A PLC (Programmable Logic Controller) is a ruggedised industrial computer used to automate machinery and process control. PLCs sit at the heart of factory automation, water treatment plants, building management systems, energy infrastructure, and any environment where deterministic, real-time control of physical equipment is needed.

In short: A PLC reads inputs from sensors (digital, analogue, or networked), executes a control program, and drives outputs to actuators, motors, valves, or relays. PLCs are designed for industrial conditions: wide temperature ranges, electrical noise immunity, long operational lifetimes, and predictable performance. Major vendors include Siemens (S7 series), Rockwell Automation (Allen-Bradley ControlLogix and CompactLogix), Mitsubishi, Schneider Electric, Beckhoff, and Omron.

PLCs typically communicate over industrial protocols rather than general-purpose IT protocols. Modbus, Profinet, EtherNet/IP, OPC UA, and increasingly MQTT are all common. The choice depends on vendor and application.

In an IoT context, a PLC is often the device a cellular router is connected to. The router provides remote access to the PLC for engineers (typically via a VPN), forwards process data to cloud platforms, and provides connectivity for remote firmware updates. For sites without fixed-line broadband, the router is the only way to monitor or manage the PLC remotely. This pattern is common in water utilities, energy infrastructure, transport telemetry, and remote industrial sites.