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PLC

What is a Programmable Logic Controller, or PLC?
If you’ve used a computer, and you’ve used an Arduino, Raspberry Pi, or any number of other microcontrollers, you might think that a computer based on the physical world is a relatively new concept. A digital input causes some sort of conditional logic to make something blink, your robot to move, a signal to be transmitted, or any number of other things.
This type of logic has, in fact, been going on for quite some time in industry. In 1968, a man named Richard E. Morley proposed the Modular Digital Controller (MODICON) to a car manufacturer. This MODICON, or programmable logic controller (PLC), was used to replace complicated relays with virtual programmable relays, making machinery changes in industry much, much faster.
This later became the commercial MODICON 084, which led to many more manufacturers making their own version. Here’s an article on the history of the PLC if you’d like more background.
How do You Program One?
Computers and microcontrollers that you may be familiar with use a typed programming language to let them perform their tasks. You might assume, as I did when I first learned to program in this fashion, that a PLC uses something similar, like C, BASIC, or Python. It doesn’t (at least not in its most basic form); instead, it uses something called “ladder logic.”
This ladder logic looks exactly like it sounds, with a ladder-like screen with characters like “X1” with a normally open contact connected to an output labeled as, say “Y0” at the end of it. These ladder “rungs” are suspended between two rails which represent electrical power. Thus an electrician with a background in setting up relay logic circuits could, in theory, read this code without additional training.

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