A programmable logic controller (PLC) is a ruggedized industrial digital computer designed to execute control logic for machines, processes, and production lines. It operates continuously in a cyclic manner, reading input signals, executing a user-defined control program, and updating output signals.
In the context of ISA‑95 and industrial automation, a PLC typically resides at Level 1 (basic control) of the hierarchy. It interfaces directly with field devices such as sensors, switches, and actuators, and exposes process data and status information to higher-level systems like SCADA, DCS, or MES.
Operationally, a PLC:
- Receives digital or analog input signals from field devices.
- Processes these inputs according to programmed logic (e.g., ladder logic, function block diagrams, structured text).
- Generates corresponding digital or analog outputs to control actuators and other equipment.
- Communicates with other PLCs and supervisory systems over industrial networks and protocols.
PLCs are configured and maintained using vendor-specific programming and diagnostic tools, and their control programs are typically stored in non-volatile memory for continuous operation.