Sample PLC Exam Problems Problem 1 1- Using the relay, timer, counter, pulse, and set/reset instructions in your handout, write a PLC ladder diagram program that will use the following I/O points: Address Name Description A momentary closure of SW-0 should be enough to Inputs start the following sequence: X0 SW-0 Normally open 1. This should be repeated five times: switch a) All three red lights (Y0, Y2, Y4) on for 0.5 sec. X1 SW-1 Normally open b) All three green lights (Y1, Y3, Y5) on for 0.5 sec. switch 2. The first red light (Y0) on for 3.0 sec. Outputs 3. The first green light (Y1) on for 2.5 sec. Y0 LT-0 Red light 4. The second red light (Y2) on for 2.0 sec. Y1 LT-1 Green light 5. The second green light (Y3) on for 1.5 sec. Y2 LT-2 Red light 6. The third red light (Y4) on for 1.0 sec. Y3 LT-3 Green light 7. The third green light (Y5) on for 0.5 sec. Y4 LT-4 Red light 8. All lights off for one second Y5 LT-5 Green light This entire sequence should be repeated three times, after which all lights should remain off. Once the sequence is running, a momentary closure of SW-1 should stop the sequence and all lights should go off. Once the sequence has been stopped with SW-1 or has been completed, a momentary closure of SW-0 should start the entire sequence from the beginning.
Problem 2 A PLC has three inputs, and one output. Inputs X1 and X2 are momentary contact normally open push button switches. X3 is a normally closed push button switch. Output Y1 is a coil for the control circuit of a meat grinder motor. For safety, the PLC is to be programmed so that a person must be pressing both X1and X2 (one with each hand) in order to start the grinder. The grinder is to remain running once both buttons are pressed and released. The grinder is to stop if X3 is momentarily pressed. Output Y2 is to operate exactly the opposite of Y1. Draw the PLC ladder diagram.
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Problem 3 A 2-pole, NO relay switches power to a 240-volt single-phase load. The relay has a 120-volt coil. The load is controlled by two SPST position switches. The load is to be off only when both switches are open. Draw a ladder diagram of the coil and load circuits.
Problem 4 1. How does the PLC execute a ladder program? What is a “program scan”? 2. How is the problem-solving process different when programming in assembly language versus programming in ladder logic? 3.
Problem 5 Convert the following ladder diagram mode to instruction mode.
Solution:
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Problem 6 Write a ladder logic program that does what is described below. - When push button (X0) is pushed, a light will flash for 10 times. - The flashing light (Y0) will be on for 2sec and off for 1 sec. - By pressing button X1 or If button (X0) has been pushed for 5 times the system will be reset.
Solution:
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Problem 7 Design a garage door controller using an SFC. The behavior of the garage door controller is as follows, - there is a single button (X0) in the garage, and a single button (X1) remote control. - when the button is pushed the door will move up (Y0) or down (Y1). - if the button is pushed once while moving, the door will stop, a second push will start motion again in the opposite direction. - there are top (X4) / bottom (X5) limit switches to stop the motion of the door. - there is an optical sensor (X3) at the bottom of the door. If any object is detected by the sensor while the door is closing the door will stop and reverse.
Solution: OUT Y0
OUT Y1
OUT Y0
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Problem 8 Draw a ladder diagram for [Pencil Length Determination System]. The pencil of new product will be send to a transmission belt for length determination. The transmission belt motor is connected with a photoelectric sensor (X1), So when the motor is rotating then the photoelectric sensor will send out a pulse. Furthermore there is a another photoelectric sensor(X0) has be set upon belt. When a pencil is detecting by the photoelectric sensor(X0) then X0=ON. When the pencil has removed then X0=OFF. Thus the pulse number which generated by X1 between X0=ON to X0=OFF equals pencil length L when 7 =< L < 12 :Good product : And the others are bad product. The good product can pass and the bad product will be eliminated by the cylinder (Y0). The cylinder has a limit switch (X2) indicates that the cylinder in advanced state.
