What Is a PLC?

A PLC, or programmable logic controller, is an industrial computer built to control machines and processes reliably for years. It is wired to field devices: inputs such as push buttons, limit switches, proximity sensors and transmitters, and outputs such as contactors, solenoid valves, motor drives and indicator lamps. A program decides which outputs to switch based on the state of the inputs.

The PLC runs that program in a repeating scan cycle. First it reads every input and stores a snapshot in memory. Then it runs the program from top to bottom against that snapshot. Next it writes the results to the outputs. Finally it handles communications and self-diagnostics, then starts again. A full scan typically takes a few milliseconds to a few tens of milliseconds, depending on program size and the processor. Because each scan follows the same order, a PLC responds in a predictable time, which is what machine control needs.

The idea came from the US car industry. In 1968, General Motors’ Hydra-Matic division asked for a programmable replacement for the banks of relays that controlled its production lines, and Modicon delivered the first such controller, the 084, in 1969. Changing a process then meant editing a program rather than rewiring a cabinet.

Whatever the size, a PLC has the same building blocks. On larger systems each one is a separate module on a backplane or a DIN rail; on small controllers they share one housing.

PLCs range from palm-sized controllers for a single machine to rack systems with thousands of I/O points. They fall into four broad groups, and many sites use more than one.

Some vendors also sell programmable automation controllers (PACs), which sit between a modular PLC and an industrial PC, with more memory and networking than a classic PLC.

Most PLCs are programmed in one or more of the languages defined in IEC 61131-3, the international standard for PLC programming. Engineers write the program on a laptop in the vendor’s software, test it and download it to the controller.

Ladder logic is still the most common way to program a PLC for discrete machine control in industrial automation, and it is the PLC language most maintenance electricians learn first. A ladder logic program is drawn as a stack of horizontal lines between two vertical rails. The left rail stands for power, each line reads like a small circuit, and when a path of true contacts reaches the coil at the right-hand end, the coil turns on.

The PLC solves the lines in order, left to right and top to bottom, once per scan, against the input snapshot taken at the start of the scan. The new coil states are written to the outputs at the end of the scan, which is why the order of the lines can matter.

The classic first example is a motor start and stop circuit. The start button is placed in parallel with a contact from the motor’s own output, and both sit in series with the stop button and an overload contact, driving the motor coil. Pressing start turns the coil on, and the parallel contact then keeps the line true after the button is released, a pattern called a seal-in or latch, until stop or an overload breaks the path.

Ladder logic is easy to read and to fault-find while the machine runs, because the programming software highlights which contacts are true in real time. It becomes unwieldy for maths, loops, text handling and data structures, so most projects mix languages: ladder logic for interlocks and sequences, and structured text or function blocks for calculations and PID loops. Soft PLC runtimes on industrial PCs offer the same IEC 61131-3 ladder diagram editors as hardware PLCs.

A PLC and an industrial PC both read inputs and drive outputs, but each suits a different job. A PLC is purpose-built for deterministic control: it boots in seconds, runs one program for years and is easy for maintenance staff to diagnose. An industrial PC brings more processing, storage and interfaces, so it suits data logging, machine vision, databases, HMI screens and AI models next to the machine.

In practice the two usually work together. The PLC keeps control of the machine, and fanless industrial PCs or DIN rail PCs beside it collect its data over Modbus TCP, OPC UA or the vendor protocol, run the HMI or forward the data to SCADA. Where a project needs extra I/O on the PC itself, digital I/O modules add isolated inputs and relay outputs. Our guide to the differences between DIO and GPIO explains which suits field wiring.

Vendor choice often follows region. Siemens SIMATIC controllers are widely used in the UK and Europe, and Rockwell Automation’s Allen-Bradley range is common in North America, so integrators on both sides of the Atlantic usually support both families.

We supply and integrate the industrial PCs, I/O and networking that sit alongside PLCs as standard, prepared with the operating system and drivers the control project needs.