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What is a PLC

2016-06-22 13:43:21 Xiamen Haiwell Technology Co., Ltd. Read

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.

Almost any production line, machine function, or process can be greatly enhanced using this type of control system. However, the biggest benefit in using a PLC is the ability to change and replicate the operation or process while collecting and communicating vital information.

Another advantage of a PLC system is that it is modular. That is, you can mix and match the types of Input and Output devices to best suit your application.


The first Programmable Logic Controllers were designed and developed by Modicon as a relay re-placer for GM and Landis.

* These controllers eliminated the need for rewiring and adding additional hardware for each new configuration of logic.

* The new system drastically increased the functionality of the controls while reducing the cabinet space that housed the logic.

* The first PLC, model 084, was invented by Dick Morley in 1969

* The first commercial successful PLC, the 184, was introduced in 1973 and was designed by Michael Greenberg.



The Central Processing Unit, the CPU, contains an internal program that tells the PLC how to perform the following functions:

* Execute the Control Instructions contained in the User's Programs. This program is stored in "nonvolatile" memory, meaning that the program will not be lost if power is removed

* Communicate with other devices, which can include I/O Devices, Programming Devices, Networks, and even other PLCs.

* Perform Housekeeping activities such as Communications, Internal Diagnostics, etc.


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.

Four Steps In The PLC Operations

1.) Input Scan

Detects the state of all input devices that are connected to the PLC

2.) Program Scan

Executes the user created program logic

3.) Output Scan

Energizes or de-energize all output devices that are connected to the PLC.

4.) Housekeeping

This step includes communications with programming terminals, internal diagnostics, etc...