ATEX · PLC · Modbus · SCADA

Control & automation glossary

Hazardous-area, PLC, industrial-network and SCADA terms, explained plainly with examples from the field.

Hazardous areas (ATEX/IECEx)

ATEX: The European directive setting requirements for equipment used in explosive atmospheres; the name comes from the French ATmosphères EXplosibles.
IECEx: The international certification scheme for equipment in explosive atmospheres, run by the IEC and based on the IEC 60079 series of standards.
Equipment Protection Level (EPL)EPL: The equipment protection level states how safe a device is as an ignition source, and maps directly to the zones where it may be used.
Hazardous-area zones (Zone 0, 1, 2): Dividing a site by the likelihood of an explosive atmosphere: Zone 0 continuously or for long periods, Zone 1 likely in normal operation, Zone 2 rarely and briefly.
Temperature class (T1 to T6): The highest temperature a device’s surface can reach; it must stay below the auto-ignition temperature of the gas on site.
Gas groups (IIA, IIB, IIC): Gases classified by how easily they ignite and pass a flame: IIA least severe (propane), IIB (ethylene), IIC most severe (hydrogen, acetylene).
Flameproof protection (Ex d)Ex d: A protection concept that lets an explosion happen inside the enclosure, which withstands it and cools the escaping flame through its gaps so the surrounding atmosphere does not ignite.
Intrinsic safety (Ex i)Ex i: A protection concept that limits the electrical energy in a circuit so it cannot produce an igniting spark or heat, even under fault conditions.

Programmable logic controller (PLC)PLC: An industrial computer built to control machines and processes: it reads sensors and switches, runs the control program, and drives motors and valves, repeatedly and reliably.
Structured Text (ST)ST: A textual programming language in IEC 61131-3 resembling Pascal, used for complex PLC logic, calculations and state machines.
PID controllerPID: A control algorithm that computes its output from the difference between the setpoint (SP) and the measured value (PV), in three parts: proportional, integral and derivative.
4-20 mA signal4-20 mA: The most widely used analogue signal in industrial measurement: 4 mA is the bottom of the range and 20 mA the top, and current, unlike voltage, is not affected by cable length.
HART protocolHART: A digital protocol carried on top of the 4-20 mA signal itself, allowing transmitter configuration, reading and diagnostics without interrupting the analogue measurement.

Modbus protocolModbus: A simple, open industrial protocol dating from 1979: a client asks, servers answer from numbered registers. It runs over RS485 (RTU) or Ethernet (TCP).
MQTT protocolMQTT: A lightweight publish/subscribe messaging protocol through a broker, ideal for carrying sensor data from remote sites to a server.

SCADA: Supervisory control and data acquisition: collects readings from distributed controllers and sensors, presents them to operators, and manages alarms, commands and history.

Edge computing: Processing data close to where it is produced, on site, instead of sending everything to the server: filtering, calculations and local decisions.