The Complete Handbook of Programmable Logic Controllers

Chapter 1: Understanding Programmable Logic Controllers

A Programmable Logic Controller, or PLC, is a ruggedized, industrial-grade computer designed to automate electromechanical processes. It is not a general-purpose device like a desktop PC or a single-board hobby computer. It is a purpose-built machine intended to operate reliably in harsh conditions - extreme temperatures, electrical noise, vibration, and humidity - and to run continuously for years without interruption. Its primary function is to read signals from input devices (sensors, switches, buttons), execute a user-written logic program, and then set outputs (motors, valves, lights, alarms) based on that logic. You can think of it as the central nervous system for a machine or a process.

To understand why the PLC is so fundamental, consider the world before it existed. In a manufacturing plant circa 1960, machine control was implemented with banks of electromagnetic relays. A single machine might require hundreds of relays, each wired into a complex electrical circuit. The logic of the machine - the sequence of operations, the safety interlocks, the timers - was physically cast in the wiring. If a production engineer wanted to add a new function or change a sequence, someone had to physically rewire the panel, a task that could take days and required a licensed electrician. This was slow, expensive, and prone to error. The PLC, introduced in the late 1960s, changed this paradigm. The logic would now be a computer program, stored in memory, and rewiring was replaced by simply editing software.

The core of