Programmable-logic controllers and operation

Programmable-logic controllers are microprocessor-based systems that receive input signals from electromechanical sensors, process the input signals, and then send output signals to perform control or monitoring functions. PLCs perform real-time control of plant systems. Essentially, the PLC has many similarities to personal computers, having a dc power supply, read-only memory (ROM), random-access memory (RAM), and a central-processor-unit chip (CPU), and it has the ability to communicate with peripherals, such as human-machine interfaces. Nowadays PLCs have the ability to be networked with other PLCs or with computer-based supervisory control systems. This chapter provides a general description of PLCs and their input/output arrangements. It discusses the microprocessor program-execution sequence, which distinguishes PLC operation from relay-logic controllers of the electro-mechanical systems they replace.