Glossary · PLC
What is Programmable logic controller (PLC)?
An industrial computer built to control machines and processes: it reads sensors and switches, runs the control program, and drives motors and valves, repeatedly and reliably.
It runs a fixed scan cycle: read inputs, execute the program, write outputs. It tolerates heat, vibration and electrical noise and runs for years without stopping.
It is programmed in IEC 61131-3 languages such as ladder, Structured Text and function block diagram. Well-known makers include Siemens, Allen-Bradley and Schneider.
Related terms
- Structured Text (ST)
- A textual programming language in IEC 61131-3 resembling Pascal, used for complex PLC logic, calculations and state machines.
- PID controller
- A control algorithm that computes its output from the difference between the setpoint (SP) and the measured value (PV), in three parts: proportional, integral and derivative.
- SCADA
- Supervisory control and data acquisition: collects readings from distributed controllers and sensors, presents them to operators, and manages alarms, commands and history.