Page 170 - Programmable Logic Controllers, Fifth Edition

CHAPTER 7 REVIEW QUESTIONS

1. Explain the difference between the timed and 7. State three advantages of using programmed PLC
instantaneous contacts of a mechanical timing timers over mechanical timing relays.
relay. 8. For a TON timer:
2. Draw the symbol and explain the operation of each a. When is the enable bit of a timer instruction
of the following timed contacts of a mechanical true?
timing relay: b. When is the timer-timing bit of a timer instruc-
a. On-delay timer—NOTC contact tion true?
b. On-delay timer—NCTO contact c. When does the done bit of a timer change state?
c. Off-delay timer—NOTO contact 9. For a TOF timer:
d. Off-delay timer—NCTC contact a. When is the enable bit of a timer instruction true?
3. Name five pieces of information usually associated b. When is the timer-timing bit of a timer instruc-
with a PLC timer instruction. tion true?
4. When is the output of a programmed timer c. When does the done bit of a timer change state?
energized? 10. Explain what each of the following quantities asso-
5. a. What are the two methods commonly used to ciated with a PLC timer instruction represents:
represent a timer instruction within a PLC’s a. Preset time
ladder logic program? b. Accumulated time
b. Which method is preferred? Why? c. Time base
6. a. Explain the difference between the operation of 11. State the method used to reset the accumulated
a nonretentive timer and that of a retentive timer. time of each of the following:
b. Explain how the accumulated count of pro- a. TON timer
grammed retentive and nonretentive timers is b. TOF timer
reset to zero. c. RTO timer

CHAPTER 7 PROBLEMS

1. a. With reference to the relay schematic diagram in
Figure 7-38, state the status of each light (on or Relay schematic diagram
off) after each of the following sequential events: L1 S1 L2
I. Power is first applied and switch S1 is open. TD1
II. Switch S1 has just closed.
III. Switch S1 has been closed for 5 s. TD2
IV. Switch S1 has just opened.
V. Switch S1 has been opened for 5 s. TD1-1
b. Design a PLC program and prepare a typical I/O (5 s) PL1
connection diagram and ladder logic program that
will execute this hardwired control circuit correctly. TD1-2
PL2
2. Design a PLC program and prepare a typical I/O (5 s)
connection diagram and ladder logic program that TD2-1
will correctly execute the hardwired relay control PL3
circuit shown in Figure 7-39. (5 s)
3. Study the ladder logic program in Figure 7-40 and TD2-2
answer the questions that follow: (5 s) PL4
a. What type of timer has been programmed?
b. What is the length of the time-delay period? Figure 7-38 Relay schematic diagram for Problem 1.

Programming Timers Chapter 7 151

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