Programmable Logic Controllers: The Hardware

In many respects, the programmable logic controller (PLC) architecture resembles a general-purpose computer with specialized input/output (I/O) modules. This chapter describes the main distinguishing characteristics of the PLC, its basic hardware and software architecture, and the method by which the program and I/O modules are scanned. The PLC processor repeatedly executes four major tasks in the following order: read the physical inputs, scan the program, write the physical outputs, and perform housekeeping tasks. Some PLC processors have a function block that can read the current state of a physical input and another function block that can immediately set the current state of a physical output. One of the unique characteristics of PLCs is their ability to override the status of a physical discrete input or to override the logic driving a physical output coil and force the output to a desired status.