Electronic computers, designed for durability in industrial environments, that are used for the control of machines and manufacturing processes through the implementation of specific functions such as logic, sequencing, timing, counting, and arithmetic. They are also known as programmable logic controllers (PLCs). Historically, process control of a single or a few related devices has been implemented through the use of banks of relays and relay logic for both the control of actuators and their sequencing.

The advent of small, inexpensive microprocessors and single-chip computers, or microcontroller units, brought process control from the age of simple relay control to one of electronic digital control while neither losing traditional design methods such as relay ladder diagrams nor restricting their programming to that single paradigm. The computational power of programmable controllers and their integration into networks has led to their having capabilities approaching those of distributed control systems, and plantwide control is now a mixture of distributed control systems and programmable controllers.

Applications for programmable controllers range from small-scale, local process applications in which as few as 10 simple feedback control loops are implemented, up to large-scale, remote supervisory process applications in which 50 or more process control loops spread across the facility are implemented. Typical applications include batch process control and materials handling in the chemical industry, machining and test-stand control and data acquisition in the manufacturing industry, wood cutting and chip handling in the lumber industry, filling and packaging in food industries, and furnace and rolling-mill controls in the metal industry.