Programmable Logic Controller – PLC Questions with Brief Answers

Programmable Logic Controller Questions with Brief Explanatory answers.

What is programmable logic controller

A programmable logic controller or PLC is a general-purpose controller that can be programmed or instructed by the end-user. A PLC may be instructed to do almost anything with the signals it receives from the input device. In the beginning, PLCs were introduced to the industry as electronic replacements for electromechanical relay controls.

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What are some devices that might be connected to input of PLCs

Typical devices connecting to a PLC’s inputs include hand switches, process switches, sensors, analog transmitters (4-20 mA), thermocouples, thermistors, and strain gauges.

What are some devices that might be connected to input of PLCs

Typical devices connecting to a PLC’s outputs include electric lamps, solenoids, relay coils, motor contactors, analog final control elements (e.g. throttling control valves, variable-speed motor drives), and audible buzzers.

Can we provide analog input to PLCs

While PLCs were originally designed for discrete (on/off) control applications such as conveyor belt management, batch sequencing, and assembly line controls, modern PLCs are equally capable of inputting and outputting analog signals as well.

What do you mean by Input/Output (I/O) capabilities

Every programmable logic controller must have some means of receiving and interpreting signals from real-world sensors such as switches, and encoders, and also be able to effect control over real-world control elements such as solenoids, valves, and motors. This is generally known as input/output, or I/O, capability.

What do you mean by monolithic or brick PLCs

Small PLC systems consist of a monolithic “brick” containing all processor, I/O, and communication functions. These PLCs are typically far less expensive than their modular cousins, but are also more limited in I/O capability and must be replaced as a whole in the event of failure.

What do you mean by modular or rack PLCs

These are the large sized PLC systems consist of a rack into which circuit “cards” are plugged. These cards include processors, input and output (I/O) points, communications ports, and other functions necessary to the operation of a complete PLC system. Such “modular” PLCs may be configured differently according to the specific needs of the application. Individual card failures are also easier to repair in a modular system, since only the failed card need be replaced, not all the cards or the whole card rack.

What is discrete data point

A “discrete” data point is one with only two states on and off.

WREF PLC What does counter implies

A counter is a PLC instruction that either increments (counts up) or decrements (counts down) an integer number value when prompted by the transition of a bit from 0 to 1 (“false” to “true”).

What are three basic types of Counter Instructions

Counter instructions come in three basic types: up counters, down counters, and up/down counters. Both “up” and “down” counter instructions have single inputs for triggering counts, whereas “up/down” counters have two trigger inputs: one to make the counter increment and one to make the counter decrement.

Article from Lessons In Industrial Instrumentation by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License

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