Definition of a PLC

What is a PLC? 

A Programmable Logic Controller, or PLC for short, is simply a special computer device used for industrial control systems. They are used in many industries such as oil refineries, manufacturing lines, conveyor systems and so on. Where ever there is a need to control devices the PLC provides a flexible way to "softwire" the components together.

The basic units have a CPU (a computer processor) that is dedicated to run one program that monitors a series of different inputs and logically manipulates the outputs for the desired control.  They are meant to be very flexible in how they can be programmed while also providing the advantages of high reliability (no program crashes or mechanical failures), compact and economical over traditional control systems.

A Simple Example 

Consider something as simple as a switch that turns on a light.  In this system with a flick of the switch the light would turn on or off.  Beyond that though there is no more control.  If your boss came along and said I want that light to turn on thirty seconds after the switch has been flipped, then you would need to buy a timer and do some rewiring.  So it is time, labor and money for any little change.

00-01 switch to light

A PLC Saves the Day 

Now consider the same device with a PLC in the middle.  The switch is fed as an input into the PLC and the light is controlled by a PLC output.  Implementing a delay in this system is easy since all that needs to be changed is the program in the PLC to use a delay timer.
00-02 switch to plc to light


This is a rather simple example but in a larger system with many switchs and lights (and a host of other devices) all interacting with each other this kind of flexibility is not only nice but imperitive.  Hopefully a light bulb has now turned on over your head.


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plc how to troubleshoot

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