Mechatronics Eng.
PLC Course
ME554: Programmable Logic Control
(PLC)
Selected Problems
Prepared By: Dr. Hani Muhsen
7/24/2019 Dr. Hani Muhsen 1
Mechatronics Eng. PLC Course
Problem 8.1
Study the ladder logic program in Figure 8-37 , and answer the questions that follow:
a. What type of counter has been programmed?
b. When would output O:2/0 be energized?
c. When would output O:2/1 be energized?
d. Suppose your accumulated value is 24 and you lose ac line power to the controller. When power is restored to your
controller, what will your accumulated value be?
e. Rung 4 goes true and while it is true, rung 1 goes through five false-to-true transitions of rung conditions. What is
the accumulated value of the counter after this sequence of events?
f. When will the count be incremented?
g. When will the count be reset?
7/24/2019 Dr. Hani Muhsen 2
Mechatronics Eng. PLC Course
Problem 8.1
7/24/2019 Dr. Hani Muhsen 3
Mechatronics Eng. PLC Course
Solution of 8.1
a) Up counter
b) When the accumulated count is equal to or greater than the preset count
c) When the accumulated count is less than the preset count
d) 024
e) Zero
f) Provided that rung 4 is false, each false-to-true transition of rung I increases the
counter’s accumulated value by I.
g) Whenever rung 4 is true
7/24/2019 Dr. Hani Muhsen 4
Mechatronics Eng. PLC Course
Problem 8.2
Study the ladder logic program in Figure 8-38 ,
and answer the questions that follow:
a. Suppose the input pushbutton is actuated from
off to on and remains held on. How will the status
of output B3:0/9 be affected?
b. Suppose the input pushbutton is now released
to the normally off position and remains off. How
will the status of output B3:0/9 be affected?
7/24/2019 Dr. Hani Muhsen 5
Mechatronics Eng. PLC Course
Solution of Problem 8.2
a) It stays on for one scan and then goes off.
b) It remains off during this transition.
7/24/2019 Dr. Hani Muhsen 6
Mechatronics Eng. PLC Course
Problem 8.3
Study the ladder logic program in Figure 8-39 , and answer the
questions that follow:
a. What type of counter has been programmed?
b. What input address will cause the counter to increment?
c. What input address will cause the counter to decrement?
d. What input address will reset the counter to a count of zero?
e. When would output O:6/2 be energized?
f. Suppose the counter is first reset, and then input I:2/6 is actuated 15
times and input I:3/8 is actuated 5 times. What is the accumulated
count value?
7/24/2019 Dr. Hani Muhsen 7
Mechatronics Eng. PLC Course
Solution of Problem 8.3
a) Up/down counter
b) 1:2/6
c) 1:3/8
d) 1:4/1
e) When the accumulated count is equal to or greater than the preset count
f) 10
7/24/2019 Dr. Hani Muhsen 8
Mechatronics Eng. PLC Course
Problem 8.4
Design a PLC program and prepare a typical I/O connection diagram and ladder logic program for the following
counter specifications:
• Counts the number of times a pushbutton is
closed.
• Decrements the accumulated value of the counter
each time a second pushbutton is closed.
• Turns on a light anytime the accumulated value of the counter is less than 20.
• Turns on a second light when the accumulated value of the counter is equal to or greater than 20.
• Resets the counter to 0 when a selector switch is closed.
7/24/2019 Dr. Hani Muhsen 9
Mechatronics Eng. PLC Course
Solution of Problem 8.4
7/24/2019 Dr. Hani Muhsen 10
Mechatronics Eng. PLC Course
Problem 8.6
Design a PLC program and prepare a typical I/O connection diagram and ladder logic program that will
correctly execute the industrial control process in Figure 8-40 . The sequence of operation is as follows:
• Product in position (limit switch LS1 contacts close).
• The start button is pressed and the conveyor motor starts to move the product forward toward position A
(limit switch LS1 contacts open when the actuating arm returns to its normal position).
• The conveyor moves the product forward to position A and stops (position detected by 8 off to-on output
pulses from the encoder, which are counted by an up-counter).
• A time delay of 10 s occurs, after which the conveyor starts to move the product to limit switch LS2 and
stops (LS2 contacts close when the actuating arm is hit by the product).
7/24/2019 Dr. Hani Muhsen 11
Mechatronics Eng. PLC Course
Problem 8.6
7/24/2019 Dr. Hani Muhsen 12
Mechatronics Eng. PLC Course
Problem 9.1
Answer the questions, in sequence, for the MCR program in Figure 9-30 , assuming the program has just been
entered and the PLC is placed in the RUN mode with all switches turned off.
a. Switches S2 and S3 are turned on. Will outputs PL1 and PL2 come on? Why?
b. With switches S2 and S3 still on, switch S1 is turned on. Will output PL1 or PL2 or both come on? Why?
c. With switches S2 and S3 still on, switch S1 is turned off. Will both outputs PL1 and PL2 deenergize? Why?
d. With all other switches off, switch S6 is turned on. Will the timer time? Why?
e. With switch S6 still on, switch S5 is turned on. Will the timer time? Why?
f. With switch S6 still on, switch S5 is turned off. What happens to the timer? If the timer was an RTO type
instead of a TON, what would happen to the accumulated value?
7/24/2019 Dr. Hani Muhsen 13
Mechatronics Eng. PLC Course
Problem 9.1
7/24/2019 Dr. Hani Muhsen 14
Mechatronics Eng. PLC Course
Solution of Problem 9.1
a) No, because the MCR instruction of rung 1 is false
b) Both outputs PL1 and PL2 will come on because rung 1, 2, and 3 are now all true.
c) Only output PL1 will de-energize; output PL2 remains energized because it is a retentive output
and as such it remains in its last state (on) when the MCR instruction goes false.
d) No, because the MCR instruction of rang 6 is false
e) Yes, because both rungs 6 and 7 are now true
f) It stops timing. The accumulated value would be maintained because it is retentive on an RTO
timer.
7/24/2019 Dr. Hani Muhsen 15
Mechatronics Eng. PLC Course
Problem 9.2
Answer the questions, in sequence, for the jump-to label program in Figure
9-31 . Assume all switches are turned off after each operation.
a. Switch S3 is turned on. Will output PL1 be energized? Why?
b. Switch S2 is turned on first, then switch S5 is turned on. Will output PL4
be energized? Why?
c. Switch S3 is turned on and output PL1 is energized. Next, switch S2 is
turned on. Will output PL1 be energized or de-energized after turning on
switch S2? Why?
d. All switches are turned on in order according to the following sequence:
S1, S2, S3, S5, S4. Which pilot lights will turn on?
7/24/2019 Dr. Hani Muhsen 16
Mechatronics Eng. PLC Course
Solution of Problem 9.2
a) Yes, because rung 3 has logic continuity and the IMP instruction is false
b) No, because the JMP instruction is true; therefore, the rung will not be scanned by the
processor
c) Energized, because it will remain in its last state when the JMP instruction is energized
d) PL2 and PL3
7/24/2019 Dr. Hani Muhsen 17
Mechatronics Eng. PLC Course
Problem 9.3
Answer the questions, in sequence, for the jump to-
subroutine and return program in Figure 9-32 . Assume all
switches are turned off after each operation.
a. Switches S1, S3, S4, and S5 are all turned on. Which pilot
light will not be turned on? Why?
b. Switch S2 is turned on and then switch S4 is turned on.
Will output PL3 be energized? Why?
c. To what rung does the RET instruction return the
program scan?
7/24/2019 Dr. Hani Muhsen 18
