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CHAPTER 9 REVIEW QUESTIONS
1. a. Two MCR output instructions are to be pro- 8. a. What does the forcing capability of a PLC allow
grammed to control a section of a program. the user to do?
Explain the programming procedure to be b. Outline two practical uses for forcing functions.
followed. c. Why should extreme care be exercised when
b. State how the status of the output devices within using forcing functions?
the fenced zone will be affected when the MCR 9. Why should emergency stop circuits be hardwired
instruction makes a false-to-true transition. instead of programmed?
c. State how the status of the output devices within 10. State the function of each of the following in the
the fenced zone will be affected when the MCR basic safety wiring for a PLC installation:
instruction makes a true-to-false transition. a. Main disconnect switch
2. What is the main advantage of the jump b. Isolation transformer
instruction? c. Emergency stops
3. What types of instructions are not normally in- d. Master control relay
cluded inside the jumped section of a program? 11. Compare standard and safety PLCs with regard to:
Why? a. Processors
4. a. What is the purpose of the label instruction in b. Input circuitry
the jump-to-label instruction pair? c. Output circuitry
b. When the jump-to-label instruction is executed, d. Power supplies
in what way are the jumped rungs affected? 12. When programming a motor starter circuit, why is
5. a. Explain what the jump-to-subroutine instruction it safer to use the starter seal-in auxiliary contact
allows the program to do. in place of a programmed contact referenced to the
b. In what type of machine operation can this output coil instruction?
instruction save a great deal of duplicate 13. When programming stop buttons, why is it safer
programming? to use an NC pushbutton programmed to examine
6. What advantage is there to the nesting of for an on condition than an NO pushbutton pro-
subroutines? grammed to examine for an off condition?
7. a. When are the immediate input and immediate 14. Explain the selectable timed interrupt function.
output instructions used? 15. Explain the function of the fault routine file.
b. Why is it of little benefit to program an immedi- 16. How is the temporary end instruction used to trou-
ate input or immediate output instruction near bleshoot a program?
the beginning of a program?
CHAPTER 9 PROBLEMS
1. Answer the questions, in sequence, for the MCR c. With switches S2 and S3 still on, switch S1 is
program in Figure 9-29, assuming the program has turned off. Will both outputs PL1 and PL2 de-
just been entered and the PLC is placed in the RUN energize? Why?
mode with all switches turned off. d. With all other switches off, switch S6 is turned
a. Switches S2 and S3 are turned on. Will outputs on. Will the timer time? Why?
PL1 and PL2 come on? Why? e. With switch S6 still on, switch S5 is turned on.
b. With switches S2 and S3 still on, switch S1 is Will the timer time? Why?
turned on. Will output PL1 or PL2 or both come f. With switch S6 still on, switch S5 is turned off.
on? Why? What happens to the timer? If the timer was an
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