PLC Basics

programmable logic controller

Introduction to Programmable Controllers

It’s always good to get an overview of where designs have been and where they are going.  To do this it’s essential to get a bird’s eye view of the concepts and processes that make the PLC so valuable in industrial control.  Pitting PLCs against other control types will also serve to show the pros and cons for different applications.

Definition of a PLC

How PLCs Work

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 “software” 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, labour 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 switches and lights (and a host of other devices) all interacting with each other this kind of flexibility is not only nice but imperative.  Hopefully, a light bulb has now turned on over your head.

How PLC’s Work

A programmable logic controller is a specialized computer used to control machines and processes.  It, therefore, shares common terms with typical PCs like the central processing unit, memory, software and communications.  Unlike a personal computer through the PLC is designed to survive in a rugged industrial atmosphere and to be very flexible in how it interfaces with inputs and outputs to the real world.

The components that make a PLC work can be divided into three core areas.

  • The power supply and rack
  • The central processing unit (CPU)
  • The input/output (I/O) section

PLCs come in many shapes and sizes.  They can be so small as to fit in your shirt pocket while more involved controls systems require large PLC racks.  Smaller PLCs (a.k.a. “bricks”) are typically designed with fixed I/O points.  For our consideration, we’ll look at the more modular rack-based systems.  It’s called “modular” because the rack can accept many different types of I/O modules that simply slide into the rack and plug in.


PLC angled with doors closed and text


The Power Supply and Rack

So let’s start off by removing all our modules which leaves us with a naked PLC with only the power supply and the rack. 


PLC empty rack with text

The rack is the component that holds everything together.  Depending on the needs of the control system it can be ordered in different sizes to hold more modules.  Like a human spine, the rack has a backplane at the rear which allows the cards to communicate with the CPU.  The power supply plugs into the rack as well and supplies a regulated DC power to other modules that plug into the rack.  The most popular power supplies work with 120 VAC or Read More…


Course Credits: PLC dev

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