plc exam

QUESTION 1

a)

Outl Outline ine the diffe differe rence ncess betw betwee een n a Prog Progra ramm mmabl ablee Logi Logicc Contr Control olle lerr and and a per perso sonal nal computer. [3]

b) c) d)

Provide Provide a diagram diagram that shows shows the the internal internal architecture architecture of a PLC. Expl Explai ain n brie briefl fly y why why a PLC PLC do do a self self che check ck eve every ry tim timee it is tur turne ned d on.[2 on.[2]] A ladder lo logic di diagram is is sh shown in in figure 1 below.

[5]

Figure 1

Explain what is wrong with this diagram and hence what will happen if it is used.[5] e) So Some PL PLCs re remove me memory wh when a battery is is re removed. Ex Explain wh why.

[5]

Question 2

Given a system that is described by the following equation X = A + ( B.( A + C ) + C ) + A B . ( D + E ) a) Simpli Simplify fy the equat equation ion using using Bool Boolean ean algebr algebra. a. b) Implement Implement the the original original then the the simplified simplified equation with with digital digital circuit. circuit. c) Implement Implement the the original original then the simplif simplified ied using using ladder ladder logic logic

Question 3

a) Describe the basic steps of operation of a PLC after it is turned on. [8] b) How do input and and output cards act act as interface interface between between the the PLC and and external external devices [3] c) Given a clear plastic bottle, list three types of sensors that could be used to detect it. [3]

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d) Describe the difference between positive and negative edge triggered

[4]

Question 4

Figure 2

The diagram in figure in figure 2 shows a ladder logic program. a) Explain the the purpose and operation of of the ladder logic pr program.

[4]

b) There are few basic types types of memory memory used used in PLCs PLCs today. State and and explain explain each. [12] c) With the aid of a diagram, describe the difference between an event based device and an event based device. [4]

Question 5

a) Using a simple diagr agram, am, describe the princi nciple of ope operatio ation n of a latch atch..

[4]

b) There are are two fundament fundamental al types types of memory memory used used in Allen-Bradle Allen-Bradley y programmabl programmablee Logic controllers. State and then explain each of these. [8]

c) The diagram shown in figur in figuree 3 below is a ladder logic program for Allen-Bradley PLCs. Provide an equivalent instruction list program. [8]



Figure 3

Question 6 a)

A ladd ladder er log logic ic can can be be used used to to desig design n a rela relay y base based d cont contro rolle llerr that that will will allow allow thr three ee swi switc tches hes in a room to control a single light. There are two possible approaches to this problem. Provide the ladder logic programs that can be used for the each of the approaches. [8]

b). The diagram in figure 4 below is a ladder logic representation of a latch instruction.



Figure 4

Explain how the instruction operates.

[5]

c) Explain with the aid of a diagram the difference between an on delay timer and an off delay timer. [5] d) State and briefly describe the two basic counters that we have.

[2]

Question 7

Consider the traffic lights in Figur in Figuree 5. 5. The normal sequences for traffic lights are a green light in one direction for a long period of time, typically 10 or more seconds. This is followed by a brief yellow light, typically 4 seconds. This is then followed by a similar light pattern in the other direction. It is understood that a green or yellow light in one direction implies a red light in the other direction. Pedestrian buttons are provided so that when pedestrians are present a cross walk light can be turned on and the duration of the green light increased.



Figure 5

By first defining the inputs and outputs and describe the system variables, develop a programmable programmable logic logic controller controller for for this system system.. [20]

END

plc exam

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