In many respects, the architecture of the programmable logic controller (PLC) resembles a general-purpose computer with specialized input/output (I/O) modules. However, some important characteristics distinguish a PLC from a general-purpose computer. First, and most importantly, a PLC is much more reliable, designed for a mean time between failure (MTBF) measured in years. Second, a PLC can be placed in an industrial environment with its substantial amount of electrical noise, vibration, extreme temperatures, and humidity. Third, plant technicians with less than a college education can easily maintain PLCs.

This chapter describes the main distinguishing characteristics of the PLC, its basic hardware and software architecture, and the method in which the program and I/O modules are scanned.