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page 212

17.2 A SIMPLE EXAMPLE

• Consider the problem below.

Develop a simple relay based controller so that three switches in a room could turn the light on or off.

17.3 PRACTICE PROBLEMS

(ans. c)

1. Write the mnemonic program for the Ladder Logic below.

201

100

101

(ans. XIC 100, XIC 101, OR, OTE 201)

2.A PLC can be used effectively to,

a)replace a few relays.

page 213

b)replace customized electronics.

c)replace a highly skilled human operator.

d)control a well understood process.

(ans. d)

5.Why would relays be used in place of PLCs? (ans. for simple designs)

6.Give an example of where a PLC could be used. (ans. to control a conveyor system)

7.Give a concise description of a PLC.

8.List the advantages of a PLC over relays.

9.Explain the trade-offs between relays and PLCs for control applications.

10.In the figure below, if a limit switch is NO, the power the power flow to the output will normally be___________________.

#4

#5

(ans. off)

11. In the figure above, if a limit switch input is NC, the power indicator in the input module should be __________________.

(ans. on)

12.TRUE or FALSE - The left-most side of the PLC’s ladder diagram simulates the power line. (ans. true)

13.In ladder diagram programs, all outputs are represented by:

a)Contact symbols

b)Coil symbols

c)a or b

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d) None of the above

(ans. b)

14. List the advantages of PLCs over relay logic.

15.

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