A PLC (programmable logic controller) is a ruggedized industrial computer that reads inputs from sensors, runs a user program, and drives outputs such as motors and valves in real time. It is the basic control device of most machines and lines.
A PLC runs a continuous scan cycle: read all inputs, execute the program, write all outputs, repeat. Scan times are short and predictable, which is why PLCs handle interlocks, sequencing, and control loops. NIST's definition lists I/O control, logic, timing, counting, PID control, communication, and arithmetic.
Hardware includes a CPU, input and output modules (digital and analog), communication ports, and a power supply, built for vibration, heat, and electrical noise. Programs are commonly written in the IEC 61131-3 languages, including ladder diagram and structured text.
PLCs are the source of most machine data: states, counts, alarms, and process values are stored as tags. That makes them central to any data project. It also makes them security-sensitive. Many were designed for isolated networks, and changing PLC logic on a running line requires strict change control. AI projects typically read from PLCs; they rarely write to them directly.
Questions about PLC
What is the difference between a PLC and SCADA?
A PLC runs the fast, closed-loop control of a machine or line. SCADA is supervisory: it collects data from PLCs and remote terminal units, raises alarms and sends setpoints or commands.
Do AI systems write to PLCs?
Rarely. AI projects typically read data from PLCs, which are the source of most machine data. Changing PLC logic on a running line requires strict change control.
PLC in real plants
- FedEx and HighByte: FedEx used HighByte Intelligence Hub to contextualize and standardize operational data from PLC systems, deploying data models across 41 sites in six months to support robotics and site automation.
Related terms
SCADA
SCADA (supervisory control and data acquisition) is a computerized system that gathers data from remote or distributed equipment and lets operators monitor and control it.
DCS (distributed control system)
A DCS (distributed control system) is a plant control system in which control functions are spread across many autonomous controllers near the equipment, linked by a network to central operator stations.
OPC UA
OPC UA (OPC Unified Architecture) is a platform-independent, open standard for exchanging industrial data securely between machines, controllers and software, published by the OPC Foundation and standardized as IEC 62541.
OT cybersecurity
OT cybersecurity is the protection of operational technology, the control systems and networks that run physical processes, against cyber threats, with safety, availability and reliable operation as the first priorities.
Industrial IoT (IIoT)
Industrial IoT (IIoT) is the network of sensors, instruments, and machines in industrial settings that are connected to computers and software to collect and use operational data.