Programmable logic controllers (PLCs) are one of the primary forms of manufacturing automation. They are used to replace relays, control analog and digital open- and closed-loop systems, and control manufacturing cells in islands of programmable automation, flexible manufacturing, and computer-integrated manufacturing. The PLC, regardless of size, speed, and memory, has components similar to any computer, that is, input modules, central processing unit (CPU), memory, and output modules. The CPU is composed of a processor, memory, and power supply. The I/O modules are the interface between the physical inputs and outputs and the CPU. Historically, the programming language common to PLCs has been relay ladder logic (RLL), the language used with relay control systems.
By Gary Conner
By George N Bullen FSME