What is a PLC?

A simple definition

A PLC (Programmable Logic Controller) is an industrial computer designed to control machines and processes. Its job can be summed up in three verbs:

1. Read signals from the real world: push buttons, sensors, level switches, transmitters.
2. Decide what to do, following a program written by a person.
3. Act on the machine: start motors, open valves, turn on warning lights.

And it repeats this over and over, hundreds or thousands of times per second, for years.

An easy way to picture it: the sensors are the machine’s senses, motors and valves are its muscles, and the PLC is the brain that follows the rules in the program. A typical rule would be: “if the start button is pressed and the safety door is closed, start the motor”.

What makes it “industrial”?

A PLC is not a desktop computer with a different name. It is built for very different conditions:

• It withstands the environment: extreme temperatures, vibration, dust and electrical noise from motors and welders.
• It speaks the language of industry: its inputs and outputs work directly with signals such as 24 VDC, 230 VAC or 4–20 mA, with no extra circuitry.
• It is predictable: it runs its program in a controlled, repeating cycle (the scan cycle), so its behaviour is always the same.
• It is built to last: it runs non-stop for years, and manufacturers keep supporting it and supplying spare parts for a long time.
• It is programmed in standard languages: the IEC 61131-3 standard defines languages designed for technicians and electricians. The classic one, Ladder, looks like an electrical drawing.

Where is it used?

Practically anywhere something moves, fills, mixes or is transported automatically:

• Production lines: bottling, packaging and assembly machines.
• Mining: conveyor belts, crushers, pumps and process plants.
• Drinking water and wastewater treatment: pumping stations and tank control.
• Food and agribusiness: fruit packing, cold storage, precision irrigation.
• Energy: substations, generation and distribution.
• Buildings and infrastructure: HVAC, booster pumps, access control.

From relay panels to PLCs

Before PLCs, machine logic was built with relays, timers and contactors wired together. Every “rule” was a physical wire. Changing how the machine worked meant rewiring the panel.

The PLC was born in the late 1960s in the automotive industry precisely to replace those panels: the logic moved from wires into a program.

Aspect	Relay logic	PLC
Changing the behaviour	Rewire the panel	Edit the program
Size	Grows with every new function	Compact; logic takes no physical space
Timers and counters	One physical device each	Hundreds or thousands, in software
Wear	Mechanical contacts wear out	Internal logic does not wear out
Troubleshooting	Check with a multimeter, wire by wire	Every signal’s state is visible on screen
Communication with other systems	None	Industrial networks, HMI panels, SCADA systems

PLC versus microcontroller (Arduino)

If you have used an Arduino, you already know the idea: a program that reads inputs and writes outputs. The difference lies in what each one was designed for.

Aspect	Development board (e.g. Arduino)	PLC
Signals	5 V or 3.3 V; needs extra circuitry for 24 V	24 VDC, 230 VAC and 4–20 mA directly, with isolation
Ruggedness	Exposed board, sensitive to electrical noise	Designed and certified for industrial environments
Programming	C/C++	IEC 61131-3 languages (Ladder, Structured Text…)
Diagnostics	Limited	Online monitoring and changes while the machine runs
Support and spares	Varies	Long life cycles and manufacturer support
Cost	Very low	Higher

There are now industrial controllers based on microcontrollers that are programmed in IEC languages, so the line has become blurry. What defines a PLC is that it is designed, tested and certified to work in industry.

The pieces of a PLC system

A PLC never works alone. Around it you will always find:

• Input devices: push buttons, selector switches, limit switches, proximity sensors, pressure or temperature transmitters.
• The PLC: the CPU, its memory and the input and output modules.
• Output devices (actuators): contactors, solenoid valves, pilot lights, variable frequency drives.
• The program: written on a computer with the manufacturer’s software and downloaded to the PLC.
• The human interface: HMI screens so operators can see and control the process.

A first example

The simplest possible program: a switch connected to input I0.0 turns on a lamp connected to output Q0.0. In Ladder it looks like this:

   I0.0                          Q0.0
──┤ ├──────────────────────────( )──

It reads from left to right: “if input I0.0 is on, turn on output Q0.0”. The ┤ ├ symbol is a contact (a condition) and ( ) is a coil (an action).

Try it in the simulatorOpen a preloaded example and experiment yourself.

Summary

• A PLC is an industrial computer that reads inputs, decides according to a program and acts on outputs.
• It replaced relay panels because changing a program is much easier than rewiring.
• Compared with a microcontroller, its strengths are ruggedness, industrial signals and standard languages.
• In the next topic you will see what is inside a PLC and how its scan cycle works.