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.
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.
HISTORY OF PLCS
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.
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,
internal diagnostics, etc...
