industrial//industrial automation//PLC

A **Programmable Logic Controller** is an industrial computer built to control machines and processes. Its architecture is a CPU, memory, inputs, outputs and communications: inputs receive information from sensors, outputs drive actuators.

A Programmable Logic Controller is an industrial computer built to control machines and processes. Its architecture is a CPU, memory, inputs, outputs and communications: inputs receive information from sensors, outputs drive actuators.

A complete automated machine in one line of reasoning: sensors feed the PLC, the PLC runs its logic, outputs act, and for a motor an output usually commands a variable frequency drive rather than the motor directly. The PLC decides what should happen; the drive solves the electrical control of the motor.

The logic runs in a fixed loop, the scan cycle, written in one of the IEC 61131-3 languages: Ladder for relay-like discrete logic, Function Block Diagram for process and control, Structured Text for algorithms.

A PLC is not a PC that "will do it when it has time". It executes logic with bounded timing, in milliseconds; for a machine it matters to know that a condition will get its response within a known interval. That is the meaning of determinism and the reason classic automation behaves so predictably.

PLCs dominate discrete automation. In large continuous processes the same role is played, in an integrated way, by a DCS. Toward the plant, the PLC talks fieldbuses and industrial Ethernet; toward the data layer, gateways and OPC servers read its tags.

A PLC is also a subsystem's boss, not its servant: a robot controller receives high-level commands from it (start program 17, part available, safe zone) and solves its own kinematics inside.