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  • David Peterson

Back to Basics - What is a PLC?

Until the late 1960’s, the hard-wired circuits were the only option for industrial control.


These were often confusing and difficult to construct and troubleshoot. They were also very slow, at least compared to a computer.


The coil-activated relay devices consumed a lot of power and took a long time to activate.


Following those late 60’s and early 70’s, the Programmable Logic Controller, or PLC, began to affect the automotive industry, replacing huge banks of relay circuits - hundreds or thousands of relays that had to work perfectly for a facility to operate.



During the early development of computers, they were still very expensive and difficult to program. But that has changed dramatically; progressing through the first decades of the new millennium when technology is at our fingertips.


The basic construction of a PLC consists of the following pieces:


1 - A Processor, or CPU.

Usually, it's a 16 or 32-bit single-core microprocessor. They usually run about 16 MHz (about 100-150 times slower than your laptop) with storage on the order of a few MB (thousands of times smaller than your laptop hard drive)


It seems pretty pathetic when compared to a modern computer, but think about it - we're controlling motors and coils, we don't really need to store complex video, music or picture files, and we don't need to process massive graphic, audio, or video instructions. Simple = reliable.


2 - A Power Supply

There are two normal power supply types -

Often they are supplied by a single-phase line cord coming from a normal 110v receptacle. This is usually a separate block on the side of the PLC, and it converts the AC down to the proper DC voltages.


Alternatively, the PLC may need to be supplied with a common industrial DC 24 volt supply. Every PLC will have an indicator of what supply is needed.


3 - Input / Output Connections

If you want to talk to the real world, you need to have a connection point. This is almost certain to be a set of screw or spring terminals for wires. Sensors have wires, and so do relay coils, motor starters, buttons, switches, and even other PLCs attached to other devices.


Inputs and Outputs, abbreviated I/O, can be grouped into just a couple categories

  • Discrete inputs and outputs work with on/off signals only, like a switch.

  • Analog inputs and outputs use signals with wider ranges of variable values like speed, temperature, pressure, etc.

4 - Communication

It's hard to imagine a machine working all by itself these days. Everything seems to communicate with some other device or machine center nearby. Even a small network of devices, like a motor drive and a touch-screen interface, must be connected to the PLC. This is the function of communications.


The most common type of interface is an Ethernet network, indicated by the familiar RJ45 8-pin square connector. Among the other common connections are USB, serial, RS-485, Modbus, Profibus, DeviceNet, and many others! Communication is very important to PLCs!



These devices are found everywhere, in every imaginable kind of industry. The skills to wire and program them, and also the ability to troubleshoot problems are invaluable. The skills are not hard to learn, but it is important to find the right training source.


Not only is it important, but with the right kind of training, it's a fun and exciting field to learn!

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