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. It is the industrial form of the loop that robotics builds from sensors, an MCU and an MPU, with rugged I/O cards, a scan time and programs an electrician can read.

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. It is the industrial form of the loop that robotics builds from sensors, an MCU and an MPU, with rugged I/O cards, a scan time and programs an electrician can read.

A complete automated machine fits in one line of reasoning.

1Sensors feed the PLC2The PLC runs its logic3The outputs acta motor through a drive

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 PC runs a task whenever it has time; a PLC 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.

A machine-level PLC typically handles tens to a hundred or so inputs and outputs, and the plain scan is only its simplest mode: periodic tasks run a fast loop every 1 ms beside a slow one every 100 ms, and hardware interrupts react to an edge within microseconds without waiting for the scan (scan cycle).

Plants keep PLCs although a PC is far more powerful, and the reasons are not computational. A PLC runs a fixed cycle of 1 to 100 ms on a few watts, survives the cabinet for decades, follows the safety and programming standards the plant's procedures assume, and is understood by the maintenance technician at three in the morning, a property worth more than it sounds when every hour of unplanned stoppage costs money. Heavy numerical work is what it does badly; that goes to an IPC or beyond (embedded system).

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.