Industrial Automation Glossary

A power electronic device that controls the speed and torque of an AC motor by varying the frequency and voltage it supplies. "AC drive" is also called a variable frequency drive (VFD) or frequency converter.

An I/O module that measures analog signals, such as a voltage or a current from a sensor or transmitter, and converts them into digital values the controller can process. Depending on the module, each input can be set to measure voltage or current, and the module can report faults such as wire break, overflow, and underflow.

A resistor connected to a drive's DC bus, usually through a braking transistor, that dissipates energy returned by the motor as heat during deceleration or when a load overhauls the motor. It is sized by braking power and duty cycle.

A procedure for restoring the insulating oxide layer inside the electrolytic capacitors of a drive that has been stored or left unpowered for an extended period. This is done by applying voltage under controlled conditions, such as powering the drive unloaded for a set period or using a current-limited external DC supply, before the drive is put into normal service. Manufacturers specify when reforming is recommended and which procedure to follow for a given drive.

A catalog number identifies a product as listed in a manufacturer's sales catalog or price book, while a part number identifies the specific manufactured item down to its engineering revision, series, or option code. Two items can share the same catalog number and still differ at the part number level if the manufacturer revised the design between production runs. Buyers matching a replacement component should confirm the full part number rather than the catalog number alone, since the catalog number may not capture a mid-production design change.

An application-layer protocol for industrial automation that defines a common set of messages and services for control, safety, motion, and configuration data, independent of the physical network carrying it. CIP is maintained by ODVA and runs over several network adaptations, including EtherNet/IP, DeviceNet, and ControlNet.

A switching device that carries current under normal conditions and automatically interrupts it under abnormal conditions, such as a short circuit or overload, to protect equipment and wiring. Unlike a fuse, it can be reset and reused after tripping.

The control unit of a CNC (computer numerical control) machine. It reads a part program, such as G-code, and turns it into coordinated motion of the machine's axes and spindle by sending commands to servo drives and motors. It also manages feed rates, tool changes, and machine functions such as coolant and safety interlocks, using feedback from the machine to keep motion accurate. The PLC that handles the machine's logic is often built into the controller.

An electrically operated switch designed to make and break current under normal operating conditions, built to switch frequently over its service life. The core building block of most motor starters.

A capacitor, or bank of capacitors, connected across the DC bus of a drive or other inverter, between the rectifier and the inverter. It smooths the rectified DC voltage, absorbs ripple, and acts as an energy buffer, storing energy during voltage peaks and releasing it during dips so the inverter sees a nearly constant DC voltage. Aluminum electrolytic, film, and ceramic capacitors are used for this job; electrolytic types give high capacitance but have a limited lifetime.

The internal DC link in a drive, between the rectifier that converts AC input to DC and the inverter that produces the variable-frequency output. Capacitors, and in many drives DC reactors, smooth it, and energy returned by the motor during deceleration appears on it. The capacitors can hold a charge after power is removed, so follow the manufacturer's discharge procedure before servicing.

A drive that controls the speed and torque of a DC motor by adjusting the voltage supplied to the motor's armature and, in many designs, its field. Traditional DC drives use silicon controlled rectifiers to convert AC supply into controllable DC.

A computerized control system for continuous and batch industrial processes in which control is spread across several process controllers placed throughout a plant. Each controller oversees a part of the process, called a unit. Input and output modules and fieldbus connections relay information between the controllers and field devices such as transmitters, actuators, and drives, and the system as a whole coordinates production across the plant. DCSs are used in industries such as chemicals, power generation, oil and gas, pulp and paper, pharmaceuticals, and mining.

I/O modules are physically located near the field devices they serve, rather than in the main controller cabinet, and connected back to the controller over a network such as PROFIBUS or Industrial Ethernet. Reduces the amount of wiring needed to run long distances back to a central panel.

Downtime is any period when a piece of equipment or a production line is not producing. Unplanned downtime, an unexpected failure rather than scheduled maintenance, is one of the costliest events in a plant, which is why fast sourcing for replacement parts matters.

The circuit board in a drive that holds the processor and firmware, reads commands and feedback, and generates the signals that switch the power section. A replacement board needs to match the drive model, series, and firmware version.

A filter installed on a drive's output that slows the rate of voltage rise (dv/dt) of the pulses reaching the motor, reducing stress on the motor's winding insulation, particularly on long cable runs. It does not produce a sinusoidal waveform; that is the role of a sine wave filter.

A method of slowing a motor in which the energy it returns during deceleration is dissipated as heat in a resistor instead of being returned to the supply. In AC drives, this is done with a brake chopper and braking resistor on the DC bus. In servo applications, dynamic braking can also refer to shorting the motor windings through resistors (often via a contactor) to stop the motor quickly on a fault or loss of power, rather than letting it coast to a stop.

A filter, often built into the drive or fitted as an option, that reduces high-frequency electrical noise a drive sends back onto the supply and helps protect it from incoming electrical disturbances. It helps the installation meet the electromagnetic compatibility requirements for adjustable speed drives set out in IEC 61800-3.

The cable that connects a motor's feedback device (encoder or resolver) to the drive's feedback input. Its conductor count and connectors depend on the feedback interface in use. Single-cable (one-cable) interfaces, such as HIPERFACE DSL, carry feedback over wires integrated directly into the motor power cable.

The lifecycle stage at which a manufacturer formally announces it is ending production of a part, often through a discontinuance notice. Limited stock or support may continue for a period afterward, but new production has stopped. "Discontinued" is used interchangeably with "end-of-life" in this sense, and a part generally becomes Obsolete Equipment once this stage has passed.

An industrial networking protocol that runs the Common Industrial Protocol (CIP) over standard Ethernet hardware. The "IP" stands for "Industrial Protocol," not Internet Protocol. Managed by ODVA.

An industrial communication network used to connect distributed I/O to a controller, so that the process signals of many devices reach the controller over a network. The IEC 61158 series of international standards specifies fieldbuses.

The version of embedded software running on a controller, drive, HMI, or module is distinct from the hardware part number. Two units with an identical part number can run different firmware versions depending on when they were manufactured or last updated, and firmware versions can affect feature availability, communication compatibility, and whether a configuration file from one unit loads on another. Buyers matching a replacement for an active system should confirm the installed firmware version alongside the part number.

A criterion for judging whether a replacement component is truly interchangeable with the original. "Form" covers physical size, shape, and mounting; "fit" covers connection points and physical compatibility with surrounding equipment; "function" covers whether the part performs the same role in the system. A replacement that matches all three can typically substitute for the original without redesigning the installation.

A graphical programming language for PLCs and one of the languages in the IEC 61131-3 suite. A program is built from function blocks, each of which performs an operation on its input values and produces output values, with the blocks connected together.

A passive or active device that reduces the harmonic currents drawn by non-linear loads, such as the rectifier section of a drive, to limit distortion on the power supply. Passive filters use tuned inductor-capacitor circuits, while active filters inject a compensating current.

The screen, panel, or software through which an operator monitors and interacts with a machine or process, viewing status, entering setpoints, or acknowledging alarms. Ranges from a small text display on a machine to a full graphical panel tied into a plant's control system.

A brake built into or fitted to a servo motor that mechanically holds the shaft when power is removed, preventing unwanted movement such as a vertical axis dropping. It is typically spring-applied and released electrically, and is intended to hold a stationary load rather than to stop a moving one.

A plug-in or rack-mounted unit that connects a controller to field signals, converting real-world signals, such as sensor readings or switch positions, into data the controller can use, and converting controller outputs into signals that operate field devices.

The international standard that specifies the syntax and semantics of a unified suite of programming languages for programmable controllers. The current edition, IEC 61131-3:2025, defines the textual language Structured Text and the graphical languages Ladder Diagram and Function Block Diagram. It also defines Sequential Function Chart elements for structuring the internal organization of programs and function blocks.

A power semiconductor module that houses insulated-gate bipolar transistor (IGBT) chips, together with diode chips and, in some types, an NTC temperature-sensing thermistor, in one package. Modules come in different circuit configurations, such as a single switch, a dual switch, or a three-phase full bridge, and are used in power electronics such as motor drives.

A seller of genuine manufacturer hardware that has no contractual arrangement with, or written authorization from, the original manufacturer to distribute its products. Manufacturer warranties and official support programs typically do not extend through an independent supplier. PLC Direct is an independent supplier, never an authorized distributor.

The general use of Ethernet-based networking, often paired with a protocol such as EtherNet/IP, PROFINET, or Modbus TCP, along with hardened hardware, for real-time communication between controllers, drives, I/O, and HMIs on a plant floor.

A personal computer designed for use in industrial environments, typically installed in a control cabinet. Industrial PCs are used for tasks such as process control, data acquisition, and visualization. They are specified with a stated supply voltage, ambient temperature range, and vibration and shock resistance tested to IEC/EN 60068 methods. They are designed to meet the generic EMC standards for industrial environments, EN 61000-6-2 (immunity) and EN 61000-6-4 (emissions), which apply to equipment intended for use in industrial locations.

A device installed in the safe (non-hazardous) area between a controller, such as a PLC, and a field device in a hazardous area. It limits the electrical energy that can reach the hazardous area to a safe level, so the circuit cannot ignite an explosive atmosphere. Barriers are used with simple devices such as push buttons, RTDs, and thermocouples. Common types are Zener barriers, isolator barriers, and converter barriers.

A graphical programming language for PLCs that represents control logic as rungs of contacts and coils between two vertical rails, visually resembling a relay wiring diagram. It is one of the standard programming languages defined for programmable controllers.

Automation hardware or software that is still in productive use on a plant floor, even though it is based on an older product generation with reduced or ended manufacturer support. A legacy system can include both active and obsolete components. “Legacy” describes what is installed, while “obsolete” describes the manufacturer’s production status.

An inductor installed in series with a drive's AC input that limits line current, reduces harmonic current, and helps protect the drive from supply-side voltage disturbances. Also called an input reactor or line choke.

A servo motor that produces straight-line motion directly, without a ball screw, belt, or other rotary-to-linear mechanism. It typically consists of a primary section containing the windings and a secondary section (magnet track) with permanent magnets, with position feedback usually provided by a linear encoder.

An openly published communication protocol that lets industrial devices exchange data over serial connections or Ethernet (Modbus TCP). Originally developed in 1979, it remains widely used in industrial automation for its simplicity and vendor-neutrality.

A device that generates motion commands, such as target positions, velocities, and trajectories, and coordinates one or more axes by sending those commands to servo drives or other drives. In PLC-based systems, motion control functions can be integrated into the PLC CPU.

Communication interfaces, digital or hybrid analog/digital, that carry position data, and often motor identification data, from a motor-mounted feedback device to the drive. EnDat and HIPERFACE are two widely used examples. The drive must support the interface used by the installed feedback device.

The combination of switching hardware and overload protection is used to start and stop a motor safely. A soft starter is one type of motor starter, and a contactor-based starter is another.

The parts, supplies, and processes used to keep production equipment and facilities running are distinct from materials that go into the final product. A replacement contactor or sensor for a production line is an MRO purchase.

One of PLC Direct’s three condition grades for surplus hardware. Never Used Surplus units have not been used or refurbished, but may or may not retain original manufacturer packaging. See PLC Direct’s condition grades guide for how this grade compares to Sealed Surplus and Used/Refurbished.

A product no longer in production by the manufacturer to its original specification, having passed through end-of-life. Obsolete does not mean old, worn, or unreliable. A part can become obsolete shortly after a manufacturer shifts production to a newer model.

The company that originally designed and manufactured a component or piece of equipment, as distinct from a reseller, distributor, or independent supplier of that same hardware. A part described by its OEM part number is being identified by the number assigned by the company that built it, regardless of who is currently selling it.

The time between placing an order through the manufacturer (or its authorized channel) and actually receiving the product. Lead times often stretch for constrained, high-demand, or late-lifecycle parts, which is a common reason buyers turn to the surplus and secondary market instead.

A controller class that combines PLC-style logic with motion, drive, and process control on one platform, using a single programming environment and shared tag database instead of separate controllers for each function. The term originated in industry analysis in the early 2000s and is not formally standardized. Vendors draw the line between "PLC" and "PAC" differently.

An industrial computer that reads inputs from sensors and field devices, applies programmed logic, and controls outputs such as motors, valves, or alarms. PLCs are built to withstand the electrical noise, temperature swings, and vibration of a plant floor, and they continuously repeat this read, process, control cycle to keep equipment running to spec.

A battery installed in a PLC's CPU that keeps the controller's memory and real-time clock powered while the main supply is off. What it preserves depends on the model: program memory, retained data, or only the clock. Manufacturers specify how long the battery lasts and when it should be replaced.

The module of a modular PLC that executes the user program. It works together with the I/O modules, communication modules, and technology modules that make up the system.

A device that converts incoming AC or DC power into a steady, regulated voltage, commonly 24 VDC, needed by controllers, I/O modules, and sensors in a control panel.

A fieldbus communication standard that links a controller to decentralized field devices, such as sensors, actuators, and drives, over a single bus cable rather than individual point-to-point wiring. Managed by PROFIBUS and PROFINET International (PI) and standardized under IEC 61158.

An Ethernet-based industrial communication standard maintained by PROFIBUS and PROFINET International (PI) that extends real-time control, diagnostics, and device configuration over standard Ethernet hardware, serving as the Industrial Ethernet counterpart to the original PROFIBUS fieldbus.

A sensor that detects whether an object is present without physically touching it, using methods such as inductive, capacitive, or photoelectric sensing. Commonly used to detect part position on a conveyor or machine without contact wear.

A method of slowing a motor in which the energy it returns during deceleration is fed back to the AC supply instead of being dissipated as heat. It requires a regenerative drive or a separate regenerative unit, because a standard drive with a diode rectifier cannot return energy to the supply.

An electromagnetic feedback device that outputs analog signals corresponding to the sine and cosine of the shaft angle, used to measure motor shaft position and speed. It contains no electronics in the sensor itself, so the drive must have a resolver input to interpret the signals.

The control unit of an industrial robot. It contains the electronics needed to control the robot arm, any additional axes, and peripheral equipment, together with the system software that provides the basic functions for operating and programming the robot. It includes a computer unit, communication interfaces, and a connection for the teach pendant, and the drive system that powers the robot's motors is either in the same cabinet or in a separate drive module.

A safety function, defined in IEC 61800-5-2, that removes the power capable of producing torque from the motor so the drive cannot cause motion. STO does not electrically isolate the drive from its supply, so it is not a substitute for lockout procedures.

A control-system architecture that gathers data from and supervises equipment spread across a wide area, such as multiple pump stations or an entire pipeline, from a central location. SCADA systems typically communicate with local PLCs in the field.

The repeating sequence in which a PLC reads its inputs, executes the user program, and updates its outputs. One pass is called a scan, or a cycle, and the time it takes is the scan time. Scan time is not the same for every pass: it varies with the program's runtime and increases when interrupts or communication tasks interrupt the cycle. Controllers report the last scan time and the largest scan time since the last reset, and the CPU monitors the cycle time against a configurable maximum.

One of PLC Direct's three condition grades for surplus hardware. Sealed Surplus units remain in their original, sealed manufacturer packaging, though the box may show shelf wear, and the manufacturing date can vary. See PLC Direct's condition grades guide for how this grade compares to Never Used Surplus and Used/Refurbished.

A graphical programming language for PLCs that is similar to a flowchart. A program is built from steps and transitions. Each step contains the actions that take place at a particular time, phase, or station, and each transition is a true-or-false condition that tells the chart when to move on to the next step. Steps can run in parallel, or the chart can choose between alternative paths. The IEC 61131-3 standard defines SFC elements for structuring the internal organization of programs and function blocks.

The product family grouping (series) and the specific design iteration within that family (revision) to which a part belongs. Manufacturers update a series over time without changing its base part number, so two units in the same series can differ in capability, firmware compatibility, or physical layout depending on revision. Confirming both the series and the revision, not just the series name, is part of matching the correct replacement.

An electronic amplifier that powers a servo motor and continuously adjusts it using feedback from an encoder or resolver, allowing precise control of position, speed, or torque. Used where a VFD's open-loop speed control is not precise enough, such as in robotics or CNC axes.

A motor designed for closed-loop control of position, speed, or torque. It is paired with a feedback device, such as an encoder or resolver, and driven by a servo drive. Servo motors are typically chosen for precise, responsive control of position and speed.

A four-layer (P-N-P-N) semiconductor switch, also called a thyristor, that conducts current in one direction only, from anode to cathode. It turns on when a gate current pulse is applied while the anode is positive, stays on after the gate signal is removed, and turns off when its current falls below the holding current, which in AC circuits happens as the current crosses zero. SCRs are used in power control, for example in solid-state relays, motor speed control, uninterruptible power supply bypass switches, and inrush-current limiting circuits.

An output filter that smooths a drive's pulsed output into a near-sinusoidal voltage, reducing motor noise, heating, and voltage stress on the motor and cable. It is typically selected based on motor current and the drive's switching frequency.

A solid-state device that limits inrush current and mechanical shock when starting or stopping an AC motor, by ramping voltage up and down rather than applying full power instantly. It differs from a VFD, which continuously varies the frequency during operation rather than only at start-up and shut-down.

A high-level, text-based programming language for PLCs, defined in the IEC 61131-3 standard. Its syntax resembles Pascal: a program is written as a list of statements, each ending with a semicolon. The statements include assignments, function block calls, conditions (IF, CASE), and loops (FOR, WHILE, REPEAT).

Genuine manufacturer hardware that is no longer held by its original purchaser or moving through the manufacturer's standard sales channel, typically because of a canceled project, inventory overstock, or a decommissioned installation. Surplus hardware is sold through independent suppliers rather than an authorized distribution channel and is graded by condition, not by age. See Sealed Surplus, Never Used Surplus, and Used/Refurbished for PLC Direct's condition grades, and PLC Direct's condition grades guide for a full explanation.

A general term for switching devices, including breakers, disconnects, and contactors, combined with the control, measuring, and protective equipment that support them, often assembled into a single enclosure or lineup.

The rate at which a drive's output transistors, typically IGBTs, switch on and off to build its output waveform, usually expressed in kHz. A higher setting gives smoother motor current and less audible noise, but increases heat in the drive, and many drives reduce their output rating at higher settings.

A hand-held operator device used to program and operate an industrial robot. With it, an operator can run robot programs, jog (move) the robot, and modify programs. It typically has a display, a joystick or keys, an emergency stop button, and an enabling device. The enabling device is a constant-pressure push-button that must be held halfway in for the robot to move; releasing it, or pressing it fully in, switches the motors off.

One of PLC Direct's three condition grades for surplus hardware. Used/Refurbished units have been previously installed, refurbished, or both, and have been inspected, tested, and restored to working condition. See PLC Direct's condition grades guide for how this grade compares to Sealed Surplus and Never Used Surplus.

A method of controlling an AC motor that regulates the torque-producing and magnetizing components of motor current separately, also called field-oriented control. It allows accurate speed and torque control, including at low speeds, and is available in closed-loop form, using a feedback device such as an encoder, and in sensorless (open-loop) form, without one.

An electronic drive that controls an AC motor's speed and torque by varying the frequency and voltage supplied to it, rather than running the motor at a fixed speed. Used to improve process control and reduce energy use in applications such as pumps, fans, and conveyors.

An arrangement of contactors and protective devices that lets a motor run directly from the AC line when its drive is out of service. The circuit is interlocked so the motor cannot be connected to the drive output and the line at the same time.