Industrial Automation Glossary

Plain-English definitions of the terms you encounter when programming, wiring, and commissioning industrial control systems.

A
AC Drive: See Variable-Frequency Drive. The term "AC drive" is sometimes used specifically for drives that use an AC motor whereas "VFD" implies variable-speed control. In practice the terms are interchangeable in most industrial contexts.
Acyclic Communication: Data exchange triggered by a specific event (e.g., a parameter read/write request) rather than sent continuously on a fixed cycle. Contrast with Cyclic Communication. Used in PROFIBUS, PROFINET, and IO-Link for configuration and diagnostics.
Analog Input (AI): A PLC or controller module that measures a continuously varying signal, most commonly a 4–20 mA current loop or 0–10 V voltage. The module's analog-to-digital converter (ADC) translates the signal into a numeric value the program can use.
Analog Output (AO): A module that converts a numeric value from the PLC program into a proportional electrical signal (typically 4–20 mA or 0–10 V) to drive a field device such as a control valve actuator or VFD speed reference.
4–20 mA Current Loop: The de-facto standard signal for process instrumentation. A 4 mA current represents 0% of the measured range and 20 mA represents 100%. Using current rather than voltage makes the signal immune to cable resistance and easy to detect wire-break faults (0 mA = fault). Defined in IEC 60381-1.
B
Backplane: The passive bus inside a PLC rack that carries power and data between the CPU and I/O modules. Module slots physically connect to the backplane; communication speed and supported module types depend on the platform (e.g., Allen-Bradley ControlLogix uses a proprietary high-speed backplane).
Baud Rate: The number of signal changes per second on a serial communication line, expressed in baud (Bd). For simple binary signals, baud rate equals bit rate in bits per second (bps). Critical for Modbus RTU configuration — all devices on the same RS-485 segment must use the same baud rate, data bits, and parity settings.
C
Category (B, 1, 2, 3, 4): A classification defined in ISO 13849-1 that describes the structural design of a safety-related control system. Category B uses basic principles; Categories 1–4 add redundancy and diagnostic coverage, culminating in Category 4, which has fully redundant channels with high diagnostic coverage and can achieve PLe.
Coil: In ladder logic, a coil is the output element — the right-hand side of a rung. When the preceding logic evaluates as true, the coil energises and sets the associated bit or tag. Common coil types include Output, Retentive (latch), Non-Retentive, Set, and Reset.
Contact: In ladder logic, a contact is an input element that examines the state of a bit. A normally-open (NO) contact passes power when the bit is 1; a normally-closed (NC) contact passes power when the bit is 0. Contacts do not change the value of the bit they examine.
CPU (Central Processing Unit): The module in a PLC system that executes the user program, manages I/O updates, and handles communication. Key CPU specifications include scan time, program/data memory size, supported languages, and the number of supported I/O points.
Cyclic Communication: Data exchange that occurs automatically on every network cycle, regardless of whether values have changed. Fieldbus protocols such as PROFINET IRT and EtherCAT rely on cyclic communication to achieve deterministic update rates. Contrast with Acyclic Communication.
D
Data Block (DB): In Siemens S7 PLCs, a data block is a memory area that stores variable data. Instance DBs are automatically created for function blocks; global DBs can be read and written by any block in the program. Equivalent concepts in other platforms are called UDTs, structures, or tag databases.
DC Fault Current: A short-circuit or ground-fault current that flows as direct current rather than alternating current. DC fault currents are harder to interrupt because DC arcs lack the natural current zero-crossing of AC arcs. DC systems require purpose-built circuit breakers or fuses rated for the specific DC voltage.
Dead Band: A range of input signal values over which no output change occurs. Used in control loops to prevent hunting (constant small corrections) near the setpoint, and in alarm management to prevent nuisance alarms when a process variable oscillates around a threshold.
DCS (Distributed Control System): A process control architecture where control functions are distributed across multiple controllers (nodes) linked by a plant-wide network. DCS platforms are common in continuous-process industries (oil and gas, chemicals, power generation) and offer integrated historian, alarm management, and engineering tools. Contrast with PLC-based systems that originated in discrete manufacturing.
Digital Input (DI): A PLC module that reads the ON/OFF state of a field device — a pushbutton, limit switch, proximity sensor output, or relay contact. The module converts the field voltage (commonly 24 V DC or 120 V AC) to a logic 1 or 0. Also called a discrete input.
Digital Output (DO): A PLC module that drives a field device to an ON or OFF state. Common output types include relay (switches any voltage up to rating), transistor/sinking (provides a switched path to 0 V DC), and sourcing (provides a switched +24 V DC). Also called a discrete output.
E
E-Stop (Emergency Stop): A manually operated, direct-opening action device that removes power from hazardous energy when actuated. E-stop requirements are defined in ISO 13850. E-stops are not a safeguard (they do not prevent access to danger zones); they are complementary protection for unexpected situations. They must be self-latching and require deliberate manual reset.
EtherCAT: An Ethernet-based real-time fieldbus developed by Beckhoff. Data frames pass through each slave node, which reads and writes its data on the fly before forwarding the frame. Achieves sub-100 µs cycle times. Defined in IEC 61158/IEC 61784.
F
FBD (Function Block Diagram): One of five IEC 61131-3 programming languages. FBD uses graphical blocks with inputs and outputs wired together to show data flow. Common in process control and motion applications. Execution order depends on data flow, not top-to-bottom rung order as in ladder logic.
Fieldbus: A digital serial communication network that connects field devices (sensors, actuators, drives) directly to a controller, replacing traditional point-to-point wiring. Early fieldbuses include PROFIBUS DP, DeviceNet, and Modbus; newer Ethernet-based variants include PROFINET, EtherNet/IP, and EtherCAT.
Function Block (FB): An IEC 61131-3 program organisation unit that has internal memory (retains state between calls). Used for encapsulating reusable logic such as PID controllers, motor starters, or valve sequences. Each call of the FB creates an instance with its own data. Contrast with a Function (FC), which has no internal memory.
Function (FC): An IEC 61131-3 program organisation unit without internal memory. Every input must be provided on each call; no state is retained between scan cycles. Suitable for pure calculations and conversions. Contrast with a Function Block (FB).
G
Ground Fault: An unintended electrical connection between a live conductor and earth ground or a conductive enclosure. In 24 V DC control circuits, a single ground fault often causes no immediate failure but creates a shock hazard; a second ground fault can create a direct short. Ground-fault monitoring (often required by NEC 230.95 and NFPA 79) detects and alarms on the first fault.
H
HART (Highway Addressable Remote Transducer): A communication protocol that superimposes a digital FSK (frequency-shift keying) signal on a conventional 4–20 mA current loop. The digital signal carries device configuration, diagnostics, and secondary process variables without disrupting the 4–20 mA signal. Defined in IEC 62591. HART 5, 6, and 7 are the most common revisions.
HMI (Human–Machine Interface): A hardware or software panel that allows an operator to monitor process status and issue commands. Hardware HMIs are standalone touch panels with embedded software; software HMIs run on industrial PCs. HMIs display live process values, alarm lists, trends, and navigation between process views.
Hysteresis: A deliberate difference between the ON threshold and the OFF threshold of a switching device. In process instrumentation, a sensor with hysteresis switches ON at, say, 50 °C and does not switch OFF until the temperature drops to 45 °C. Prevents chattering in threshold applications.
I
IEC 61131-3: The international standard defining five programming languages for PLCs and industrial controllers: Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL — deprecated in edition 3), and Sequential Function Chart (SFC). Provides a vendor-neutral programming model so logic can be ported across compliant platforms.
IO-Link: A standardised point-to-point serial communication protocol (IEC 61131-9) for connecting sensors and actuators to an IO-Link master. Unlike fieldbus, IO-Link is not a network; each device connects to a dedicated port on the master via an unshielded 3-wire cable. Supports bidirectional data, remote parameterisation, and event diagnostics.
IP Rating (Ingress Protection): A two-digit classification defined in IEC 60529 indicating the degree of protection an enclosure provides against solid particles (first digit, 0–6) and liquids (second digit, 0–9K). For example, IP65 is fully dust-tight and protected against water jets from any direction; IP67 adds protection against temporary immersion.
L
Ladder Logic (Ladder Diagram): A graphical PLC programming language derived from relay control diagrams. Programs are arranged in horizontal rungs between two vertical power rails. Logic is read left-to-right: when all contacts on a rung evaluate to true, the output coil on the right is energised. Defined in IEC 61131-3 as Ladder Diagram (LD). The dominant language in North American discrete manufacturing.
Latch / Set-Reset: A memory function that retains its state after the energising condition is removed. A Set (S) coil turns ON and stays ON until a corresponding Reset (R) coil is executed. Used for interlocks and alarms that must be acknowledged before clearing. Also called a retentive coil.
M
MCC (Motor Control Centre): A modular assembly of motor starter units (buckets) in a common enclosure with a shared busbar. Each bucket includes a starter (contactor plus overload relay, or a soft starter/VFD), branch circuit protection, and a disconnect. MCCs reduce floor space and simplify maintenance compared to individual panel-mounted starters.
Modbus: A serial communication protocol published by Modicon in 1979. Modbus RTU uses RS-232/RS-485; Modbus TCP encapsulates RTU frames in TCP/IP. A master-slave protocol: the master polls each slave's coils and registers; slaves respond. Remains one of the most widely supported protocols in industrial automation despite its age.
Modbus RTU: The serial variant of Modbus. Data is encoded in binary with CRC error checking. Up to 247 slave devices can share a single RS-485 segment. The "RTU" (Remote Terminal Unit) designation refers to the binary framing as opposed to the ASCII framing used in Modbus ASCII. See the Modbus RTU vs TCP guide.
Modbus TCP: Modbus protocol transported over TCP/IP on standard Ethernet (port 502). Removes the slave address limit of RS-485 and supports multiple simultaneous connections. Adds a MBAP (Modbus Application Protocol) header in place of the serial CRC. Does not guarantee real-time delivery. See the Modbus RTU vs TCP guide.
Motor Starter: A combination of a contactor (switching element) and an overload relay (thermal or electronic protective device). The contactor switches motor power on/off under control signals; the overload relay monitors current and trips the starter on sustained overcurrent, protecting the motor from thermal damage.
N
NEMA Enclosure Rating: A classification system published by the National Electrical Manufacturers Association defining protection levels for electrical enclosures in North American markets. NEMA 4 is roughly equivalent to IP65; NEMA 4X adds corrosion resistance. NEMA ratings include additional considerations (icing, oil immersion, hazardous locations) not covered by IP ratings alone. See the NEMA Enclosure Ratings guide.
Normally Closed (NC): A contact that is closed (conducting) in its de-energised or un-actuated state and opens when actuated. NC contacts are preferred for safety-critical functions such as e-stop buttons and door interlock switches, because a broken wire produces a safe (open) state.
Normally Open (NO): A contact that is open (non-conducting) in its de-energised state and closes when actuated. NO contacts are the default for most control signals such as start pushbuttons and selector switches.
O
OPC UA (Unified Architecture): A platform-independent, service-oriented communication standard (IEC 62541) for industrial automation. Provides a secure, structured data model with built-in authentication, encryption, and fine-grained access control. Increasingly used for machine-to-cloud and MES/ERP integration. The publish-subscribe transport (OPC UA PubSub) enables efficient data distribution over MQTT and AMQP.
Overload Relay: A protective device in a motor starter that monitors the motor running current and trips the starter if sustained overcurrent is detected. Thermal overload relays use bimetallic strips heated by motor current; electronic overload relays use current transformers and digital algorithms for more accurate protection and remote diagnostics.
P
PID Controller: A control loop algorithm that calculates an output based on three terms: Proportional (proportional to current error), Integral (proportional to accumulated error over time, eliminates steady-state offset), and Derivative (proportional to rate of change of error, damps oscillation). The most common closed-loop control algorithm in process industries. Parameters are tuned by methods such as Ziegler–Nichols or lambda tuning.
PLC (Programmable Logic Controller): A ruggedised industrial computer designed to control manufacturing processes and machines. A PLC repeatedly executes a user program in a fixed scan cycle: read inputs, execute program, write outputs. PLCs are rated for harsh environments (vibration, temperature, EMI) and support a wide range of I/O and communication modules. Programmed to IEC 61131-3 languages.
PL (Performance Level): A five-level discrete classification (PLa through PLe) defined in ISO 13849-1 that describes the ability of a safety-related control system to perform its safety function under foreseeable conditions. PLe is the highest level. PL is determined by the architecture category, diagnostic coverage (DC), mean time to dangerous failure (MTTFd), and common-cause failure (CCF) measures.
PROFIBUS: A serial fieldbus standard developed by Siemens and standardised in IEC 61158/IEC 61784. PROFIBUS DP (Decentralised Periphery) is the most common variant, used for cyclic exchange of I/O data between a master and slave devices at up to 12 Mbit/s over RS-485. PROFIBUS PA uses Manchester Bus Powered (MBP) physical layer for process instrumentation in hazardous areas.
PROFINET: The Ethernet-based successor to PROFIBUS, defined in IEC 61158/IEC 61784. PROFINET IO is the most common variant for I/O; PROFINET IRT (Isochronous Real-Time) adds hardware-scheduled communication for sub-1 ms cycle times in motion control. Uses standard Ethernet hardware but requires managed switches for IRT.
Program Scan: The continuous cycle a PLC executes: (1) read all physical inputs into the input image table, (2) execute the user program from top to bottom, (3) write the output image table to physical outputs, (4) perform housekeeping tasks (communication, diagnostics). Scan time is typically 1–50 ms depending on program size and platform.
PT100 / PT1000: Resistance temperature detectors (RTDs) made from platinum with a nominal resistance of 100 Ω (PT100) or 1000 Ω (PT1000) at 0 °C. Follow the IEC 60751 standard defining the resistance-temperature curve. PT100 sensors are the most common industrial RTD type; PT1000 offers higher sensitivity for small temperature changes.
R
Relay: An electromechanical switch where an electromagnetic coil, when energised, mechanically moves contacts to open or close a circuit. Industrial control relays (ICRs) are used to interface PLC outputs to field loads at different voltages, to multiply contact availability, and to provide galvanic isolation. Relay output PLC modules contain one relay per output point.
RTD (Resistance Temperature Detector): A temperature sensor whose resistance changes predictably with temperature. The most common industrial RTDs are platinum-based (PT100, PT1000). RTDs are more accurate and stable than thermocouples over the –200 °C to +850 °C range but require a Wheatstone bridge or specialised input module and are fragile compared to thermocouples. See the RTD vs thermocouple guide.
S
Safety Relay: A specialised relay module designed to implement safety functions (e-stop, light curtain, door interlock). Safety relays use force-guided contacts, cross-monitoring of dual-channel inputs, and timed outputs to achieve functional safety requirements up to SIL 3 / PLe. They provide a simpler and lower-cost alternative to a safety PLC for straightforward guarding applications.
SCADA (Supervisory Control and Data Acquisition): A system architecture for monitoring and controlling geographically dispersed assets (pipelines, utilities, substations). A SCADA system collects data from remote terminal units (RTUs) or PLCs, presents it to operators via HMI screens, stores it in a historian, and can send control commands. Contrast with a standalone HMI, which typically monitors a single machine or cell.
SFC (Sequential Function Chart): One of five IEC 61131-3 programming languages. SFC describes a process as a sequence of steps connected by transitions. Only the active step(s) execute; a transition fires when its condition is true, deactivating the current step and activating the next. Ideal for batch processes, machine cycles, and state machines.
SIL (Safety Integrity Level): A four-level classification (SIL 1 to SIL 4) defined in IEC 61508 and IEC 62061 that specifies the required probability of failure on demand (PFD) for a safety-instrumented function (SIF). SIL 4 has the most stringent requirements (PFD <10⁻⁴). SIL applies to safety instrumented systems (SIS); ISO 13849 uses Performance Level (PL) for machinery.
Sourcing / Sinking: Two conventions for 24 V DC I/O wiring. A sourcing output provides current (sources +24 V) through the load to common; a sinking output provides a path to 0 V (sinks current) and requires the load to be powered from +24 V. A PNP sensor sources current; an NPN sensor sinks current. See the NPN vs PNP wiring guide.
ST (Structured Text): One of five IEC 61131-3 programming languages. Structured Text is a high-level language resembling Pascal or C, with IF/THEN/ELSE, CASE, FOR, WHILE, and REPEAT constructs. Better suited than ladder logic for complex calculations, string handling, and data manipulation. Increasingly popular as PLC vendors improve debugger support. See the Structured Text vs Ladder Logic guide.
T
Thermocouple: A temperature sensor consisting of two dissimilar metals joined at one end. The junction voltage (Seebeck effect) is proportional to the temperature difference between the measurement junction and a reference (cold) junction. Common types: J (iron-constantan, –40 to +750 °C), K (chromel-alumel, –200 to +1260 °C), T (copper-constantan, –200 to +350 °C). Rugged, wide range, but less accurate than RTDs. See the RTD vs thermocouple guide.
Timer: A PLC function block that measures elapsed time. Common types: TON (On-delay: output activates after a set time when the input is energised), TOF (Off-delay: output stays active for a set time after the input de-energises), TP (Pulse: output activates for a set time on each rising edge of the input, regardless of input state).
Torque: A rotational force, measured in Newton-metres (N·m) or foot-pounds (ft·lb). In motor and drive applications, rated torque is the continuous torque the motor can produce at rated speed. Peak (breakaway) torque is the maximum the motor can develop. VFD control modes include V/Hz (constant voltage-to-frequency ratio), vector control, and direct torque control (DTC).
U
UDT (User-Defined Data Type): A custom data structure created by the programmer to group related variables. For example, a "MotorStatus" UDT might contain Running (BOOL), Fault (BOOL), Speed (REAL), and Current (REAL) members. Instances of the UDT can be created for each motor in the system, keeping the tag database organised and enabling template-based programming.
V
VFD (Variable-Frequency Drive): A power electronics device that converts incoming AC mains power to a variable-frequency, variable-voltage output, allowing precise control of AC motor speed. The rectifier stage converts AC to DC; the inverter stage synthesises the output waveform using pulse-width modulation (PWM). VFDs reduce energy consumption in fan and pump applications by allowing speed to track demand. Also called an inverter, adjustable-speed drive (ASD), or AC drive. See the VFD overvoltage fault guide.
W
Watchdog Timer: A hardware or software timer in a PLC or safety system that must be reset by the running program within a defined interval. If the program fails to reset the watchdog (due to a CPU fault, infinite loop, or software hang), the watchdog triggers a fault and initiates a safe state. Required by IEC 61508 for safety-critical applications.
Wiring Diagram: A technical drawing that shows the physical connections between electrical components. In panel design, wiring diagrams (also called connection diagrams) show wire routing, terminal numbers, wire colours, and conductor sizes. Distinct from a schematic, which shows functional relationships without regard to physical layout. Created to IEC 61346 and IEC 81346 reference designation standards.

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