motion of the earth with respect to

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inertial space. The earth rotates with respect to inertial space at a rate of one rotation per 24 hours as it spins from west to east on its own axis, plus one rotation per year as it revolves around the sun. The sum of these two rates is equivalent to an angular rate of 15.04° per hour. The microprocessor


compensates for this rate by subtracting this value, which is stored in memory, from the signal measured by whichever gyro or gyros are pointed eastward.

Without this "earth rate" compensation, an IRS operating at the equator would mistakenly think that it is upside down after 12 hours of navigation. At other places on the earth, the system would develop similar errors in pitch, roll and heading.

Earth' s Spherical Shape

The major effect imposed by the earth' s spherical shape is somewhat similar to that caused by the earth' s rotation. As an aircraft travels across the surface of the earth, its path becomes an arc due to the shape of the earth. Consequently, the gyros - particularly the pitch axis gyro - measure a rotational rate, because travelling in a curved path always involves rotation.

This rate, called the transport rate is being measured by the gyros, and subtracts that value from their measurements. There are many other effects that are compensated for in commercial inertial reference systems, and even more in systems used in military applications. These effects have not been considered in this book as they get more complicated the higher the precision.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Inertial Reference Unit

The Inertial Reference Unit (IRU) is the heart of the IRS. It provides all the required inertial reference outputs for the aircraft avionics. The outputs are: ♦

Primary attitude - pitch, roll.

♦

Heading - true, magnetic.

♦

Body linear accelerations - lateral, longitudinal, normal.

♦

Body angular rates - pitch, roll, yaw.

♦

Inertial velocity - N-S, E-W, ground speed, track angle, vertical rate.

♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦

f o g y n r i r a t e e e i n r i p g o n r P SE A M Navigation position - latitude longitude, inertial altitude, Wind data - wind speed, wind angle, drift angle.

Calculated data - flight path angle and acceleration, along track and cross track acceleration. Flight management computer.

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Flight control computer.

Thrust management computer. Stability augmentation system. Weather radar.

Anti-skid and auto-brake systems. Attitude Director Indicator (ADI).

Horizontal.Situation Indicator (HSI). Vertical Speed Indicator (VSI).

Radio direction magnetic indication. Flight Data Recorder (FDR).





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Figure 15.54 IRS Interface with Other Systems



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The primary sources of information for the IRU are its own internal sensors -three laser gyros, and three inertial accelerometers. The only other inputs required are initial position, barometric altitude, and true airspeed. Initial position is required because present position is calculated from the distance and direction travelled from the starting point.

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Barometric altitude stabilises the vertical navigation and thereby stabilises the vertical velocity and inertial altitude outputs. The true airspeed input allows the IRU to calculate wind speed and wind direction.

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In the IRS the principle is exactly the same as in the INS. Accelerations are sensed by the accelerometers and the values are integrated to provide velocity


and distance data. The signal is obtained from the accelerometer by measuring the amount of force required to re-centre the mass. This being proportional to the acceleration (force balance). The signal is integrated once to give velocity and a second time to give distance. Providing starting position was entered then present position can be calculated, exactly like the INS.

So in general, the gyro' s sense the attitude of the aircraft and pass this information to the microprocessor to allow it to adjust the accelerometers output. The accelerometer output needs to be adjusted as it is hard mounted on the aircraft, it will be sensing other acceleration effects other than horizontal accelerations. The compensated output from the three accelerometers (triple-axis) are vectorially added to determine the actual direction of travel and distance travelled.

When the system has to go through an alignment procedure the alignment determines the local vertical and the direction of true north. The accelerometers are sensing accelerations due to the earth' s gravity at this time, but as these are always perpendicular to the earth' s surface then the microprocessor can use this to establish the local vertical.




f o g y n r i r a t e e e i n r i p g o n r P SE A M Figure 15.55 Single Axis Computation


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Figure 15.56 Triple Axis Computation


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Accelerometers

Since the accelerometers are the heart of the inertial navigation system it should come as no surprise that they use inertia to measure acceleration. There are a number of different types of accelerometer, but the one most commonly used in aircraft inertial systems is the pendulous force balance type. If a

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freely suspended pendulum is subjected to acceleration it will lag, due to inertia, in a direction opposite to that of the acceleration. The accelerometer incorporates a pendulous mass that is constrained so that it responds only to acceleration or deceleration along its sensitive axis.

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In the example illustrated in figure 15.57 the pendulous mass is suspended by two light leaf springs between two pick-off transformers. Ferrite armatures


are attached to the ends of the mass and the transformer primary coils are supplied with low voltage alternating current at a frequency of about 12 kHz. Whilst the accelerometer is not subjected to acceleration, the suspended mass is positioned exactly midway between the two transformers. Under this condition the secondary voltage is identical in both transformers and there is consequently no current flow in the secondary circuit.

When the accelerometer is subjected to an acceleration or deceleration along the axis of the pendulous mass, the mass is deflected by inertia, reducing the gap between one armature and its transformer and increasing the gap at the other end. This causes the voltages induced in the transformer secondary windings to be unbalanced, with a resultant current flow between them. This current flow is amplified and fed back to the force coil surrounding the pendulous mass, creating an electro-magnetic field around the mass. This field reacts with the field produced by the permanent magnets on either side of the mass, to return the mass to its mid, or null, position. The secondary current required to achieve this is directly proportional to the acceleration that caused the deflection and it therefore serves as the output signal from the accelerometer.

As previously stated, the purpose of the gyro-stabilised platform is to provide a mounting for the accelerometers that is earth horizontal at all times and that remains aligned with true north. The system uses rate integrating gyroscopes to sense platform horizontality and alignment. The platform is mounted on gimbals which are controlled by pick-off signals from the rate integrating gyros. Servo motors drive the gimbals to keep the platform level and aligned irrespective of aircraft manoeuvres. For Training Purposes Only




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Figure 15.57 Accelerometer Principle of Operation



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Let us assume that the aircraft is stationary on the ground and accept for the moment that the system will maintain the platform both level and aligned. If the pilot now taxies the aircraft away from its stand, still on a northerly heading, the north-south accelerometer will sense the northerly acceleration as the aircraft moves away and the integrators will convert this to speed and distance for the computer and its cockpit display. The east-west accelerometer senses no acceleration and so the system continuously computes distance traveled north from the starting point.

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On arrival at the eastern end of the east-west runway (very convenient airfield, this), the pilot turns the aircraft left onto a heading of 2700T. The frame of

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the stable platform, being attached to the airframe, will have turned through 900 with the aircraft, as illustrated in plan view in figure 15.58.


Any tendency of the stable platform to turn away from its north-south alignment will be immediately sensed by the azimuth gyro, which will precess and send an error signal to the azimuth servo motor. The platform is therefore turned relative to the airframe to maintain northsouth alignment. The amount by which it is turned gives the change of aircraft heading for the INS computer.

As the aircraft accelerates along the east-west runway on the take-oft roll, the acceleration is sensed by the east-west accelerometer and integrated to show increasing velocity and distance traveled westward. The aircraft nose is pitched up and this nose-up pitch is maintained during the climb, which for simplicity we will assume continues on a westerly heading. Any tendency of the stable platform to tilt in pitch will be immediately sensed by the north gyro, since on this heading the pitch axis is coincident with the north-south axis. The north gyro will send an error signal to the pitch servo motor to correct the tilt until the north gyro signal is nullified.





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Figure 15.58 Gyro-Stabilised Platform – Aircraft on Westerly Heading



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Finally in this example, let us assume that the aircraft levels out at the top of the climb at constant speed (Le. no acceleration) and turns left onto a heading of 225°T. During the turn the aircraft will, of course, bank and any tilting of the stable platform will be sensed by both north and east gyros, which will signal the pitch and roll servo motors to maintain the platform level. North alignment during the turn is again maintained by the azimuth gyro and azimuth servo motor, turning the platform relative to the airframe and generating the heading change for the INS computer.

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Whenever the aircraft is on a non-cardinal heading, control of platform levelling is shared by the north and east gyros and the pitch and roll servo motors.

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North-south alignment is at all times maintained by the azimuth gyro and its servo motor. The situation is illustrated in plan view in figure 15.59.





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Figure 15.59 Gyro Stabilised Platform – Aircraft on Non-Cardinal Heading



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Position Calculation

The ultimate function of an inertial navigation system is to provide the pilot with navigational data such as track, groundspeed and present position in terms of latitude and longitude. The block diagram in figure 15.60 illustrates how this is achieved.


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Figure 15.60 INS Calculations



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Track and Speed

The track calculated by the INS is a great circle track. The speed calculated at any instant is groundspeed, because the INS bases its calculations upon accelerations of the aircraft over the surface of the earth.

Latitude and longitude

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Distance travelled north-south in nautical miles converts directly to change of latitude, since each nautical mile along a meridian equates to one minute of


latitude. Distance travelled eastwest is known as departure and is calculated using the equation:

Departure E-W (nm) =change of longitude xcosine latitude

Since, in the case of the INS computation, departure is known the calculation carried out by the INS computer to determine change of longitude is therefore:

Change of longitude = departure (nm) x secant latitude



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) INS Self-Alignment

The gyro-stabilising platform is self-levelling and self-aligning, but these functions can only normally be carried out in non-military aircraft with the aircraft stationary on the ground. In order to compute the speed, distance travelled and position of the aircraft the INS must first be referenced to north at the

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current aircraft position, which is fed into the computer by the pilot from airfield information. The process of self-alignment is performed in two distinct phases; first the platform must be levelled so that the accelerometers are not influenced by gravity and then it must be aligned with true north so that they only sense horizontal accelerations on the northsouth and east-west axes.


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Levelling and alignment are initiated by switching the INS to STANDBY and inserting the present position in terms of latitude and longitude, and then selecting ALIGN mode.

Levelling

Conventional systems reduce the time taken to level the platform by using gravity to achieve coarse levelling, followed by fine levelling using the accelerometers.

Coarse leveling

During coarse levelling the pitch and roll gimbals are typically driven by the servo motors until they are mutually perpendicular and the platform is brought to within about 10 of level using either gravity or reference to the airframe.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Fine leveling With the aircraft stationary and the platform level there should be no output from the accelerometers. If there is an output from either or both accelerometers this indicates that the platform is not level and that acceleration due to gravity is being sensed. The output signals from the accelerometers are used to torque the north and east gyros, which in turn use the pitch and roll servo motors to drive the platform about the pitch and roll axes until output from the accelerometers is zero.

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However, it will be appreciated that a platform levelled to earth horizontal using spatial referenced gyroscopes will not remain horizontal over an earth that is rotating. From figure 15.61 it will be seen that a level platform above the poles would suffer drift about the vertical axis at earth rate (15.040/hr), and one


at the equator would suffer topple about the north-south axis at the same rate. At intermediate latitudes the platform would drift and topple at a rate dependent upon the latitude. To correct for this the north gyro is biased for topple at 15.04 x cos lat/hr and the azimuth gyro is biased for drift at 15.04 x sin lat/hr.

Consequently, the platform will only be level when the output of the east-west accelerometer is zero and the output of the north-south accelerometer is also zero with the correct bias applied. The INS computer is able to apply the correct bias, because current latitude is inserted before the levelling procedure begins. Fine levelling typically brings the platform to within 6 seconds of arc of earth horizontal and takes about 1 ½ minutes.





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Figure 15.61 Effect of Each Rotation on Stable Platform



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Gyro-compassing

The process of aligning the platform with the local meridian is usually referred to as gyro-compassing or azimuth alignment and it is the final stage of the alignment procedure. Most modern north-referenced gyro-stabilised platforms use a system called open loop gyro-compassing. Since both earth rate and

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the local latitude are known, any misalignment between the platform north-south axis and the local meridian can be calculated. When the inertial navigation system is switched from the alignment mode to the navigation mode the platform is rotated by the azimuth servo motor through the computed misalignment angle.


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Many earlier systems use what is called closed loop gyro-compassing to achieve the same result. In this, any gravitational acceleration due to tilt of the platform sensed by the north accelerometer after fine levelling results in an output signal that is used to torque the azimuth gyro until the signal is nullified, which will only occur when the platform north-south axis is aligned with the local meridian. During this process the fine levelling process remains operative.

With either system the gyro-compassing phase takes between 6 and 10 minutes and typically achieves an alignment accuracy of about 6 minutes of arc. The time taken for self-alignment and the accuracy achieved increases with latitude. Close to the poles it becomes impossible to align a north-referenced platform; in UK latitudes self-alignment usually takes between 10 and 15 minutes.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Mode Selector Panel

The various modes of operation of the stable platform INS are selected by means of a simple panel.

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STANDBY (STBY) is the mode selected to switch the system on. In this mode power is supplied to the system and the gyros are warmed and spun up to operating speed. Whilst this is taking place it is usual to insert the current position to the nearest 6 seconds of arc. ALIGN selects the alignment mode,

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during which the levelling and alignment procedure described above takes place. When this is completed the READY NAV annunciator illuminates, indicating that the INS is ready for use.


NAV is the navigation mode, used throughout the period of ground movement and flight and during which the INS will make all its navigation calculations and display them as required on the control and display unit, to be described shortly.

ATT is the attitude reference mode and is only used when the INS computer fails to provide its navigational information. In this mode the stable platform is used to provide heading, pitch and roll information.

The INS has its own internal battery, which is capable of supplying power to the system for a limited period, typically about 15 minutes, in the event of loss of the normal power supply. The BATT annunciator will illuminate red when the battery power falls to a predetermined level, warning the pilot that the INS is about to fail.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Control and Display Unit (CDU)

The unit comprises an alphanumeric light-emitting diode (LED) display, a rotary selector switch and a keyboard for the insertion of data. In addition, there is the facility to set up a route by inserting waypoint latitudes and longitudes at which track changes are to be made. Waypoints are identified in numerical

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sequence using the selector wheel, starting with waypoint 1 as the departure airfield. Each waypoint is entered using the keyboard to set its latitude in the left alphanumeric display and its longitude in the right alphanumeric display. Insertion is made by pressing the INSERT button. If a change of track is authorised en-route, say direct from waypoint 5 to waypoint 8, this is made using the TK CHG pushbutton.


CDU selections

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The displays associated with the various rotary display selector switch positions are listed and explained in the following paragraphs:

TK / GS (track and groundspeed)

The INS computed track, usually referenced to magnetic north, is displayed to the nearest tenth of a degree in the left display and the groundspeed in knots in the right display. For example, a current track of 1350M and a groundspeed of 467 knots would appear as 135.00 and 0467.

HDG / DA (heading and drift angle)

The heading obtained from the angle between the platform frame and north reference is displayed to the nearest tenth of a degree in the left display. The angular difference between heading and track (drift angle) is displayed to the nearest tenth of a degree in the right display, preceded by the letter R or L to indicate whether drift is right or left. Thus, a heading of 1370M on a track of 135°M would be presented as 137.00 and L 02.00.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) XTK / TKE (cross track distance and track error angle) Cross track distance is the distance by which the aircraft is displaced right or left of the desired great circle track and is displayed in the left display to the nearest tenth of a nautical mile. The track error angle is the angular difference, right or left, between the desired great circle track and the actual track being made, to the nearest tenth of a degree. If the aircraft were displaced 1% nm to the left of the desired track of 135°M, the left display would read L 01.5. If the track being made good happened to be 1300 M, the right display would read L005.0°.

POS (present position)

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The aircraft' s current latitude and longitude are shown to the nearest 6 minutes of arc in the left and right displays, respectively. Suppose it happens to be


at 51.15.7N, 04.23.6W, this would appear as 51 °15.7' N in the left display and 04°23.6' W in the right display.

WPT (waypoint positions)

The position of each inserted waypoint is shown as latitude in the left display and longitude in the right display by selecting WPT on the rotary selector switch and scrolling through the waypoint numbers with the waypoint selector wheel.

DIS / TIME (distance and time to the next waypoint)

The distance in nautical miles from the present position to the next waypoint is shown in the left display and the time at present groundspeed to the nearest tenth of a minute in the right display.

WIND (wind speed)

The INS is able to compute wind direction and speed and these are displayed in the left and right displays, respectively, to the nearest degree of arc and knot.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) DSR TK / STS (desired track and status) The great circle track from one waypoint to the next changes as the aircraft progresses between the two and the INS computes the present desired magnetic track based upon distance from the waypoints, magnetic variation and the assumption that the aircraft is on track. This will appear in the left display to the nearest tenth of a degree and the right display will be blank. The status function is for use only whilst the INS is in ALIGN mode and it shows

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a numerical display in the right window that indicates the status of the alignment procedure. The display typically shows 99 at the start of alignment and counts down to 0, when alignment is completed and READY NAV is illuminated.

Other CDU controls


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It will be noted from figure 15.62 that there are additional controls and annunciators and their functions are as follows: •

The waypoint selection controls are situated immediately below the left LED display. A thumbwheel is rotated to select the number of a waypoint that is to be inserted or amended. An LED display indicates the current from/ to situation; the illustration depicts the display that would appear when flying between waypoints 2 and 3. The track change (TK CHG) push button is used when altering the pre-planned sequence of waypoints. Conventionally waypoint 0 is the aircraft' s current position. Suppose ATC has cleared you to fly direct to, say, waypoint 5 then the TK CHG button is pressed until 0-5 appears in the display. The DIM thumbwheel adjusts the brightness of the LED displays.

•

The AUTO/MAN/RMT rotary switch is used to select the type of flight control to be used in flying from waypoint to waypoint. In AUTO the INS will automatically change the from/to display as each waypoint is overflown and would normally be used in conjunction with automatic flight. In MAN (manual) the pilot is required to enter the from/ to display as each waypoint is reached. The RMT (remote) position is used when two or more INS are fitted and enables the waypoint information to be transferred from one INS to the other(s).

•

The three annunciators situated below the right LED display serve to draw attention to specific events. The ALERT annunciator illuminates amber as the aircraft approaches the next waypoint, typically when about 2 minutes short of it, and will continue to flash until either cancelled by the pilot or, when in AUTO mode, by over-flying the waypoint. The BATT (battery) annunciator will illuminate amber when the INS is operating on battery



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) power, reminding the pilot that the system will only operate for a limited time on internal power. The WARN annunciator illuminates red in the event of a system failure. At the same time the status display will show a number that cross refers in the system manual to the nature of the failure.


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Figure 15.62 INS Control and Display Unit



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.10 Traffic Alert Collision Avoidance System (TCAS)

Introduction

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It became obvious that if secondary surveillance radar could be used to provide information to a ground station about the aircraft carrying a transponder, then it might be possible to provide the same information to an airborne station. The vast increase in computer power over the years since SSR came into

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general use has given that idea reality. It has also allowed systems to be developed which can process the information received from these transponders and give directions to an aircraft fitted with one of them to prevent it colliding with the transponding aircraft.


The generic term for all these systems is ' Airborne Collision Avoidance System'or ACAS. This includes the systems developed in the United States called ' Traffic Alert and Collision Avoidance System'or TCAS. TCAS is at the time of writing the only system which provides the requirements of ACAS. ICAO defines the requirements for ACAS in Annex 10, and that term will be used throughout for their required equipment. Examination questions can be expected to follow the same principle. References to TCAS apply to available equipment; references to ACAS apply to the requirements.

Principle of Operation

Basically, the airborne equipment consists of an interrogator (transmitter) and receiver on SSR frequencies (1030 MHz and 1090 MHz respectively). The transmitter need not have great power, because a pilot is unlikely to be interested in traffic at long ranges. For the same reason, it requires a short PRF and can sweep quite quickly. The system sends an SSR Mode ' S'interrogation (which includes interrogations in Modes ' A'and ' C' ) and receives the code from the other aircraft. It uses directional antennas to determine the relative direction of the transponded signal, and calculates the range from the time between transmission and reception (less the normal transponder fixed delay).



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) The equipment sends out interrogation pulses and awaits replies. When a signal from a Mode ‘A'transponder is received, the direction can be determined and range calculated. The information on range and bearing can be displayed on a screen for the pilot, and if the range is below a certain ' trigger distance' an audible and/or visual warning can alert him to the presence, direction and range of the other aircraft. He can then looks in that direction and take any avoiding action he deems necessary.

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The information from a Mode ' C'transponder can be decoded, and the transponding aircraft' s relative altitude displayed on an instrument. Alerts need not

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be given about traffic which may be conflicting in plan but have adequate vertical separation, although the display will normally still show its position. A system which can display this information and give ' traffic advisories (TAs)'like this is called a TCAS I or ACAS I system. ACAS I gives approximately 40


seconds warning of a possible collision.

Despite being superseded by ACAS II for international air transport operations, ACAS I equipment is simpler and cheaper to install, and provides TAs which a pilot can use to direct his eyes towards a possible threat. This has been realised as a distinct benefit to aircraft operating under Visual Flight Rules. Helicopters especially have a technical difficulty in using TCAS II equipment, and lightweight ACAS I systems have been developed to warn pilots of approaching aircraft. While tracking the transponding aircraft it calculates not only the range and relative altitude, but also the rate of change of both. The processor can then calculate the time to, and the separation at, the closest point between the two aircraft. If the closest point is likely to be within a certain distance, it generates a TA to advise the crew of an aircraft of that fact. It will ignore signals from aircraft outside 10000 ft of its own altitude.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) ACAS I

ACAS systems are an extension of the SSR transponder. They have a sophisticated computer processor which tracks the relative position of the aircraft. If the ' intruder'continues to close, the TA symbol changes once the intruder is within 6nm horizontally and 1,200 ft vertically. This traffic is defined as ' proximate' , although there may be no direct threat of collision.

ACAS II


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ACAS II requires the system to provide avoiding action if appropriate. If the computer calculates that the approaching traffic is a potential collision threat, it then calculates the best method of avoiding the confliction, an ' escape manoeuvre' . Because the directional antennas are not particularly accurate, the only safe escape manoeuvres are in the vertical plane, so the directions are to climb or descend at nominated rates. These directions are referred to as ' Resolution Advisories'or ' RAs' .

A RA will give quite exact directions to avoid the collision. A climb or descent if advised should be entered and continued at set rates, as indicated below. The advisory information will continue, and the advice will change during the manoeuvre, until the confliction is past. If two aircraft equipped with ACAS II appear to be conflicting with each other, their Mode ' S'datalinks can ' talk'to each other, passing data in their Mode ' S'transmissions. This will coordinate their RAs to optimise their separation and minimise disruption. ACAS II equipment can track up to 45 aircraft, and display up to 30 of them. It can coordinate RAs to avoid up to 3 conflicting aircraft.

If an aircraft' s ACAS II equipment generates a RA, its Mode ' S'transponder can inform the ground interrogator of the manoeuvre automatically. Otherwise the crew must inform ATC of any deviation from the flight path which ATC have allocated it. Pilots are expected to make altitude changes to follow RAs, provided they tell ATC immediately, using standard phraseology.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Below 1000 ft above ground level, the ACAS II system will not generate any RAs. Nor will an ACAS aircraft be given a RA to descend below 1,000 ft agl. Any intruder which has an apparent vertical speed above 10,000 ft/min up or down will not generate a RA.

ACAS III


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ICAO has issued more advanced specifications for a future system to be called ACAS III. This will use the Mode ' S'transponded signal from the other aircraft, with the intention of providing horizontal RAs. However, at the time of writing that lies in the future.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) ACAS II Details

Air Traffic Control Requirements

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The European Civil Aviation Conference (ECAC) states decided that by 1 January 2000, every fixed wing aircraft with turbine engines and more than 30 seats in their area must carry a serviceable TCAS II system with software version 7. The same applies to any aircraft with a maximum certificated takeoff

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mass greater than 15000kg. In addition, all UK registered aircraft in those mass or capacity brackets must carry it wherever they are in the world. Although the commercially available TCAS II system was intended to fulfill ACAS II requirements, version 7 was the first to comply fully with the ICAO ACAS II


requirements. Because of equipment availability problems, the JAA allowed certain temporary exemptions. However, the requirement will be extended to include all aircraft over 5700 kg or with more than 19 passenger seats from 1 January 2005.

ACAS II Functions

According to ICAO Annex 10, ACAS II is designed to have the following functions: •

surveillance and generation of traffic advisories

•

detection of collision threats and generation of resolution advisories to avoid them

•

co-ordination with other suitably equipped aircraft

•

communication with ground stations



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) ACAS II Inputs

To function properly, ACAS II requires the following inputs: •

aircraft address code

•

air-air Mode ' S'transmissions received by the Mode ' S'transponder

•

own aircraft' s maximum cruising true airspeed capability

•

pressure altitude

•

radio altitude


Intruder's Requirements

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The main problem with ACAS is that it can only detect and alert to aircraft which have operating transponders. It can only deconflict with aircraft which have operating and serviceable altitude reporting (Mode ' C' ) transponders. It is therefore important that all aircraft, even those not receiving a radar service, have their transponders switched on in both Mode ' A'and Mode ' C' .



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Airborne Equipment

Antennas ACAS II uses two antennas, one above and one below the aircraft. The top antenna is a direction finding one, as is often the bottom one. Several

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installations may use an omnidirectional receiver antenna on the bottom surface, but no directional information is then available if the transponded signal is not received by the top antenna. These antennas are kept separate from the normal SSR antenna.

Control Unit


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A typical TCAS control unit is similar to an SSR controller, with the additional functions of selecting either Traffic Alert or both Traffic Alert and Resolution Advisory functions. A TCAS/ Mode ' S'transponder control unit is shown below, in figure 15.63. The VFR function allows an automatic change in transponder code to a preselected VFR code (7000 in the UK).

Figure 15.63 TCAS Control Unit



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Displays There are four different symbols which may appear on the TCAS II cockpit display. These appear in a position on the display relating to the relative position of the other aircraft, although if the system cannot determine the bearing of that aircraft it will show the relevant symbol in a convenient position with the words ' no bearing' . Manufacturers claim a bearing accuracy of 20.

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The symbol displayed is selected by the equipment depending on the intruder' s position and closing rate. A TA which is not a threat will appear initially as an open white (or sometimes blue) diamond on the display. This means it is more than 6 nm or more than 1,200 ft vertically (if a Mode ' c'signal is received)

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away from your aircraft. A solid diamond indicates that the other traffic is within that safe zone (Le. ' proximate' ), but the computer calculates it is still not a threat. If the computer calculates that the intruder is potentially hazardous, the symbol will change to a solid yellow circle. A voice TA will be given of ' Traffic


Traffic' . About 10-15 seconds later, if the intruder is assessed as an actual collision threat, the symbol will change to a red square, and a voice command will give an indication of the computed RA, which will be displayed exactly on the cockpit display. If the intruder is transponding with Mode ' C' , the symbol will have beside it the relative altitude in hundreds of feet. If the relative altitude is changing by more than 500 ft/min an arrow will also appear. Examples of the symbols are given in figure 15.64 below.


Figure 15.64 TCAS II Symbols Issue 2 Revision 0 Oct 2011 Page 471

PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Aural Advisory Messages

TCAS II has a vocabulary of phrases which it uses to pass its advisory commands to the pilot. These are listed in Table 15.6 with the actions which the pilot must carry out to avoid the confliction.


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Table 15.6



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Pilot' s Actions

The system is designed for the aircraft to follow the exact avoiding action within 5 seconds of an initial command and 2-3 seconds for subsequent commands. The required maneuvers are not meant to be violent; a maximum of 0.25g is intended. If the commands are followed exactly, including the

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' softening'commands issued to reduce rates of climb or descent after the initial command, the system is designed to restrict altitude changes to a maximum of 500 ft. If an ACAS RA is given when an aircraft is under ATC control, the pilot is required to follow the command and inform ATC ' ACAS climb'

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or ' ACAS descent'as appropriate. Once the danger is past, on receipt of the message ' clear of conflict' , he must return to his cleared level as soon as practicable.


Monitoring System

A system of monitoring both the ACAS transmission power and the necessary inputs is provided. This monitoring also covers any failure in the equipment which would adversely affect the interrogator, warns the crew, and sends a Mode ' S'message to ground interrogators.

ACAS II Errors

It is possible, but progressively more unlikely, for false RAs to be generated by other transmissions on the transponder frequency. These ' parrots'from ships or parked aircraft (those without the Mode ' S'facility of switching the transponder off on the ground) can be minimised by switching the system to a ' TA only'mode at low level. The same action can avoid RAs being given from aircraft on parallel runway approaches.



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Light Sensitive Warning System

Realising the benefits of collision warning systems, but attempting to reduce cost for light aircraft, a system was developed to detect not a radio signal, but high intensity lights, such as are carried by many aircraft as anti-collision beacons. The strobe warning systems has light detectors, similar to solar

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electricity generating panels, arranged to give coverage around the planform of the aircraft, usually one on each wingtip and one on the tail. The simple display consists of lights at compass points around a representation of the aircraft, which illuminate to indicate the approximate direction of the light which

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has been detected. No indication of distance can be given, and the range depends on atmospheric conditions, such as dust haze and water droplets in cloud or precipitation.





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5.15.11 Integrated Modular Avionics (IMA) Typical Arrangement (Airbus A380) Thanks to the new avionics concept Integrated Modular Avionics (IMA), most of the conventional LRU functions are done by avionics applications. These independent applications are hosted in shared IMA modules, called Core Processing Input/Output Modules (CPIOMs). As a consequence, IMA concept reduces the maintenance cost due to less spare computers.

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Each CPIOM integrates new hardware and software technologies and hosts these independent applications on the same computing resource, and also supplies an Input/Output interface service to some of the conventional avionics. Moreover, in order to satisfy the high demand of conventional avionics, this service capability has been increased thanks to additional IMA modules called Input/Output Modules (IOMs). CPIOMs and IOMs are Line Replaceable Modules (LRMs). These LRMs dialogue through the Avionics Data Communication Network (ADCN) supported by a communication technology developed from a non-aeronautical standard, which has been adapted to aviation constraints. This technology is called Avionics Full DupleX Switched Ethernet (AFDX).



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Figure 15.65 IMA – Typical Arrangement




Core Processing Input/Output Modules (CPIOM) The CPIOM integrates shared memory and computing resource to execute independently its hosted avionics applications. In addition the CPIOM processes independently specific input/output data for each application. This data is AFDX data when the applications dialogue through ADCN and nonAFDX data when they dialogue directly with conventional LRUs. There are 7 types of CPIOM, each one identified by a letter (A to G). Each type is associated to a specific part number. Within a given type, all CPIOMs are interchangeable but may require a software reconfiguration.


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Figure 15.66 CPIOM



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Input/Output Modules (IOM) The IOM does not host avionics applications. The IOM converts non-AFDX data, coming from conventional LRUs, into AFDX data used within the Avionics Data Communication Network (ADCN) and vice versa. There is only one type of IOM identified by the letter A. All IOMs are fully interchangeable.


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Figure 15.67 IOM



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.12 Cabin Systems 5.15.12.1 General The CIDS (Cabin Intercommunication Data System) is the cabin core system. It fulfills an easy interface with some aircraft and cabin support systems for the cabin crew, the passengers, the maintenance personnel and the cockpit crew. The CIDS lets them accomplish the functional control, the testing and the monitoring of these systems through four different types of function related to: - Communication, - Indicating, - Control, - Programming.


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Figure 15.68 CIDS



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.12.2 System Architecture (Airbus A380) The CIDS is designed in a modular way, that means the number of installed components will be adapted to the cabin layout and functional requirements. The general CIDS system architecture is based on a controller, bus lines and network concept. Within this concept the role of the controllers is taken over by the CIDS directors. Directors

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For redundancy reasons, the CIDS is equipped with 3 identical directors, wired in parallel, which are the heart of the system. One director is active, the two others are in hot standby. The active director controls, operates and monitors passenger and cabin crew related functions as well as the cabin support systems. For that, the active director exchanges data with them through an onboard CIDS network or directly. Moreover the 3 directors are connected to the cockpit controls and indicating and to dedicated control panels (FAPs and optional MINI-FAPs) to give interactivity to cockpit and cabin crew. They are also connected to some A/C systems to make an automatic activation of some of the CIDS functions.


Flight Attendant Panels (FAPs)

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The FAPs let the cabin and maintenance crew control and monitor in the whole cabin; the various cabin support systems and, the passenger and cabin crew related functions. In baseline configuration, there are two independent touch-screen Flight Attendant Panels (FAPs) installed: one in the Upper Deck and one in the Main Deck. However the number of FAPs can be enlarged up to 10 FAPs. Decoder/Encoder Units type A (DEUs A)

The interface between the active director and the passenger related functions, is ensured via Decoder/Encoder Units type A (DEUs A). The CIDS uses each DEU A to control the cabin lighting and all the Passenger Service Units (PSUs) functions (PAX individual lighting, PAX signs and calls and the loudspeakers). In baseline configuration, there are 85 DEUs A installed in the whole cabin (a maximum of 192 can be installed). DEUs B

The interface between the active director and the cabin crew related functions, is ensured via Decoder/Encoder Units type B (DEUs B). The CIDS uses each DEU B to control the Area Call Panels (ACPs), the Attendant Indication Panels (AIPs), the optional Additional Attendant Panels (AAPs) and the handsets. In baseline configuration, there are 21 DEUs B installed in the whole cabin (a maximum of 72 can be installed).



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) CIDS Cabin Crew Related Equipment The ACPs are used as a far call facility to inform cabin attendants of a PAX or interphone call, of a lavatory smoke or of an EVAC signaling. They are mainly located on the cabin ceiling above aisles. The AIPs display dial and call information from Passenger Address (PA), interphone and PAX. They display also additional cabin systems information like the LAV smoke location.The optional AAPs let the attendants control certain cabin support systems and the passenger related functions in a specific cabin zone. The optional AAPs and AIPs are mainly located near the attendant stations. Each attendant station is also equipped with a handset for passenger address and interphone functions.


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Figure 15.69 System Architecture



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.12.3 Communication Functions The CIDS system ensures several communication functions, which are: - Passenger Address - Cabin interphone - Service interphone - Crew signalling and alerting Passenger Address (PA)

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The Passenger Address (PA) is one of the main functions of the CIDS. It supplies one way voice communication to perform announcements from the cockpit or from a cabin crew station to the passengers. These announcements are initiated from the cockpit using either a handset or acoustic devices and from the cabin using cabin crew stations handset. They are then broadcast to the passengers either through passengers headset via In Flight entertainment (IFE) or through all cabin loudspeakers. A PA announcement can be direct or prerecorded voice information and it can also be used to supply prerecorded music.


Cabin Interphone

The cabin interphone system is used for the telephone communication between all cabin crew stations or between the cockpit and the cabin crew stations. From the cockpit the communication is established via the cockpit handset or via any acoustic device. From the cabin the communication is established via any cabin crew station handset. Service Interphone

The service interphone system is used for the telephone communications, on ground only, between the service interphone jacks or between the service interphone jacks, the cockpit and the cabin crew stations. The service interphone jacks are located within the service areas. Crew Signaling and Alerting

There are different kind of cockpit and cabin crew signaling and alerting functions depending on the situation. The area ready function lets the cabin crew inform the purser that a dedicated cabin area is ready for takeoff/landing. This function is activated thanks to the Flight Attendant Panels (FAPs) and the optional MINI-FAPs. The cabin ready signaling function lets the purser inform the cockpit crew that all the cabin areas are ready for takeoff/landing. This function is activated thanks to the FAPs. The sterile cockpit lets the cockpit crew inform the purser that they do not want to be disturbed. This function is activated through a sterile cockpit P/B. The optional Emergency Crew Alerting System (ECAS) is used to indicate a possible violent incident against For Training Purposes Only


PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) passengers, cabin or cockpit crew members. This function is activated from the cabin thanks to a cabin alert P/B and from the cockpit via a cockpit alert P/B.


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Figure 15.70 CIDS Communication



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.12.4 Indicating Functions The CIDS system fulfils several indicating functions related to: - Smoke Detection Function (SDF), - Emergency lighting power supply, - Ice protection and control, - Trolley lift, - Electrical load management, - Galley cooling, - IFE and seat power, - Doors/slides, - Vacuum System Control Function (VSCF). SDF


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The directors Smoke Detection Function (SDF) receives information directly from the fire protection system to monitor the smoke detectors in the cargo and in the avionics compartments. This information is also used to monitor the fire extinguishing system condition. Moreover the SDF receives also information from the fire protection system through DEUs B to display the status of the cabin smoke detectors through the FAPs and the optional MINIFAPs. In all cases, when a smoke is detected or after a successful ignition of extinguishing agent the SDF reports to the Flight Warning System (FWS). Emergency Lighting Power Supply

The CIDS directors fulfil via DEUs B an interface with the emergency lighting system for failure indicating through FWS and for testing. Ice protection and Control

The CIDS directors have an interface with the potable and waste water ice protection system via DEUs B to report through FAPs failures related to the protection against freezing of the potable water/waste system and failures related to the floor panel heating as well. In addition this interface is used for the floor panel temperature display and selection through the FAPs and the optional MINI-FAPs. Trolley Lift

The CIDS directors fulfil via DEUs B an interface with the trolley lift system to report related failures through the FAPs.





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Figure 15.71 Indicating Functions



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Electrical Load Management Indication The CIDS directors fulfil via Aircraft Data Communication Network (ADCN) an interface with the electrical load management application to indicate the shedding status of some cabin support systems (such as air conditioning, IFE or light systems) on the FAPs. Galley Cooling

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The CIDS directors fulfil via ADCN an interface with the Supplemental Cooling System (SCS) to display the galley cooling system status and its related parameters through the FAPs. IFE and Seat Power


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The CIDS directors fulfil via the ADCN an interface with the secondary power distribution system to display through the FAPs, the IFE and the seat energizing status. In addition, this function is used to control: - the FAP initiated power switching, - the IFE power switching and, - the seat power switching. Doors/Slides

The CIDS directors fulfil via ADCN an interface with the Door and Slide Management System (DSMS) to indicate through the FAPs, the doors and the slides status. VSCF

The CIDS directors Vacuum System Control Function (VSCF) has an interface with the water/waste system to display through the FAPs the monitoring of the potable and waste water tanks filling level.





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Figure 15.71 Indicating Functions



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.12.5 Control Functions The CIDS system fulfils several control functions related to: - Cabin and passenger lighting, - EVAC, - Lighted signs, - PAX call, - IFE, - Air conditioning, - VSCF. Cabin and Passenger Lighting


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The CIDS controls the cabin general lighting and the passenger reading lights independently in each cabin zone, deck and room. Centralized control commands are entered via the FAPs, the optional AAPs and the optional MINI-FAPs. In addition, for the passenger reading light, individual control commands are entered via the PSUs and the IFE. EVAC

The CIDS Emergency eVACuation signaling (EVAC) function controls the evacuation signaling in all cabin areas and in the cockpit. In case of emergency A/C evacuation, the appropriate signaling can be activated either from the cockpit (via the EVAC panel) or from the cabin (via FAPs, optional AAPs or optional MINI-FAPs). Lighted Signs

The CIDS lighted signs function controls directly the illumination of the exit signs or, via DEUs A, the illumination of the No Smoking (NS), or the optional Portable Electronic Devices (PED), the Fasten Seat Belts (FSB) and the Return to Seat (RTS) signs. In addition it controls, via DEUs B, the illumination of the lavatory occupied signs. These signs can be manually activated from the cockpit signs panel or automatically according to A/C systems data. Pax Call

The CIDS passenger call function is activated from the passenger seats (via IFE) and from the lavatories and is reset from the attendant stations via FAPs, optional AAPs or optional MINI-FAPs. Once activated, this function supplies the illumination of the passenger call indications (via the ACPs and the AIPs) and the broadcasting of passenger call chimes in the cabin. For Training Purposes Only




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Figure 15.72 Control Functions



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) IFE The CIDS exchanges with the IFE, control commands doing CIDS PAX Call and reading lights operation from PAX seats and potential IFE operation from FAP. In addition the CIDS exchanges with the IFE, control commands and audio signals, for broadcasting PA announcements at the PAX seats or IFE video related audio signals in the cabin. Air Conditioning

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The CIDS has an interface with the air conditioning system via the ADCN to remotely control the cabin temperature in a given range. The temperature can be set via the FAPs, the MINI-FAPs (optional) or the AAPs (optional). The actual temperature of all cabin and optional zones is shown on the FAPs. VSCF


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The CIDS directors VSCF supplies through the FAPs to control the water/waste system. It makes the water depressurization, the auto flush, and the shut down of the water system. Moreover it does, through the FAPs, the pre-selection of the water quantity for potable water tank refilling.





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Figure 15.72 Control Functions



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.12.6 Programming Functions The CIDS system fulfils several programming functions which are: - Software loading - Layout selection - Cabin programming - Loudspeakers level adjustment Software Loading

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Via a dedicated FAP menu page, the Software Loading function makes the update of the software of all loadable CIDS components (directors, FAPs, optional MINI-FAPs, handsets and DEU Bs). This function is only available on ground.


Layout Selection

Via a dedicated FAP menu page, the CIDS cabin layout selection function is made to choose one out of a maximum of three predefined and three modifiable cabin layouts. This function is protected by an access code and is only available on ground. Cabin Programming

Several functions of the CIDS operate in relation to different cabin zones. The configuration of these zones can be changed via a dedicated programming page on the FAP. Through this page three cabin programming modes are available on ground or in flight: - Cabin Zones Programming, - No Smoking Zones Programming - Non Smoker Aircraft Programming They can be all protected by an access code. Loudspeakers Level Adjustment

Via a dedicated FAP menu page, the CIDS loudspeaker level adjustment function is made to adjust manually the cabin loudspeakers level for announcements and chimes. This function is protected by an access code and is available on ground or in flight.





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Figure 15.73 Programming Functions



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) 5.15.13 Information Systems (Ref: Airbus A380) 5.15.13.1 General New technologies are used for data communication. Inside the aircraft two so called onboard worlds are defined: - the avionics world and - the open world.

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In the avionics world you can find all avionics systems necessary to fly the aircraft. The avionics world links aircraft systems together in a secured environment.


In the open world you can find electronic documentation and non-critical applications for aircraft safety called Onboard Information System (OIS) which is hosted by an on-board hardware platform called Network Server System (NSS). The open world is like an extension of the aircraft systems to the outside for airline operations and passengers’ needs. The purpose of the NSS is to offer a cost effective and versatile network for hosting the OIS. The NSS and the OIS will improve the airline operations related to flight, maintenance and cabin operations, and, to implement additional services for passengers. The NSS, based on high-speed communication protocol, mainly supplies: -A server’s platform, -Connections to Human Machine Interfaces (HMIs) for various end users, which are the flight crew, the maintenance staff, the cabin crew and the passengers, -Secured data communication with the aircraft systems in the avionics world through firewalls, -An external data communication capability with Airline Operational Control (AOC) centers and with service providers (e.g.: electronic mail). The OIS mainly improves the airlines operations on ground and in flight by:

-Supplying electronic forms (e.g.: logbook) and documentations, replacing the use of paper media, -Offering a set of customized applications, documentation and database developed either by the aircraft manufacturer, the airlines or a third party.





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Figure 15.74 General



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) These enhancements give: -The flight crew an easy, intuitive and quick access to the data they may require to make a decision, etc., -The maintenance staff, tools to facilitate maintenance operations improving the autonomy of the aircraft and leading to the reduction of the failure isolation time, -The cabin crew an easy access to their electronic documentation used for cabin operations, -The passengers a worldwide electronic mail service. 5.15.13.2 OIS Services

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The OIS ensures various services to improve flight, maintenance and cabin operations and to ensure additional services for passengers. The OIS is shared out into 3 NSS domains:


-The AVIONICS domain, -The optional FLIGHT OPERATIONS domain, -And the optional COMMUNICATION and CABIN domain. Avionics Domains

The OIS part hosted in the avionics domain, supplies:

-An onboard/ground communication capability between the aircraft and the Airline Operational Control (AOC) centers, -Tools to support Maintenance operations dedicated to the avionics world and to the avionics domain, -Electronic Documentation related to flight, maintenance and cabin operations, -Servicing tool dedicated to refueling operation, -Data storage used for the maintenance operations. Flight Operation Domain

The OIS part hosted in the flight operations domain, supplies:

-Performance computation tools, -Operational electronic Documentation related to flight operations, -Tools to support the Mission for aircraft navigation. For Training Purposes Only




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Figure 15.75 OIS Services



PART 66 CAT B1.1/B2 MODULE 5 DIGITAL TECHNIQUES ELECTRONIC INSTRUMENT SYSTEMS TYPICAL ELECTRONIC / DIGITAL AIRCRAFT SYSTEM (DCAM 5.15 L2) Communication and Cabin Domain The OIS part hosted in the communication and cabin domain, supplies: -Pax services such as electronic mail, -An air/ground and gate link Communication capability between the NSS, the AOC centers and service providers, -Tools to support Maintenance operations dedicated to this domain and to its connected systems, -Data storage used for the maintenance operations.


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easa - Module 5

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