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What is a Programmable Logic Controller (PLC)?

By Dan Blacharski
Updated May 17, 2024
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The automation of many electromechanical processes, such as the movement of machinery on an assembly line, is done through the use of small computers called programmable logic controllers (PLCs). A PLC contains a programmable microprocessor that is programmed using a specialized computer language. Typically, the program for the automated process is written on a computer and then is downloaded onto the programmable logic controller directly through a cable connection. The program is stored in the programmable logic controller in non-volatile memory.

Inputs and Outputs

Programmable logic controllers typically contain a variable number of input/output (I/O) ports and usually employ reduced instruction set computing (RISC), which consists of simplified instructions that are intended to allow for faster execution. PLCs are designed for real-time use and often must withstand harsh factory environments, such as excessive vibration and high noise levels. The programmable logic controller circuitry monitors the status of multiple sensor inputs, which control output actuators such as motor starters, solenoids, lights, displays and valves.


This type of controller has made a significant contribution to factory automation. Earlier automation systems had to use thousands of individual relays, timers and sequencers, which had to be replaced or rewired whenever the automated process needed to change. In many cases, a programmable logic controller allows all of the relays and timers within a factory system to be replaced by a single controller. Modern PLCs deliver a wide range of functionality, including basic relay control, motion control, process control and complex networking. They also can be used in a distributed control system (DCS).


There are several types of interfaces that are used when people need to interact with programmable logic controllers to configure them or work with them. The interface might be configured with simple lights or switches, or it might include a text display. A more complex system might use an Internet-based interface on a computer running a supervisory control and data acquisition (SCADA) system.


PLCs were first created to serve the automobile industry. The first programmable logic controller project was developed in 1968 for General Motors to replace hard-wired relay systems with electronic controllers. PLCs have remained widely used in the early 21st century within manufacturing sectors such as the automobile industry.

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Discussion Comments
By anon313821 — On Jan 14, 2013

Does a PLC code need to be controlled as a document in an ISO 9001:2008 QMS?

By anon282605 — On Jul 30, 2012

PLC now is a common application for controlling processes and operations for industrial and domestic uses. Nice explanation about PLC.

By anon178454 — On May 21, 2011

why is PLC used in a hydropower plant?

By anon158615 — On Mar 07, 2011

Why don't we use higher end workstations instead of PLC?

By anon102518 — On Aug 08, 2010

Please tell me what is actual meaning of plc? And where is the institute of plc in Delhi (South Extension)?

By eddakecik — On Jul 17, 2010

Identify and elaborate on the advantages and disadvantages of each PLC type.

Include some pictures and diagrams show the differences between all three.

By anon89746 — On Jun 12, 2010

what is the difference between DCS and PLC?

By anon87263 — On May 29, 2010

1. What is the difference between PID(proportional integral and derivative) algorithm and PLC?

2. What is the relationship between PID and PLC?

By anon42252 — On Aug 20, 2009

Give me some basics of plc. for example, what is plc and its application and its working?

By anon37641 — On Jul 21, 2009

how to distingwish b/w different model of plc's of plc in a perticular make?

By anon22901 — On Dec 12, 2008

I might be interning at an electrical company this summer, and they do some work with PLC's. I want to be a computer or software engineer however, could interning at this electric company and working with PLC's help me in my hopeful field? Or should I just forget it and look for something else?

By anon10821 — On Apr 03, 2008

Either proximity switch can be OK...depending on program in PLC. Proximity must match what the program was written for.

By anon8112 — On Feb 07, 2008

what are the types of plcs and if i want to attached npn and pnp proximity then which type should i use?

By riswandoko — On Apr 23, 2007

I got the problem with old PLC. Th type is telemecaniq type TSX-7.

Any body have information regarding this soft ware, please let me know, where I have to buy or down load if possible.

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