What is a PLC?

The PLC, or Programmable Logic Controller, is a computer with a single mission.  Most commonly used in industrial applications, it usually lacks a monitor, keyboard, and a mouse, as it is normally programmed to operate a machine or system using but one program.  The machine or system user rarely, if ever, interacts directly with the PLC's program.  When it is necessary to either edit or create the PLC program, a personal computer is usually (but not always) connected to it.

The PLC has a remarkable capacity for real-world I/O connectivity.  Most PLC models feature a vast assortment of interchangeable I/O modules which allow for convenient interfacing with virtually any kind of industrial or laboratory equipment.

Most PLCs are programmed in a special language called Ladder Logic.  Ladder Logic is essentially a Boolean logic-solving program with a graphical user interface designed to look like an elementary wiring diagram, familiar to all industrial electricians.