A programmable logic controller (PLC) is a relatively small industrial computer used to automate real-time processes - controlling machines or production lines in a factory. It is characteristic for PLC that the program is executed in so-called cycles. In the modern sense, PLC is replaced by PAC (Programmable Automation Controller), although PLC designation is widely used worldwide and will continue to exist.

PLCs are different from conventional computers not only because they process the program cyclically, but also because their peripherals are directly adapted for connection to technological processes. The majority of peripherals in this case are digital inputs (DI) and digital outputs (DO). Analog inputs (AI) and analog outputs (AO) for continuous signal processing are designed for further signal processing and technology connection. With the advancement of industrial automation, other PLC-connected peripheral modules are also called function modules (FM) for positioning, communication processors (CP) for data acquisition and transmission, and other specific modules depending on the manufacturer of the particular system.

[Source: www.wikipedia.org]