Those in the control industry, and those looking for control solutions, bandy the term PLC about regularly, but what is a PLC?

PLC is the acronym for Programmable Logic Controller. Okay, we can see where the PLC comes from, but then what is a programmable logic controller?

A PLC is a computer similar in concept to, but different from, a desk top or laptop computer. It has internal programs, data storage and input devices like the desktop or laptop computer, but it provides more capabilities than just those basic computers.

The PLC will have the ability to accept incoming information from a variety of devices on a piece of machinery, not just from the mouse or keyboard found on the home computer. The incoming information might be from push buttons, timers, toggle switches, proximity switches; the incoming signal can be from many different sources and these sources can feed information to the PLC through the PLC’s Inputs.

Once the PLC receives an input from somewhere, it executes the control program that has been programmed into it by the PLC programmer. This program then may create an output signal of some sort.

The Outputs are part of the PLC as well, and allow a broad range of equipment to be connected to them. As the CPU executes the program, it will send information to the correct PLC Output (if it’s been programmed correctly), and that output signal will then cause some other external device on the machine to function.

Even though the PLC has a central CPU (central processing unit) just like the home computer, it differs by having more than just two inputs (mouse & keyboard), and it differs by being able to send control commands out from itself to initiate action somewhere else on a machine.

PLC’s are often quite small (in comparison to their P.C. cousins) though those with large numbers of Inputs or Outputs (I.O. points) can get quite large. They are installed inside of a control panel enclosure on or near the machine they control. Wires leading from the various Input devices to the PLC, and those from the Output’s of the PLC, enter or exit the enclosure through holes drilled in the enclosure wall.