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Question-1. What is full form of PLC?
Answer-1: Programmable logic controller.
Question-2. What is the purpose of PLC?
Answer-2: The purpose of a PLC was to directly replace electromechanical relays as logic elements, substituting instead a solid-state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks.
Question-3. What is a PLC?
Answer-3: A Programmable Logic Controller (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.
Question-4. How Does A PLC Operate?
Answer-4: There are four basic steps in the operation of all PLCs; Input Scan, Program Scan, Output Scan, and Housekeeping. These steps continually take place in a repeating loop.
Question-5. What is the function of PLC?
Answer-5: A PLC consists of a Central Processing Unit (CPU) containing an application program and Input and Output Interface modules, which is directly connected to field I/O devices. The program controls the PLC so that when an input signal from an input device turns ON, the appropriate response is made. The response normally involves turning ON an out put signals to some sort of output devices.
Question-6. What is the function of CPU?
Answer-6: The central processing unit (CPU) is the part of a programmable controller that retrieves, decodes, stores, and processes information. It also executes the control program stored in the PLC?s memory.
Question-7. Explain Execution process of PLC?
Answer-7: All PLC have four basic stages of operations that are repeated many times per Second. Initially when turned on the first time it will check it?s own hardware and software for faults. If there are no problems it will copy all the input and copy their values into memory, this is called the input scan. Using only the memory copy of the inputs the ladder logic program will be solved once, this is called the logic scan. While solving the ladder logic the output values are only changed in temporary memory. When the ladder scan is done the outputs will updated using the temporary values in memory, this is called the output scan. The PLC now restarts the process by starting a self check for faults.
Question-8. What are the advantages of PLC?
Advantages of PLC control
Rugged and designed to withstand vibrations, temperature, humidity, and noise.
Have interfacing for inputs and outputs already inside the controller.
Easily programmed and have an easily understood programming language.
Question-9. What are the disadvantages of PLC?
Answer-9: Disadvantages of PLC control
Too much work required in connecting wires.
Difficulty with changes or replacements.
Difficulty in finding errors; requiring skillful work force.
When a problem occurs, hold-up time is indefinite, usually long.
Question-10. What is ladder logic?
Answer-10: Ladder logic can be thought of as a rule-based language, rather than a procedural language. A "rung" in the ladder represents a rule. When implemented with relays and other electromechanical devices, the various rules "execute" simultaneously and immediately. When implemented in a programmable logic controller, the rules are typically executed sequentially by software, in a loop. By executing the loop fast enough, typically many times per second, the effect of simultaneous and immediate execution is obtained.
Question-11. What are the functions of left and right vertical lines?
Answer-11: The left vertical line of a ladder logic diagram represents the power or energized conductor. The output element or instruction represents the neutral or return path of the circuit. The right vertical line, which represents the return path on a hard-wired control line diagram, is omitted. Ladder logic diagrams are read from left-to-right, top-to-bottom.
Question-12. What normally open (examine if open) Contact means?
Answer-12: Normally Open Contact means at normal condition the contacts are open (OFF). So, if we apply electrical supply in present state, it will not operate. The moment you apply an external source (electrical or, mechanical) to it, the contacts are in close (ON) state.
Question-13. What Normally Closed (examine if closed) Contact means?
Answer-13: In normal condition this component will be close (ON), thereby, electrical supplies flow through the circuit.
Question-14. What is Latch bit and Unlatch bit?
Answer-14: L (Latch Bit): Latch bit is used to set the output high until it is not reset. U (Unlatch Bit): Unlatch bit is used to reset the output low until it is set.
Question-15. What is the function of TON?
Answer-15: This instruction counts time when the enabling input is on. When the accum value is greater than or equal to the preset value timer bit is on. The on delay timer current value is cleared when the enabling input is off. This timer is continues counting after preset is reached.
Question-16. What is the function of counter in PLC?
Answer-16: Counters used in PLCs Serve functions similar to mechanical counters. They compare accumulated values with preset values in order to control circuit functions.
Question-17. How many types of counters are there?
Answer-17: There are two types of counters which can be used for particular application:
Up Counters (CTU)
Down Counters (CTD)
Question-18. Why PLC language is designed?
Answer-18: PLC languages are designed to emulate the popular relay ladder diagram format. This format is read and understood worldwide by maintenance technicians as well as by engineers.
Question-19. What is the function of down counter?
Answer-19: The down counter counts down from the preset value (PV) each time CD transitions from logic 0 to logic 1. When the current value is equal to Zero, the counter output bit (O) turns on.
Question-20. What are various inputs used for PLC?
Answer-20: Examples of inputs are push buttons, switches, toggle switches, limit switches, proximity switches, and contact closures, sensors.
Question-21. What are various outputs used from PLC?
Answer-21: Lights, horns, motors, and valves are all good examples of output devices
Question-22. Which types of scanning is used in Execution process of PLC?
Answer-22: Self Test, Input Scan, Logic Solve/Scan, Output Scan
Question-23. What do you mean by input scan?
Answer-23: When the inputs to the PLC are scanned the physical input values are copied into memory.
Question-24. What do you mean by output scan?
Answer-24: When the outputs to a PLC are scanned they are copied from memory to the physical outputs.
Question-25. What is the function of opto coupler?
Answer-25: This electrically isolates the external electrical circuitry from the internal circuitry.
Question-26. What is the internal structure of PLC?
Answer-26: PLCs have internal clocks and "watch-dog timers" built into their operations to ensure that some functional operation does not send the central processor into the "weeds". The first priority of the CPU is to scan the I/O for status, to make sequential control decisions (as defined by the program), to implement those decisions, and to repeat this procedure all within the allotted scan time.
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