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Distributed Control System and Programmable Logic Control

Course Aim

The aim of this training course is to build up the procedural and declarative knowledge required to be recognized by projects engineer that don not have past background of DCS or PLC. This will help them to supervise projects dealing with control systems systems with a strong bac background. kground. In this course, the training cycle is divided in five steps that necessitate the cooperation between the instructor and the trainees. These steps are shown in figure below, they are summarized as follows: 1. Define the knowledge and skills required to be developed. 2. Define the elements of each knowledge or skill. 3. Formulate a verbal phrase for the learning objective of each element. 4. Choose an adequate instructional activity to present each element. 5. Set up an indicator to measure the outcomes of the course and modify the training skills to adapt the vocational needs.

Define Knowledge & Skills

Determine Elements

Learning Objectives

Measure & Correction Instruction Activity


































Knowledge and Elements





Illustrate DCS & PLC Benefits, Usage and History. 

Overview of control system history.



Control system benefits and usage.



Types of control

Develop Knowledge of DCS Components (Hardware & Software). 

Infrastructure [ Communication Bus, Interfaces, Controllers, Gateways, RTU, Others ].



Hardware and technologies.



Software [Configuration, Graphics, Alarming, Trending, System Management, Others ].





Extend Knowledge of DCS installation and Maintenance. 

Site Installation, Commissioning and Startup.



Diagnostics, Spares, Tools and Power Distribution.



Maintenance [Backup, Replacements and System Installation ].

Develop Knowledge of PLC Components. 

PLC fundamentals.



PLC Logic.


































Table of Contents

Section I

Chapter 1

Introduction

Chapter 2

Regulatory Control Section II

Chapter 3

DCS Infrastructure

Chapter 4

DCS Hardware

Chapter 5

DCS Software Section III

Chapter 6

Installation

Chapter 7

Maintenance

Chapter 8

Power Distribution Section IV

Chapter 9

PLC Fundamentals.

Chapter 10

Ladder Logic And SFC

Appendices

A

Electrical Relay Diagram And P&ID Symbols

B

Serial Communication

C

Networking

D

Software Engineering



































































Chapter 1 Control Systems 1.1 Automation System Structure Although applications differ widely, there is little difference in the overall architecture of their control systems. Why the control system of a power plant is not sold also for automating a brewery depends largely on small differences (e.g. explosion-proof), on regulations (e.g. Food and Drug Administration) and also tradition, customer relationship. The ANSI/ISA standard 95 defines terminology and good practices Level 4

Business Planning & Logistics

Level 3

Manufacturing Operations & Control

Plant Production Scheduling Operational Management, etc.

Dispatching Production, Detailed Product Scheduling, Reliability Assurance,...

Level 2,1,0 Batch Control

Continuous Control

Discrete Control

Enterprise Resource

Manufacturing Execution

Control & Command

1.1.1 Large Control System Hierarchy



Administration: Production goals, planning



Enterprise: Manages resources, workflow, coordinates activities of different sites, quality supervision, maintenance, distribution and






































Supervision: Supervision of the site, optimization, on-line operations. Control room, Process Data Base, logging (open loop)



Group (Area): Control of a well-defined part of the plant. closed loop, except for intervention of an operator)



o

Coordinates individual subgroups

o

Adjusting set-points and parameters

o

Commands several units as a whole

Unit (Cell): Control (regulation, monitoring and protection) of a small part of a group (closed loop except for maintenance).



o

Measure: Sampling, scaling, processing, calibration.

o

Control: regulation, set-points and parameters

o

Command: sequencing, protection and interlocking

Field: Sensors & Actors, data acquisition, digitalization, data transmission, no processing except measurement correction and built-in protection.

4

Planning, Statistics, Finances

3

Workflow, Resources, Interactions

2

Supervisory

SCADA Supervisory And Data

administration enterprise supervision


































1.1.2 Response Time and Hierarchical Level ERP

Planning Level

(Enterprise Resource Planning)

MES Execution Level

(Manufacturing Execution System)

SCADA

(Supervisory Control and Data Acquisition)

Supervisory Level

DCS (Distributed Control System)

Control Level

PLC (Programmable Logic Controller)

ms

seconds

hours

days days

weeks

month month

years

Figure 1.2 Response Time And Hierarchical Level

1.2 What is DCS?



A DCS is an integrated set of modules with distributed functions. – Multi-loop controllers (10’s -100’s) that connect to field

devices – Supervisory coordinating controllers

Multi-loop operator stations and engineering stations


































Remote Users

www

System Server

Supervisory Controller

Operator Stations Engineering Station

Remote Server Control Network

Multi-loop Controller Direct I/O Module

Other Industrial Devices

Figure 1.3 DCS Hierarchy



A DCS, throughout the whole system, must provide: – Performance: control must be faster than the process. – Determinism: control must always take the same time. – Fault tolerance: redundancy; must fail to a known state. – Security: must have access restrictions/controls.

Even though performance, ease of use, and interoperability are key evaluation criteria for any control system software package, the following is intended to provide the manufacturing engineer with a concise list of


































3. DISASTER PROCESSING.

This component is defined by the efficiency with which the software provides the operator with system failure information and the ease at which the operator is permitted to bring the system back to maximum operation after system failure. 4. DATABASE.

This refers to the software’s ability to maintain the system’s

database. 5. PROCESSES/DATA.

This criterion is concerned with the variety of processes and data that can be controlled by the SCADA package. 6. DIAGNOSTICS.

The SCADA package’s ability to assist in the resolution of system

failures is evaluated by this diagnostic utility. 7. SECURITY.

This component is concerned with the levels of security provided by the software. 8. MONITORING/CONTROL

Monitoring of a given process in real-time and control of that process, within preset parameters, is evaluated by this criteria. 9. ALARM MANAGEMENT/LOG MANAGEMENT/LOGGING. GING.


































13. TRENDING.

The software’s ability to display trending plots using historical and

current data is considered in this category. 14. REPORT GENERATION.

The production of logs and reports using current real-time data and data retrieved from historical files is evaluated under this category.

Due to the advancements in computer technology and low cost, a personal computer-based distributed control system can be installed for a fraction of the cost required just a few years ago. However, prior to selecting any piece of DCS equipment, first examine the existing equipment, in particular the smart controllers, for network compatibility. Then, examine and select the software to be employed.

1.3 What is PLC? A programmable logic controller, also called a PLC or programmable controller, is a computer-type device used to control equipment in an industrial facility. The kinds of equipment that PLCs can control are as varied as industrial facilities themselves. Conveyor systems, food processing machinery, auto assembly lines…you name it




































The control program is the computer program stored in the PLC’s memory that tells the PLC what’s supposed to be going on in the system.

The use of a PLC to provide the wiring connections between system devices is called soft-wiring. Let's say that a push button is supposed to control the operation of a motor. In a traditional control system, the push button would be wired directly to the motor. In a PLC system, however, both the push button and the motor would be wired to the PLC instead. Then, the PLC's control program would complete the electrical circuit between the two, allowing the button to control the motor.


































essence, the CPU is the “brains” of a programmable controller. It

functions much the same way the CPU of a regular computer does, except that it uses special instructions and coding to perform its functions. The CPU has three parts: 

The processor



The memory system



The power supply

The processor is the section of the CPU that codes, decodes, and computes data. The memory system is the section of the CPU that stores both the control program and data from the equipment connected to the PLC. The power supply is the section that provides the PLC with the voltage and current it needs to operate.


































The I/O system consists of two main parts: 

The rack The rack is an enclosure with slots in it that is connected to the CPU.



I/O modules I/O modules are devices with connection terminals to which the field devices are wired.

Together, the rack and the I/O modules form the interface between the field devices and the PLC. When set up properly, each I/O module is both securely wired to its corresponding field devices and securely installed in a slot in the rack. This creates the physical connection between the field equipment and the PLC. In some small PLCs, the rack and the I/O modules come prepackaged as one unit.


































1.4 How is a DCS different from a PLC system?

DCS

PLC

Mfr sells a complete system of integrated

Mfr sells some components; an SI

components.

acquires others and engineers the system.

Mfr supports the system.

Mfr supports the components.

On-line repair/ maintenance are the norm.

Off-line repair/ maintenance are the norm.

System management built-in.

System management designed per project.

Users expect to evolve/upgrade/expand a

System is a one-off project (like a house).

system over 10/20/30 years.

Upgrades / expansions are new projects.

1.5 Redundancy and Fault Tolerance

1.5.1 Redundancy 

Hardware redundancy – add extra hardware for detection or tolerating faults




































Acceptable check – Cost – Reasonable check, e.g. monitor rate of change



diagnostics – Performed – Performed “by system on system components” – E.g. power-up diagnostics

1.5.2.2 Damage Confinement 

Error might propagate and spread



Identify boundaries to state beyond which no information exchange has occurred

1.5.2.3 Error Recovery 

Backward recovery – State is restored to an earlier state – Requires checkpoints




































1.5.2.5 Fault Coverage 

Measure of system’s ability to perform: – Fault detection – Fault location – Fault containment – (and/or fault recovery)



Note: – Recovery implies that the system as a whole is operational – This – This does not imply that a “repair” occurred – E.g. duplex system with benign fault can recover to continue

operation on one non-faulty processor




































Parallelism – Results are voted on requirements: N >= 3



TMR (Triple Modular Redundancy)

1.5.2.8 Fault tolerant structures

Fault tolerance allows continuing operation in spite of a limited number of independent failures. Fault tolerance relies on work redundancy.

1.5.2.9 Static redundancy: 2 out of 3 

Workby of 3 synchronised and identical units.


































Input

Primary unit

Standby unit

Switch Output

Figure 1.8 Dynamic Redundancy

1.5.2.11 Workby and Standby Workby

on-line

sync

Hot standby

workby

on-line

sync

Cold standby

standby


































1.5.2.13 Hybrid Redundancy Mixture of workby (static redundancy) and standby (dynamic redundancy).

workby

workby

workby

standby

standby

voter

Reconfiguration (self-purging redundancy)

workby

failed

voter

Figure 1.11 Hybrid Redundancy

workby

workby

standby


































Input is obtained from the keyboard and mouse, output is sent to the screen, and the disk and memory are used for both input and output for storage. (Note: the directions of these arrows are very important to engineers, always pay attention to indicate where information is flowing.) This figure can be redrawn as in Figure 1.13 to clarify the role of inputs and outputs.






































Computer - the 686 CPU is like a PLC CPU unit



Output circuits - a graphics card is like a triac output card



Outputs - a monitor is like a light



Storage - memory in PLCs is similar to memories in personal

computers




































Chapter 2 Regulatory Control 2.1 Learning Objectives




































Figure 2.1 Response of simple dynamic process to step input change




































2.3 PID Control

2.3.1 Feedback Control

The principle of feedback is one of the most intuitive concepts known. An action is taken to correct a less satisfactory situation then the
















































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































FOUNDATION fieldbus is covered by standards from three major organizations: 





ANSI/ISA 50.02 IEC 61158 CENELEC EN50170:1996/A1

The technology is managed by the independent, not-for-profit Fieldbus Foundation, whose 150+ member companies include users as well as all major process automation suppliers around the globe. Some suppliers have even donated fieldbus-related fieldbus -related patents to the Fieldbus Foundation to encourage wider use of the technology by all Foundation members.

Interoperability simply means that FOUNDATION fieldbus devices and host systems can work together while giving you the full functionality of each component.

4.10 Role Play Each trainee should introduce one of the main components: 5. Fieldbus Module and Interconnection 6. Fieldbus Processor and Clusters. 7. Foundation Fieldbus technology


































Chapter 5 DCS Software 5.1 Learning objectives



To be familiar with main software components of DCS.



Understand main tasks for each application.

5.2 Standard Application Packages 5.2.1 System Management

Features include: 

Display of equipment information for the station and its associated input/output devices, buses, and printers.





Capability for change actions directed to the associated equipment. Processing of station alarm conditions and messages.

5.2.2 Database Management




Storage, retrieval, and manipulation of system data files. A run-time license for the embedded use of the Relational Database Management System.



A spreadsheet package.

5.2.3 Historian


Maintenance

of

a

history

of

values

for

process-related

measurements that have been configured for retention by the




































Maintenance of a history of application messages that have been sent to the Historian.



Maintenance of a history of alarms and error conditions which generate messages for the Historian.



Access to all Historian data by display and report application programs.

5.2.4 View Display Manager




Presentation of the operating environment. Setting of the overall operating environment according to the type of user. Process engineers, process operators, and software engineers have access to specialized functions and databases suited to their specific requirements and authorizations.









Dynamic and interactive process graphics. Display and processing of current process alarms. Group and default displays for control blocks. Execution of embedded trending within displays.

5.2.5 Draw Display Builder


Graphical display configuration for viewing and control of process operation.



Access to graphical object palettes allowing easy inclusion of pumps, tanks, valves, ISA symbols, and similar complex objects. ob jects.



Ready modification of existing displays using a mouse pointer, menu items, and quick-access toolbars.




































Dynamic variation of object attributes such as fill level, color, position, size and visibility with changes in the associated process variable.



Inclusion of operator control elements such as pushbuttons and sliders into displays.



A library of faceplates which may be configured by simply specifying the compound and block name of the block to which the faceplate is to be connected.

5.3 Alarm System

Figure 5.1 Alarm manger

Alarm Manager provides an easy-to-use graphical interface of preconfigured alarm displays for viewing and quickly responding to process alarm conditions. The alarm display windows present alarm messages initiated by the control blocks and related to digital input, state change, absolute analog, deviation, rate of change, device status mismatch, and other alarm conditions. Accessible from any environment, the Alarm Manager Display windows




































Quick, easy access to the most recent alarm messages via the Most Recent Alarm display or Current Alarm display



Alarm status and value information dynamically updated from the control station



Color-coded priority and status indicators that allow you to quickly focus in on critical alarms



Summary displays for different views of the alarm database based on alarm status





An historical list of alarms a larms The capability to view subsets of alarms based on specific userdefined criteria



The capability to silence or temporarily mute workstation and annunciator horns.



Secured access to alarming functions dependent on user or system responsibility

This set of resizable alarm displays providing a variety of current and historic views of the process alarm database includes: 



A multi-page list of all the current alarms A single page of the most recent, active, unacknowledged alarms with dynamically updating value and status fields



Three summary displays specific to alarm status also with updating values and statuses:



o

all active, unacknowledged alarms

o

all unacknowledged alarms that have returned to normal

o

all active, acknowledged alarms

A list of historized alarms related to the selected historian database An operations display for silencing horns, temporarily muting




































These displays allow you to respond to alarm conditions, filter and analyze specific alarm data, and maintain alarm message files for reporting purposes. The Process or Alarm button in the Display Manager (DM) window indicates

the

presence

of

alarms

(both

acknowledged

and

unacknowledged) and provides access to Alarm Manager Displays. Initially, the Current Alarm Display (CAD) appears and the other displays are easily accessible from the CAD via its default Displays menu: 











Most Recent Alarm display (MRA) New Alarm display (NEWALM) Unacknowledged Alarms display (UNACK) Acknowledged Alarms display (ACKALM) Alarm History display (AHD) Operations display (OPR)

These easy-to-use displays support the following features: 



A pre-configured number of alarms per screen or page Pre-configured alarm message information and formatting per alarm type



A status area for indication of current Alarm Manager and display status, such as horns muted, match active, display paused, initial call-up time




































A scroll bar and Go To Page option for moving easily through the alarm list

Although a preconfigured set of alarm displays is provided, many aspects of the displays and alarm message content are user configurable to accommodate different process control applications and operational needs. See the section on Alarm/Display Manager Configurator.

5.4 Historian The Historian collects, stores, processes, and archives process data from the control system to provide data for trends, Statistical Process Control (SPC) charts, logs, reports, spreadsheets, and application programs. The Historian software is an easy-to-use data collection tool that allows the user to organize and enforce a plant data collection philosophy. The Historian provides extensive data collection and management functions, and data display functions for use by process engineers or operators. Typical historical data are process analog and/or digital variables (points). The Historian can also collect and display application generated messages. You can use the Historian to collect data in support of the following production control functions:












































Maintenance reporting Material accounting Process analysis Production reporting Quality control

The Historian can: 

Retrieve variables from process databases or accept data from production control databases maintained by user application programs.





Perform built-in calculations on the collected data. Store calculated (reduced) data in a real time, relational database.

Application software in a plant-wide control system can access the Historian database to obtain historical data for process control, production control, and management information reporting. You can use SPC chart displays of Historian data to monitor process variables on-line via the Statistical Process Control Package (SPCP). You can build displays for trending historical data via the Display Builder and Display Configurator with Trending software. Using the Report Writer, you can generate detailed reports of historical data for management information. Examples of Industrial Software that interface with the Historian are: Batch Plant Management










































Spreadsheet Statistical Process Control Package System Monitor Report Writer

5.5 Draw

Figure 5.2 Draw

Draw is a display builder and configurator that allow you to create and maintain dynamically updating process displays. Displays can represent the plant, a process area or a detailed portion of the process. You can draw basic objects using Draw's toolbars, menu items and shortcut keys. You assign graphic attributes such as color and line style to the objects, and then configure them to reflect process variable changes or


































5.5.1 Configuration

There are two ways of configuring a display object. You can: 1. Choose the Dynamic Update tab to connect one of the object's attributes, such as visibility or fill level, to a process variable or a file. With this type of configuration, changes in an attribute are triggered dynamically by changes in the process variable. No operator intervention is necessary. 2. Choose the Operator Action tab to connect the entire object to an action, such as opening a display or executing a command. An operator triggers the action by selecting the object. An individual object can have both types of connections, although it can have only one operator action.

5.5.2 Operator Actions

In a display configured for operator action, an operator can trigger events by selecting an object (typically a button), moving a slider, or typing text or a numeric value. In response to an operator action, variables can be modified, a new display can open or an overlay can appear.


















































Display Command Relative Pick Momentary Contact Ramp Connect Variable Move Horizontal or Vertical Numeric/Text Entry

5.5.3 Faceplates

A faceplate is a dynamic representation of control block parameters. Draw provides a complete library of faceplates, ready to be connected to any control block in the control database. In addition, you can build your own faceplates using the standard Draw drawing tools. To configure a faceplate, you need only define the Compound:block to which the faceplate is connected. Draw automatically determines the proper configuration attributes for the associated Compound:block.


































5.6 View

Figure 5.3 View

View is a window into the system software, providing a userfriendly interface to the total process. You can interact with any or all of the real-time plant, field, and process data available in the system. View provides: 



Direct access to dynamic process displays. Entry into user-configurable operating environments specific to each user - the process engineer, process operator, and software engineer.


































o

Access to the four most recently used displays.

Additionally, with View you have: 

Flexibility in customizing environments to conform to your site requirements.







Rapid access to View while in other applications. Screen print utility. Window sizing options.

The multi-window capability of Solaris and an d Windows NT operating systems allows you to monitor the information on a process control display as well as access other applications without closing any window.

5.6.1 View Window

View Window contains the following features: 

A top menu bar for accessing displays, configurators, and other applications as specified by the environment.


































5.6.2 Operating Environments

A collection of programs, utilities, and displays related to user tasks is provided for each of the following: process operator, process engineer, and software engineer. These environments, including menu bars, menu content, and Display Bar content, can be modified to conform to your site requirements. You can easily switch from one configured environment to another. To secure environments against unauthorized use, environment passwords can be configured and menu entries disabled based on the environment.

5.7 Operator Action Journal The Operator Action Journal is a record of specific operator actions taken during process control operations. These actions generally consist of manipulating certain Control Processor, and gateway parameters as


































changing block statuses, acknowledging block alarms, and other actions such as horn muting.

Information logged as a result of each database change includes: 

Name of the Display Manager, FoxView, or Alarm Manager that requested the database change.







Compound:Block.Point for which the change was made. The "old value" TO "new value" text for non-packed Boolean. Current mask and data value for packed Boolean/long.

Following is an example of an Operator Action Journal Report. Operator Action Journal Report

Tue Aug 08-02-97 08-02-97 08-02-97 08-02-97 08-02-97 08-02-97 08-02-97

1 1997 17:04:05 Page 1 07:57:08 GC3E31 GC3E31 SCRIPT /usr/fox/hi/init.cmds /usr/fox/hi/init.cmds 07:57:15 GC3E31 ChgEnv ChgEnv Init_Env Init_Env ->Init_env 07:58:19 GC3E31 ChgEnv ChgEnv Init+Env ->Proc_Eng_Env 08:00:34 CG3E31 UC01_LEAD :SINE .OUT 16.18 to 46.18 08:00:54 GC3E31 UC01_LEAD :SINE .MA Manual to Auto 08:00:57 GC3E31 UC01_LEAD :SINE .LR Remote to Local 08:01:01 GC3E31 UC01_LEAD :SINE .MA Auto to Manual

5.8 Control Configuration




































Within this structure, any block in any compound can be connected to any other block in any other compound in the system. The entire compound structure can be viewed through the workstation display. The block contains parameters that have values of the types: Real, Boolean, Packed Boolean, Boolean Long, Integer, or String.


































nuisance alarms, alarms can be inhibited at the compound level on a priority level basis. Alarms can also be inhibited at the block level, on either an alarm type basis, or an overall basis. Alarms are initiated by the blocks within the compound. Alarm messages are then sent to groups of stations or applications (for example, Workstations, Historians, Printers) according to configured alarm groups. The UNACK alarm acknowledge output parameter allows the user to propagate alarm acknowledge actions to all blocks in a compound. Stations, applications, and devices corresponding to various alarm destination groups are configured at the compound level or at the station level in the case of station compounds.


































5.8.5 Compound/Block Parameters

Compound and block parameters contain values that are of one of the types Real, String, Integer, Short Integer, Long Integer, Boolean, Packed Boolean, Packed Long, or Character. Additionally, parameters are defined as being configurable, and either connectable/settable, not connectable/not settable, or a combination that is dependent upon the compound, block, and state.

5.8.5.1 Configurable Configurable Parameters


































5.8.5.3 Input Input Parameters

Input parameters are connectable types that are the receivers of data from other connectable parameters via a path connection. If no source path is specified during configuration, then the resident data of the value record is the actual "source" of data. It can be either the initial default or configured value, or a new value through a SET call to the input parameter. If a source path is specified, then the data value is an output parameter of the same or another block, or a shared variable, thereby securing the input. By linking a shared variable to a block input during configuration,


































5.8.5.5 Nonconnecta Nonconnectable ble Parameters

Nonconnectable parameters have no value records and are not linkable. They mainly consist of string-type variables like NAME, or nonsettable parameters that are used in the configurator only, for example,

block

options.

Local

algorithm

variables

are

also

nonconnectable. Nonconnectable parameters are generally accessible through GET calls. There is also a class of nonconnectable noncon nectable input parameters that comprise the block user interface which can be manipulated through SET calls. An example is an alarm deadband.







































































Chapter 6 Installation 6.1 Learning objectives







































































6.3.2 Unpacking Procedure

The following unpacking procedure applies, in general, to all system units: 

Inspect the exterior of the shipping carton for obvious damage. (Any noticeable damage should be indicated in the shipper's bill of lading.)




































6.3.4 Industrial Enclosures Mounting Procedures

Figure 6.2 shows a single dual-height modular mounting structure area for containing processors and modules in an Industrial Enclosure.






































2. If the enclosure is the vented type and conduit entry is to be from






































6. Position the enclosure, with mounting gasket and enclosure plate,






































Chapter 7






































7.3.1 Enclosures






































7.3.3 Modular Industrial Workstations




















































































































































































































































































































































































































































































































































































































































































































































































































































Appendix B Serial Communication B.1 Introduction Multiple control systems will be used for complex processes. p rocesses. These control systems may be PLCs, but other controllers include robots, data terminals and computers. For these controllers to work together, they must communicate. This chapter will discuss communication techniques between computers, and how these apply to PLCs. The simplest form of communication is a direct connection between two computers. A network will simultaneously connect a large number of computers on a network. Data can be transmitted one bit at a time in series, this is called serial communication. Data bits can also be sent in parallel. The transmission rate will often be limited to some maximum value, from a few bits per second, to billions of bits per second. The communications often have limited distances, from a few feet to thousands of miles/kilometers. Data communications have evolved from the 1800’s when

telegraph machines were used to transmit simple messages using Morse code. This process was automated with teletype machines that allowed a user to type a message at one terminal, and the results would be printed on a remote terminal. Meanwhile, the telephone system began to emerge as a large network for interconnecting users. In the late 1950s Bell Telephone introduced data communication networks, and Texaco began to use remote monitoring and control to automate a polymerization plant. By the 1960s data communications and the phone system were being


































Internet, called ARPAnet. Before the 1980s the most common computer configuration was a centralized mainframe computer with remote data terminals, connected with serial data line. In the 1980s the personal computer began to displace the central computer. As a result, high speed networks are now displacing the dedicated serial connections. Serial communications and networks are both very important in modern control applications. An example of a networked control system is shown in Figure B.1. The computer and PLC are connected with an RS-232 (serial data) connection. This connection can only connect two devices. Devicenet is used by the Computer to communicate with various actuators and sensors. Devicenet can support up to 63 actuators and sensors. The PLC inputs and outputs are connected as normal to the process.

Figure B.1 Communication example

B.2 Serial Communication Serial communications send a single bit at a time between computers. This only requires a single communication channel, as


































channels are often wire based, but they may also be can be optical and radio. Figure B.2 shows some of the standard electrical connections. RS232c is the most common standard that is based on a voltage change levels. At the sending computer an input will either be true or false. The line driver will convert a false value in to a Txd voltage between +3V to

+15V, true will be between -3V to -15V. A cable connects the Txd and com on the sending computer to the Rxd and com inputs on the receiving

computer. The receiver converts the positive and negative voltages back to logic voltage levels in the receiving computer. The cable length is limited to 50 feet to reduce the effects of electrical noise. When RS -232 is used on the factory floor, care is required to reduce the effects of electrical noise - careful grounding and shielded cables are often used.


































The RS-422a cable uses a 20 mA current loop instead of voltage levels. This makes the systems more immune to electrical noise, so the cable can be up to 3000 feet long. The RS-423a standard uses a differential voltage level across two lines, also making the system more immune to electrical noise, thus allowing longer cables. To provide serial communication in two directions these circuits must be connected in both directions.

To transmit data, the sequence of bits follows a pattern, like that shown in Figure B.3. The transmission starts at the left hand side. Each bit will be true or false for a fixed period of time, determined by the transmission speed.

A typical data byte looks like the one below. The voltage/current on the line is made true or false. The width of the bits determines the possible bits per second (bps). The value shown before is used to transmit a single byte. Between bytes, and when the line is idle, the Txd is kept true, this helps the receiver detect when a sender is present. A single start bit is sent by making the Txd false. In this example the next eight bits are the transmitted data, a byte with the value 17. The data is followed by a parity bit that can be used to check the byte. In this example there are two data bits set, and even parity is being used, so the parity bit is set. The parity bit is followed by two stop bits to help separate this byte from the next one.


































Figure B.3 a serial data byte

Some of the byte settings are optional, such as the number of data bits (7 or 8), the parity bit (none, even or odd) and the number of stop bits (1 or 2). The sending and receiving computers must know what these settings are to properly receive and decode the data. Most computers send the data asynchronously, meaning that the data could be sent at any time, without warning. This makes the bit settings more important.

Another method used to detect data errors is half-duplex and fullduplex transmission. In half-duplex transmission the data is only sent in one direction. But, in full-dup transmission a copy of any byte received is sent back to the sender to verify that it was sent and received correctly. (Note: if you type and nothing shows up on a screen or characters show




































The transmission speed is the maximum number of bits that can be sent per second. The units for this are baud . The baud rate includes the start, parity and stop bits. For example a 9600 baud transmission of the data in Figure B.3 would transfer up to 800 bytes each second. Lower baud rates are 120, 300, 1.2K, 2.4K and 9.6K. Higher speeds are 19.2K, 28.8K and 33.3K. (Note: When this is set improperly you will get many transmission errors, or garbage on your screen.) Serial lines have become one of the most common methods for transmitting data to instruments: most personal computers have two serial ports. The previous discussion of serial communications techniques also applies to devices such as modems.

B.3 RS-232 The RS-232c standard is based on a low/false voltage between +3 to +15V, and an high/true voltage between -3 to -15V (+/-12V is commonly used). Figure B.4 shows some of the common connection schemes. In all methods the txd and rxd lines are crossed so that the sending txd outputs are into the listening rxd inputs when communicating between computers. When communicating with a communication device (modem), these lines are not crossed. In the modem connection the dsr


































Figure B.4 Common RS-232 Connection Schemes

Common connectors for serial communications are shown in Figure B.5. These connectors are either male (with pins) or female (with holes), and often use the assigned pins shown. The DB-9 connector is


































Figure B.5 Typical RS-232 Pin Assignments and Names

The handshaking lines are to be used to detect the status of the sender and receiver, and to regulate the flow of data. It would be unusual for most of these pins to be connected in any one application. The most common pins are provided on the DB-9 connector, and are also described below. TXD/RXD - (transmit data, receive data) - data lines DCD - (data carrier detect) - this indicates when a remote device is


































Appendix C Networking C.1 Introduction A computer with a single network interface can communicate with many other computers. This economy and flexibility has made networks the interface of choice, eclipsing point-to-point methods such as RS-232. Typical advantages of networks include resource sharing and ease of communication. But, networks do require more knowledge and understanding. Small networks are often called Local Area Networks (LANs). These may connect a few hundred computers within a distance of hundreds of meters. These networks are inexpensive, often costing $100 or less per network node. Data can be transmitted at rates of millions of bits per second. Many controls system are using networks to communicate with other controllers and computers. Typical applications include; 

Taking quality readings with a PLC and sending the data to a database computer.



Distributing recipes or special orders to batch processing


































C.2 Topology The structure of a network is called the topology. Figure C.1 shows the basic network topologies. The Bus and Ring topologies both share the same network wire. In the Star configuration each computer has a single wire that connects it to a central hub.

Figure C.1 Network Topologies

In the Ring and Bus topologies the network control is distributed between all of the computers on the network. The wiring only uses a single loop or run of wire. But, because there is only one wire, the network will slow down significantly as traffic increases. This also requires more sophisticated network interfaces that can determine when a computer is allowed to transmit messages. It is also possible for a




































For a factory environment the bus topology is popular. The large number of wires required for a star configuration can be expensive and confusing. The loop of wire required for a ring topology is also difficult to connect, and it can lead to ground loop problems. Figure C.2 shows a tree topology that is constructed out of smaller bus networks. Repeaters are used to boost the signal strength and allow the network to be larger.


































message originating in an application program in computer #1 must travel through all of the layers in both computers to arrive at the application in computer #2. This could be part of the process of reading email.


































The Physical layer describes items such as voltage levels and timing for the transmission of single bits. The Data Link layer deals with sending a small amount of data, such as a byte, and error correction. Together, these two layers would describe the serial byte shown in the previous chapter. The Network layer determines how to move the message through the network. If this were for an internet connection this layer would be responsible for adding the correct network address. The Transport layer will divide small amounts of data into smaller packets, or

recombine them into one larger piece. This layer also checks for data integrity, often with a checksum. The Session layer will deal with issues that go beyond a single block of data. In particular it will deal with resuming transmission if it is interrupted or corrupted. The Session layer will often make long term connections to the remote machine. The Presentation layer acts as an application interface so that syntax, formats

and codes are consistent between the two networked machines. For example this might convert ’ \ ’ to ’/’ in HTML files. This layer also




































Network Interface Hardware - The network interface may already

be built into the computer/PLC/sensor/etc. These may cost $15 to over $1000. 

The Media - The physical network connection between network

nodes. 10baseT (twisted pair) is the most popular. It is a pair of twisted copper wires terminated with an RJ-45 connector. 10base2 (thin wire) is thin shielded coaxial cable with BNC connectors. 10baseF (fiber optic) is costly, but signal transmission and noise properties are very good. 

Repeaters (Physical Layer) - These accept signals and retransmit

them so that longer networks can be built. 

Hub/Concentrator - A central connection point that network wires

will be connected to. It will pass network packets to local computers or to remote networks if they are available.


































Figure C.4 Network devices and the OSI model




































Appendix D Software Engineering

D.1 Introduction

A careful, structured approach to designing software will cut the total development time, and result in a more reliable system.

D.2 Fail Safe Design




































Sabotage - For various reasons, some individuals may try to damage a system. These problems can be minimized preventing access.



Random failure - Each component is prone to random failure. It is worth considering what would happen if any of these components were to fail.

Some design rules that will help improve the safety of a system are listed below.

Programs


































D.3 Debugging Most engineers have taken a programming course where they learned to write a program and then debug it. Debugging involves running the program, testing it for errors, and then fixing them. Even for an experienced programmer it is common to spend more time debugging than writing software. For PLCs this is not acceptable! If you are running the program and it is operating irrationally it will often damage hardware. Also, if the error is not obvious, you should go back and reexamine the program design. When a program is debugged by trial and error, there are probably errors remaining in the logic, and the program is very hard to


































2. Check indicator lights on I/O cards, see if they match the system. i.e., look at sensors that are on/off, and actuators on/off, check to see that the lights on the PLC I/O cards agree. If any of the light disagrees

with

the

physical

reality,

then

interface

electronics/mechanics need inspection. 3. Consult the manuals, or use software if available. If no obvious problems exist the problem proble m is not simple, and requires a technically skilled approach. 4. If all else fails call the vendor (or the contractor) for help.

D.5 Forcing


































References 1. I/A series Foxboro documentation. 2. HoneyWell Experion process knowledge system, "Honeywell Training.ppt" 3. "Automation Hierarchy", By: Prof. Dr. H. Kirrmann, ABB Research Center, Baden, Switzerland, " AI_14_Hierarchy.ppt" 4. http://newton.ex.ac.uk , http://newton.ex.ac.uk , By: C.D.H. Williams 5. "Electrical Relay Diagram And P&ID Symbols", From Industrial Text and Video Co. The Leader in Electrical, Motor Control and

DCS Course

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