PLC

A programmable logic controller (PLC) is an industrial digital computer designed for the control of manufacturing processes or robotic devices. PLCs monitor inputs from sensors and switches, make logic decisions, and control outputs to actuators in real time. They are built to operate in harsh industrial environments, with robust enclosures, wide temperature ranges, and power protection.

PLCs emerged in the late 1960s to replace relay-based control systems for automobile assembly lines. The first PLCs were developed by Modicon (now Schneider Electric) and were used to reprogram logic without rewiring by altering the stored program. Over time, PLCs became distributed, modular, and networked, enabling complex automation across plants.

A typical PLC consists of a central processing unit, memory, and input/output modules (digital and analog). The CPU executes a user program stored in memory and performs the scan cycle: read inputs, execute the program, update outputs. Programs are commonly written in ladder logic, but IEC 61131-3 standard languages include ladder diagram, function block diagram, structured text, and sequential function charts.

PLCs connect to sensors and actuators via digital I/O and analog I/O, and communicate over industrial networks such as Ethernet/IP, PROFINET, Modbus, and Foundation Fieldbus. Safety PLCs provide fail-safe operations for critical processes and must meet safety integrity requirements.

Used widely in manufacturing, processing plants, packaging, water treatment, and building automation, PLCs range from compact micro PLCs to large modular systems. They are often complemented by programmable automation controllers (PACs), which add advanced computation and networking capabilities.