Programmable Logic Controllers (PLC)

A programmable logic controller (PLC) is a ruggedised industrial computer designed to control machinery and processes in factories, utilities and buildings. It reads the state of input devices such as switches, sensors and push-buttons, executes a stored control program, and drives output devices such as relays, valves, motor starters and indicator lamps. PLCs are built to operate reliably in harsh environments, tolerating electrical noise, vibration, temperature extremes and dust that would disable an ordinary computer.

The PLC works by repeatedly running a fixed sequence called the scan cycle: it samples all inputs, evaluates the program logic, and then updates all outputs, repeating many times per second. The classic programming language is ladder logic, a graphical notation that resembles the relay-and-contact wiring diagrams it originally replaced. Each rung represents a logical condition, with contacts standing for inputs and coils for outputs, so electricians familiar with relay control can read it easily. Other standardised languages, such as function block diagram and structured text, are also widely used.

PLCs communicate with sensors, drives and supervisory systems over industrial networks known as fieldbuses, which allow many devices to share wiring and exchange data deterministically. Siemens controllers are often programmed with the STEP 5 and STEP 7 software families, and many vendors offer their own ecosystems. Related to industrial automation is home and building automation, sometimes called domotics, where similar control principles are applied through dedicated bus systems to manage lighting, heating and access in residential and commercial buildings.

Frequently asked questions

What is ladder logic?

Ladder logic is a graphical PLC programming language that mimics relay wiring diagrams. Inputs appear as contacts and outputs as coils arranged on horizontal rungs, making it intuitive for control engineers.

What is a PLC scan cycle?

It is the repeating sequence in which the PLC reads inputs, executes the program, and writes outputs. The cycle runs continuously, so the controller responds to changing conditions in near real time.

What is a fieldbus?

A fieldbus is an industrial communication network that links controllers with sensors, actuators and drives over shared wiring, exchanging data reliably and predictably between distributed devices.