PLC Questions and Answers for Viva

Answer-1: Programmable logic controller.

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.

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.

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.

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.

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.

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.

Answer-8:
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.

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.

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.

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.

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.

Answer-13: In normal condition this component will be close (ON), thereby, electrical supplies flow through the circuit.

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.

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.

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.

Answer-17: There are two types of counters which can be used for particular application:
Up Counters (CTU)
Down Counters (CTD)

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.

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.

Answer-20: Examples of inputs are push buttons, switches, toggle switches, limit switches, proximity switches, and contact closures, sensors.

Answer-21: Lights, horns, motors, and valves are all good examples of output devices

Answer-22: Self Test, Input Scan, Logic Solve/Scan, Output Scan

Answer-23: When the inputs to the PLC are scanned the physical input values are copied into memory.

Answer-24: When the outputs to a PLC are scanned they are copied from memory to the physical outputs.

Answer-25: This electrically isolates the external electrical circuitry from the internal circuitry.

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.