PLC · Controls Foundations

PLC & Automation Glossary

Every term a beginner needs, defined in plain English. Working through the field? Follow the learning path alongside it.

Showing 59 of 59 terms

Actuator

Device that produces motion or action from a signal.

Alarm

A notification that something is abnormal.

Analog signal

Continuous value, e.g. 4–20 mA or 0–10 V.

As-built

Final documentation reflecting what was actually installed.

Circuit breaker

Protective device that trips on overcurrent or short circuit.

Coil

Output element that is energised by a rung.

Commissioning

Bringing a machine online and proving it works.

Contact (NO/NC)

Switch element in logic: normally-open or normally-closed.

Contactor

Heavy-duty relay for switching motors and large loads.

Control panel / cabinet

Enclosure that houses the control equipment.

Counter

Instruction that counts events.

Digital signal

On/off (1/0) signal.

DIN rail

Standard metal rail that components clip onto.

Emergency stop

Hardwired stop that removes power safely.

Fault

A failure or abnormal state.

Field device

Any sensor or actuator out on the machine.

Fieldbus

Industrial network connecting field devices.

Fuse

Sacrificial overcurrent protection element.

Gateway

Device that translates between two protocols.

Grounding / earthing

Safety connection to earth potential.

Historian

Database that logs process values over time.

HMI

The operator's touchscreen interface.

Indicator / pilot light

Status light on the panel front.

Input

Signal coming into the PLC from the field.

Interlock

A condition that must be met before an action is allowed.

Ladder logic

The most common PLC language, drawn as rungs like a ladder.

Limit switch

Mechanical switch triggered when movement reaches a position.

Modbus

Simple, widely-used industrial protocol.

Motor

Converts electrical energy into rotation.

Output

Signal the PLC sends out to actuators.

Overload relay

Protects a motor from drawing too much current.

Photoelectric sensor

Detects objects using a light beam.

PLC

Industrial computer that runs the machine's control logic.

Pneumatic cylinder

Air-driven actuator that pushes or pulls.

PNP / NPN

Sensor output wiring types — sourcing or sinking.

Power supply

Converts AC mains to DC (usually 24 V).

Proximity sensor

Detects an object without touching it.

Push button

Operator button — start, stop, reset.

Register

A 16-bit memory location in Modbus.

Relay

Electrically-operated switch for small loads and isolation.

Rung

One horizontal line of ladder logic.

Safety relay

Monitors safety devices (E-stop, guards) to a rated level.

SCADA

Supervisory system that monitors and controls many machines.

Scan cycle

PLC repeatedly reads inputs, solves logic, writes outputs.

Seal-in / latch

A rung that keeps itself energised after the button is released.

Selector switch

Multi-position operator switch, e.g. auto/manual.

Sensor

Detects a condition in the field: presence, level, position.

Sequence of operations

Written description of how the machine runs its cycle.

Setpoint

The target value the system aims for.

Single-phase

Single AC supply, common for control and small loads.

Sinking / sourcing

The direction current flows at a digital I/O point.

Solenoid valve

Electrically-actuated valve for air or fluid.

Tag / address

The name or location of a signal in PLC memory.

Terminal block

Where wires are connected and landed in the panel.

Three-phase

Three-conductor AC power for industrial motors.

Timer

Instruction that delays or times an action.

VFD / inverter

Controls motor speed by varying the supply frequency.

Wire / cable

Conductor carrying current or signals.

Wiring diagram / schematic

Drawing that shows how the panel and machine are wired.