Sample/practice exam, questions and answers PDF

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PLC Practice Questions 1) Name the following logic gate functions, convert them to ladder logic diagrams, and write a truth table for each case: a)

b)

c)

d)

e)

f)

2) For each of the following ladder logic diagrams, draw timing diagrams to show when the output of the timer would go on and when it would go off: a)

30s

b)

30s

c)

40ms

d)

20ms

e)

20s

3) Complete the timing diagrams for the following ladder logic diagrams:

CNT

R

0

DIR R ON=OFF

R

CTUD

4) Design a ladder logic circuit for a motion detecting light system. The system uses a normally closed motion detection sensor (MOTION) for input. The output is an LED light (LED). The light should turn on if there is motion detected for at least 0.5 seconds, and should stay on for 10 seconds after the motion has ceased. The system also has a normally open activation switch (ACTIVE), which when open should keep the light off. 5) Design a ladder logic diagram for simple PLC based carpark monitoring/automation system. The system should count the number of cars entering and exiting the carpark, and light a red warning lamp to indicate when the carpark is full. There is an automatic boom gate at both the entry and exit of the carpark and these should open when a car is detected at the entry or exit. The boom gate at the entry should not open if the carpark is full. The capacity of the carpark is 2000 spaces. The inputs to the system are: - Two normally closed car detection sensors (CarEntering) and (CarExiting), to detect the presence of a car as it enters or as it exits respectively. The outputs of the system are: - Two outputs (BoomEntry) and (BoomExit) to control the boom gates (actuating the output opens the boom gate) - Carpark full warning lamp (CarparkFull)

6) A state machine can be described as follows. By default, the machine is in the “OFF” state. When a normally open pushbutton (PB) is pressed, the machine enters the “ON” state. While in the “ON” state, a red LED (LED) is flashed on and off at a rate of 1 Hz. When the pushbutton (PB) is pressed again (while in the “ON” state), the machine re-enters the “OFF” state. While in the “OFF” state, the LED should stay off. Draw a ladder diagram for this state machine.

7) A new pedestrian crossing (with traffic lights) needs to be installed at the front of the university. You are to develop a PLC ladder logic control solution for the crossing. The inputs to the system are: - Normally open activation buttons (Button1 and Button2) The outputs from the system are: - Three traffic light outputs (Red, Green and Amber) - Red ‘stop’ light output for pedestrians (Stop) - Green ‘walk’ light output for pedestrians (Walk) - Activation button indicators output (Indicators) - Safe to walk sound output (Sound) The system should work as follows: The initial state of the system is with the Green traffic lights on, and the pedestrian Stop light on. All other outputs should be off. While in the initial state, if either of the activation buttons (Button1 or Button2) is pressed, the activation button Indicators should turn on, the Green traffic lights should turn off, and the Amber traffic lights should turn on. This is state two. After three seconds in state two, the Amber traffic lights should turn off, the Red traffic lights should turn on, the Stop light should turn off, the Walk light and Sound outputs should turn on. This is state three. After 10 seconds in state three, the Walk light and Sound outputs should turn off, the Stop light should turn on, and the Activation button Indicators should turn off. This is state four. After 3 seconds in state four, the system should return to the initial state. The Activation buttons should have no effect for 30 seconds after the system transitions from the fourth to the initial state....