Controls and automation glossary

125 terms from PLC programming, instrumentation, SCADA, control panels, industrial networking, OT security, and water and wastewater systems. Each definition stands on its own, and each links through to the reference material behind it.

By subject

Instrumentation

11 terms

SCADA & HMI

13 terms

Control Panels

11 terms

Networking

14 terms

Water & Wastewater

15 terms

Cybersecurity

7 terms

Standards

12 terms

All terms

#

4-20 mA: The standard analog signal in process control: a current between 4 and 20 milliamps, where 4 mA is 0% of the calibrated range and 20 mA is 100%.

A

Air gap: A claimed complete physical separation between a control network and any other network.
Alarm flood: More than about ten alarms in ten minutes at one operating position, usually following a single initiating event that cascades.
Alarm rationalization: The step in the alarm management lifecycle where each candidate alarm is tested against the philosophy and given a documented cause, consequence, corrective action, response time, and priority.
Alternation: Rotating which pump takes the lead role, so run time and starts are shared rather than accumulating on one machine.
AWIAAmerica's Water Infrastructure Act: United States legislation requiring community water systems above a certain population served to conduct risk and resilience assessments and maintain emergency response plans.

B

Bumpless transfer: Switching a loop from manual to automatic without the output jumping, by having the controller pick up from the current output value.

C

Calibration: Comparing an instrument against a known reference and, where necessary, adjusting it so it reads correctly across its range.
Cavitation: Vapor bubbles forming and collapsing inside a pump because suction pressure fell below the vapor pressure of the liquid.
CIPClean In Place: A cleaning sequence that circulates chemical solution through equipment without dismantling it, used on membranes and process piping.
Coil: A ladder output instruction that sets a bit on while its rung is true and off when the rung goes false.
Compliance voltage: The minimum voltage a two-wire transmitter needs across its own terminals to operate. Whatever the supply provides above it is available to drive current through the rest of the loop.
Compression: A historian technique that stores a value only when it changes by more than a configured amount, rather than at every scan.
Contact: A ladder instruction that tests a memory bit. Examine-if-closed is true when the bit is on; examine-if-open is true when it is off.
Control narrative: The document describing what a control system does: the setpoints, the sequences, the interlocks, and the failure behavior, in words rather than code.
Control power supply: The panel component converting incoming power to the DC voltage, usually 24 V, that the controller, I/O, and field devices run on.

D

Deadband: A gap between the value that activates something and the value that deactivates it, so a measurement sitting on the threshold does not chatter.
Defense in depth: Layering independent controls so that no single failure exposes the system, rather than relying on one strong perimeter.
DMZDemilitarized Zone: A network segment between the control and business networks where data is exchanged without either side connecting directly to the other.
DNP3Distributed Network Protocol 3: A protocol built for utility telemetry, with unsolicited reporting, time-stamped events, and tolerance of constrained and intermittent links.
Duplex mismatch: A fault where one end of an Ethernet link runs half duplex and the other full, almost always because one end auto-negotiates against a hard-set partner.

E

Enclosure type rating: A classification describing what an enclosure protects against: Type 1 indoor, Type 12 dust and dripping liquid, Type 3R rain, Type 4 hosedown, Type 4X adding corrosion resistance.
Engineering unitsEU: The physical units a scaled value carries, such as feet of level, psi, or gallons per minute.
EtherNet/IP: An industrial protocol carrying the Common Industrial Protocol over standard Ethernet, using implicit messaging for cyclic I/O and explicit messaging for on-demand data.

F

Faceplate: A reusable pop-up on an HMI showing the full detail and controls for one device, opened from its symbol on a process display.
False echo: A reflection a level instrument receives from something that is not the liquid surface, such as a ladder, a pipe, or a bracket.
Force main: A pressurized pipe carrying wastewater from a lift station to a discharge point, as opposed to a gravity sewer.
Function block diagramFBD: A graphical PLC language where blocks representing functions are wired together by signal flow.

G

Ground loop: An error caused by a signal circuit being connected to earth at more than one point, where the potential difference drives current through the signal wiring.
Guided wave radarGWR: A level measurement that sends a radar signal down a probe rather than through open space, concentrating the energy along the probe.

H

HARTHighway Addressable Remote Transducer: A digital protocol superimposed on a 4-20 mA loop, carrying device configuration, diagnostics, and secondary variables without disturbing the analog value.
High performance HMI: A design approach where displays are built to make abnormal conditions immediately obvious: grey backgrounds, muted process elements, and saturated color reserved for alarms.
High service pump: A pump that takes treated water from ground storage and delivers it into the distribution system at pressure.
Historian: A time-series database that stores process values over time so they can be trended, compared, and reported on.
HMIHuman-Machine Interface: The operator interface itself: the displays a person runs the plant from.
HOAHand-Off-Auto: A selector switch giving an operator direct manual control of a device, disabling it entirely, or passing control to the controller.

I

ICSIndustrial Control System: The general term for the combined control equipment running an industrial process, including PLCs, SCADA, and the networks between them.
IEC 61131-3: The international standard defining the PLC programming languages: ladder diagram, function block diagram, structured text, sequential function chart, and instruction list.
IEC 62443: The international series of standards for security of industrial automation and control systems, covering programs, system design, and component requirements.
IGMP snooping: A switch feature that learns which ports want a given multicast stream and forwards it only there, instead of flooding every port.
Integral windup: The condition where a PID output is saturated but the integral term keeps accumulating, so the loop massively overshoots when the process finally responds.
Interlock: A condition that stops or prevents operation whenever it is present, including while equipment is already running.
Interposing relay: A relay placed between a PLC output and a load, so the controller switches a small coil rather than the load itself.
ISA-5.1: The instrumentation symbology standard, defining the tag bubbles, line types, and identification letters used on P&IDs.
ISA-18.2: The alarm management standard for the process industries, defining an alarm as an indication of an abnormal condition requiring a timely operator response, and setting out a lifecycle from philosophy through monitoring.
ISA-101: The human-machine interface standard, covering the lifecycle of operator display design including style guides and consistency.

L

Ladder logicLadder Diagram (LD): A graphical PLC programming language drawn to resemble relay ladder diagrams, where a rung energizes its output when a continuous path of true conditions exists across it.
Latch and unlatch: Instructions that set or reset a bit and hold that state when the rung goes false, unlike an ordinary coil.
Lead/lag: A pump control scheme where one pump is designated to start first and another starts when the first cannot keep up.
Lift station: A facility that collects wastewater in a wet well and pumps it up to a higher gravity line or into a force main, where topography stops gravity flow working.
Live zero: A signal scheme where the bottom of the range is a non-zero value, so a genuine zero reading is distinguishable from a dead circuit.
Loop check: A test that proves a signal path works end to end, from the field device through the wiring and input card to the value the operator sees.
Loop powered: A transmitter that draws its operating power from the same two wires that carry its 4-20 mA signal, normally from a 24 VDC supply.
Low flow cutoff: A threshold below which a flow reading is forced to zero, to stop noise near the bottom of the range accumulating into a totalizer.

M

Magnetic flowmeter: A flowmeter that applies a magnetic field across the pipe and measures the voltage the moving conductive liquid induces, following Faraday law of induction.
Managed switch: An Ethernet switch that can be configured and monitored, supporting VLANs, spanning tree, IGMP snooping, and per-port statistics.
MLSSMixed Liquor Suspended Solids: The concentration of suspended solids in an aeration basin, a primary indicator of the biological population in an activated sludge process.
Modbus: A simple, widely implemented industrial protocol in which a master reads and writes registers on addressed slave devices.
Modbus RTU: Serial Modbus, normally over RS-485, with binary messages, a CRC-16 check, and frames delimited by a silent interval of at least 3.5 character times.
Modbus TCP: Modbus carried over an Ethernet TCP connection on port 502, with a seven-byte MBAP header replacing the serial framing and CRC.
Motor overload: A protective device that trips when motor current stays above its rating long enough to threaten the windings.

N

NAMUR NE 43: A recommendation defining how a transmitter signals a fault: by driving its output outside the measurement range, below 3.6 mA or above 21 mA.
NFPA 70E: The standard for electrical safety in the workplace, covering safe work practices, risk assessment, and personal protective equipment for energized work.
NPSHNet Positive Suction Head: The suction-side pressure available to a pump above the vapor pressure of the liquid. Available NPSH must exceed the pump required NPSH.

O

One-shot: An instruction that is true for exactly one scan when its condition transitions from false to true.
OPC UAOPC Unified Architecture: A platform-independent industrial data exchange standard with a structured information model and built-in security.
ORPOxidation-Reduction Potential: A millivolt measurement of a solution tendency to oxidize or reduce, used in disinfection control to indicate oxidant activity.
OTOperational Technology: The hardware and software that monitors and controls physical processes and equipment, as distinct from the information technology that runs a business.

P

P&IDPiping and Instrumentation Diagram: The drawing showing process equipment, piping, instrumentation, and control loops, and the primary reference for how a plant is meant to work.
PACProgrammable Automation Controller: A vendor term for a higher-end controller with more memory, structured data types, and better handling of motion or batch. Functionally it is still a PLC.
Panel heat load: The total heat dissipated inside an enclosure, which must be shed through its surface or by active cooling to keep components within their ratings.
Permissive: A condition that must be satisfied before equipment is allowed to start.
pH: A measure of how acidic or alkaline a solution is, on a logarithmic scale where 7 is neutral.
PIDProportional-Integral-Derivative: A control algorithm that adjusts an output based on the size of the current error, the accumulated error over time, and the rate the error is changing.
PLCProgrammable Logic Controller: A ruggedized industrial computer that reads field inputs, solves a control program, and drives outputs on a repeating, deterministic cycle.
Poll rate: How often a SCADA system requests data from a device or remote site.
Process variablePV: The measured value a control loop is regulating, such as discharge pressure or wet well level.
Purdue model: A reference architecture organizing industrial systems into levels, from field devices at Level 0 through control and supervisory layers to enterprise networks at Level 5.

R

Radar level: Level measurement that times a microwave signal reflected from the liquid surface and subtracts the distance from a configured reference height.
RAS and WASReturn and Waste Activated Sludge: Return activated sludge is recycled from the clarifier back to the aeration basin to maintain the biological population; waste activated sludge is removed to control it.
Raw counts: The integer an analog input module produces from its analog-to-digital converter, before scaling into engineering units.
Register addressing: The two conventions for naming a Modbus register: the traditional data model numbering that starts holding registers at 40001, and the protocol address that goes on the wire starting at 0.
Report by exceptionRBE: A reporting scheme where a remote device sends data only when a value changes beyond a threshold, rather than answering continuous polls.
Retentive memory: Controller memory that keeps its value through a power cycle, as opposed to memory that clears on startup.
Reverse osmosisRO: A membrane process that forces water through a semi-permeable membrane under pressure, leaving dissolved salts behind in a concentrate stream.
RPIRequested Packet Interval: How often an EtherNet/IP connection exchanges cyclic I/O data between a controller and a device.
RS-485: A differential serial signalling standard used as the physical layer for Modbus RTU and other industrial buses.
RTURemote Terminal Unit: A controller at a remote site, optimized for low power, a wide temperature range, and reporting over a constrained link such as radio or cellular.
Rung: One horizontal line of ladder logic: input conditions on the left, an output on the right, read left to right.

S

SCADASupervisory Control and Data Acquisition: The layer that collects data from controllers across a plant or service area, presents it to operators, records it, raises alarms, and allows intervention.
Scaling: Converting a raw analog count into an engineering unit with a linear map from the card range to the transmitter calibrated range.
Scan time: How long a PLC takes to complete one full cycle of reading inputs, solving the program, writing outputs, and housekeeping.
SCCRShort-Circuit Current Rating: The maximum available fault current a control panel can safely withstand, determined by its weakest power-circuit component and required on the panel label.
Septicity: The anaerobic condition wastewater reaches when held too long, generating hydrogen sulfide that corrodes concrete and metal and creates odor and safety hazards.
Sequential function chartSFC: A graphical PLC language that expresses a process as steps and the transitions between them.
SetpointSP: The value a control loop is trying to hold the process at.
Shelving: An operator temporarily silencing a known nuisance alarm, with an automatic expiry and a visible record.
Shielded twisted pair: Instrument cable where the conductors are twisted together and wrapped in a shield, grounded at one end only.
Signal isolator: A device that passes a 4-20 mA value across an optical or transformer barrier, so the input and output sides share no conductive path.
Signal validation: Logic that checks whether an analog reading can be trusted, using out-of-range, frozen-value, and rate-of-change tests.
Situational awareness: An operator understanding of what the plant is doing now, how it got there, and what is about to happen.
Soft starter: A device that ramps motor voltage on starting to limit inrush current and mechanical shock, without providing continuous speed control.
Span: The difference between the upper and lower range values of an instrument. A transmitter ranged 0 to 25 feet has a span of 25 feet.
Square root extraction: Taking the square root of a differential pressure signal to obtain flow, because the differential is proportional to the square of flow.
SSOSanitary Sewer Overflow: A discharge of untreated wastewater from a collection system before it reaches the treatment plant, with regulatory reporting consequences.
State machine: A control structure with explicit named states and defined transitions, rather than a tangle of interlocking conditions.
Structured textST: A high-level, Pascal-like PLC programming language defined by IEC 61131-3, suited to math, loops, and string handling.
Surge protective deviceSPD: A device that diverts transient overvoltage to ground, protecting equipment from lightning and switching surges.

T

Tag: A named reference to a value in a controller or SCADA system, such as Pump_1_Run_Cmd.
Tag quality: An indicator on a SCADA value saying whether it can be trusted, alongside the value itself and a timestamp.
Task: A scheduling container in a controller. A continuous task runs whenever the processor is free; a periodic task runs on a fixed interval; an event task runs on a trigger.
TDHTotal Dynamic Head: The total pressure a pump must develop, combining static lift with friction losses through the piping at a given flow.
Termination: A resistor at each physical end of a bus that matches the cable impedance and absorbs the signal, preventing reflections.
Transmitter: A field device that measures a process variable and converts it into a standard signal, most often 4-20 mA.
Turbidity: A measure of water cloudiness caused by suspended particles, reported in nephelometric turbidity units.
Turndown: The ratio between the largest and smallest measurement an instrument can make within its stated accuracy.

U

UL 508A: The United States safety standard for the construction of industrial control panels, covering component selection, spacing, conductor sizing, marking, and enclosure suitability.
UL 698A: The standard for industrial control panels serving hazardous locations, where the panel provides intrinsically safe field circuits.
UPSUninterruptible Power Supply: A battery-backed supply that carries a load through a power interruption. In control panels this is often a 24 VDC module rather than an AC unit.

V

VFDVariable Frequency Drive: A drive that varies motor speed by varying the frequency and voltage supplied to it.
VLANVirtual Local Area Network: A logical network segment on a managed switch, separating traffic that shares the same physical infrastructure.

W

Watchdog: A timer that faults the processor when a scan takes longer than its configured limit, on the assumption that the program has stopped making progress.
Wet well: The chamber at a lift station that collects incoming wastewater and from which the pumps draw.

Z

Zero: The measured value an instrument reports at the bottom of its calibrated range.
Zones and conduits: The IEC 62443 model of grouping assets with similar security requirements into zones and controlling every communication path, or conduit, between them.

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Content on this site is general technical reference information. It is not engineering advice for a specific installation. Always follow the applicable codes, standards, manufacturer documentation, and your own site safety procedures. Verify against the equipment in front of you before you act.