Programmable Logic Controller (PLC): An Industrial Control System - Prof. Reid, Cheat Sheet of Engineering

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Programmable Logic Controller

(PLC)

What is a PLC?

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.

WHAT IS INSIDE A PLC?

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.

HOW DOES A PLC OPERATE?

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,