Page 198 - Programmable Logic Controllers, Fifth Edition - Mobile version

CHAPTER 8 REVIEW QUESTIONS

1. Name the three forms of PLC counter instructions, 9. Identify the type of counter you would choose for
and explain the basic operation of each. each of the following situations:
2. State four pieces of information usually associated a. Count the total number of parts made during
with a PLC counter instruction. each shift.
3. In a PLC counter instruction, what rule applies to b. Keep track of the current number of parts in a
stage of a process as they enter and exit.
the addressing of the counter and reset instructions? c. There are 10 parts in a full hopper. As parts
4. When is the output of a PLC counter energized? leave, keep track of the number of parts remain-
5. When does the PLC counter instruction increment ing in the hopper
or decrement its current count? 10. Describe the basic programming process involved
6. The counter instructions of PLCs are normally in the cascading of two counters.
retentive. Explain what this means. 11. a. When is the overflow bit of an up-counter
7. a. Compare the operation of a standard Examine- set?
on contact instruction with that of an off-to-on b. When is the underflow bit of a down-counter
transitional contact. set?
b. What is the normal function of a transitional 12. Describe two common applications for counters.
contact used in conjunction with a counter? 13. What determines the maximum speed of transitions
8. Explain how an OSR (one-shot rising) instruction that a PLC counter can count? Why?
can be used to freeze rapidly changing data.

CHAPTER 8 PROBLEMS

1. Study the ladder logic program in Figure 8-39, and d. Suppose your accumulated value is 24 and you
answer the questions that follow: lose ac line power to the controller. When power
a. What type of counter has been programmed? is restored to your controller, what will your ac-
b. When would output O:2/0 be energized? cumulated value be?
c. When would output O:2/1 be energized? e. Rung 4 goes true and, while it is true, rung 1
goes through five false-to-true transitions of rung
Ladder logic program conditions. What is the accumulated value of the
counter after this sequence of events?
I:1/1
CTU f. When will the count be incremented?
Rung 1 COUNT-UP COUNTER CU g. When will the count be reset?
Counter C5:1 DN
Preset 50 2. Study the ladder logic program in Figure 8-40, and
Accumulated 0 answer the questions that follow:
a. Suppose the input pushbutton is actuated from
C5:1/DN O:2/0 off to on and remains held on. How will the
Rung 2 status of output B3:0/9 be affected?
b. Suppose the input pushbutton is now released to
C5:1/DN O:2/1 the normally off position and remains off. How
Rung 3 will the status of output B3:0/9 be affected?

I:1/2 C5:1 3. Study the ladder logic program in Figure 8-41, and
Rung 4 Res answer the questions that follow:
a. What type of counter has been programmed?
b. What input address will cause the counter to
Figure 8-39 Program for Problem 1. increment?

Programming Counters Chapter 8 179

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