4–20 mA loop
A standard analog transmission method where current between 4 and 20 milliamps represents 0% to 100% of an engineering span.
FIELD GLOSSARY
Words that make
the wiring clearer.
Short definitions, field context, and the distinction that usually matters five minutes later when working with industrial I/O.
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field definitions
field definitions
Live zero
The 4.0 mA baseline in a 4–20 mA loop that allows the controller to differentiate between a valid zero process measurement and an open or severed cable (0 mA).
NAMUR NE43
An international standard defining diagnostic thresholds for current loops: <3.6 mA indicates a wire break or transmitter fault; >21.0 mA signals sensor saturation or hardware failure.
Loop power
A two-wire circuit topology where DC supply voltage and the return analog current signal share the exact same two-conductor copper path.
Headroom
The remaining DC voltage available at the transmitter terminals after subtracting all voltage drops across the series load resistor, barriers, and cable resistance from the cabinet supply.
Burden resistance
The cumulative resistance of all receivers, indicators, safety barriers, and cabling wired in series within an analog current loop.
Shunt resistor
A precision resistor (commonly 250.0 Ω ±0.1%) placed across an analog voltage input to convert a 4–20 mA current signal into a 1.0–5.0 V DC reading.
PNP (Sourcing)
A DC switching output where an internal transistor connects the output terminal to positive supply (+24 V DC) when active.
NPN (Sinking)
A DC switching output where an internal transistor connects the output terminal to 0 V Common when active.
Sourcing input
A digital PLC input module that provides +24 V DC at its terminal, waiting for an external sinking (NPN) field device to pull it to 0 V.
Sinking input
A digital PLC input module that references internal 0 V Common, designed to receive +24 V DC current sourced by an external PNP sensor.
Push-pull output
An active transistor stage containing both high-side and low-side switches, capable of driving either PNP or NPN inputs without external pull-up resistors.
Hysteresis
The intentional physical or electrical difference between a sensor’s trip point (activation) and reset point (deactivation), preventing contact chatter when a target hovers at the threshold.
Debounce
A software filter or hardware RC delay that suppresses false multiple triggers caused by mechanical contact bounce in relays or limit switches.
Flyback diode
A reverse-biased diode wired parallel across an inductive DC coil (relay, solenoid) to clamp destructive high-voltage inductive kickback spikes when de-energized.
Galvanic isolation
The complete physical and electrical separation of two circuits using optical, magnetic, or capacitive coupling, preventing ground loops and cross-channel noise transfer.
Ground loop
An unwanted circulating current formed when two interconnected system points are tied to ground potentials that differ by even fractions of a volt.
Common mode voltage
A voltage that appears simultaneously on both signal conductors relative to local earth or cabinet reference ground.
CMRR (Common Mode Rejection Ratio)
The metric defining an analog input’s ability to suppress unwanted electrical noise shared identically across both differential input lines.
Differential signaling
A transmission technique using two paired conductors carrying inverted voltages (+ and −), allowing the receiver to subtract common-mode electromagnetic noise.
Shield drain
An uninsulated tinned copper conductor laid in continuous physical contact with foil cable shielding, providing a low-impedance termination path to protective earth.
Twisted pair
A cable construction where signal and return wires are twisted together so external electromagnetic interference couples equally into both conductors and cancels out.
Ferrite bead
A passive magnetic ceramic cylinder clamped over conductors to suppress high-frequency common-mode electromagnetic interference (EMI).
RTD (Resistance Temperature Detector)
A temperature transducer whose electrical resistance increases predictably with temperature, typically utilizing pure platinum wire.
Pt100
A platinum RTD having an exact electrical resistance of 100.00 Ω at 0 °C, defined with a standard temperature coefficient of α = 0.00385 Ω/Ω/°C (IEC 60751).
Pt1000
A platinum RTD having an electrical resistance of 1,000.0 Ω at 0 °C, offering tenfold higher sensitivity and reduced susceptibility to field lead wire resistance.
Three-wire RTD
A bridge wiring technique that uses a third matched lead wire to cancel out conductor resistance in long field cable runs.
Four-wire RTD (Kelvin)
A precision RTD wiring method that supplies excitation current on two dedicated leads and measures voltage on two separate leads, eliminating lead resistance entirely.
Thermocouple
A temperature sensor formed by joining two dissimilar metals that generates a minute thermoelectric voltage (Seebeck effect) proportional to the junction temperature difference.
Cold junction compensation (CJC)
The measurement and algorithmic compensation of temperature at the terminal block where thermocouple alloy wires meet copper instrumentation terminals.
Seebeck effect
The physical phenomenon whereby a temperature gradient along a conductor produces a measurable electromotive force (voltage).
RS-485 (TIA/EIA-485)
A balanced, differential multipoint serial standard supporting high-speed data transmission up to 1,200 meters across a single twisted pair.
Modbus RTU
A compact, binary serial communications protocol widely deployed over RS-485 buses using master/slave polling architecture.
Termination resistor
A resistor (typically 120 Ω) installed across differential bus lines at both extreme physical ends of an RS-485 network to absorb signal reflections.
Bus biasing
A resistor network that pulls RS-485 differential lines to a known idle state (typically >200 mV) when all transmitters on the bus are in high-impedance mode.
Signal reflection
The bouncing back of electrical energy that occurs when a high-frequency signal wavefront strikes an unterminated cable end or impedance discontinuity.
Baud rate
The rate at which symbol or electrical transitions occur along a communication line, commonly 9600, 19200, 38400, or 115200 bps on industrial serial buses.
IO-Link
A point-to-point digital industrial communications standard (IEC 61131-9) extending standard 3-wire sensor cabling to deliver cyclic process data, diagnostics, and parameterization.
IODD (IO Device Description)
An XML-based device description file containing all parameter names, communication speeds, process data layouts, and diagnostic codes for an IO-Link device.
Master port
The physical port on an IO-Link master module capable of operating in standard digital input (DI), standard digital output (DO), or digital IO-Link point-to-point mode.
Raw counts
The unscaled, raw integer value output by an analog-to-digital converter (ADC) register before PLC engineering unit scaling is applied (e.g., 0–27648 or 0–32767).
ADC (Analog-to-Digital Converter)
The semiconductor integrated circuit inside an analog input module that quantizes continuous field voltages or currents into discrete digital numbers.
Resolution
The smallest increment of an input signal that an ADC can detect, typically expressed in bits (e.g., 16-bit resolution provides 65,536 distinct steps).
Engineering units
The real-world physical values represented by a signal measurement, such as bar, psi, °C, gallons/minute, or millimeters of stroke.
Two-point scaling
A linear calibration formula: Y = Y_min + (X - X_min) * ((Y_max - Y_min) / (X_max - X_min)) applied in PLC logic to convert raw ADC counts into engineering units.
SELV (Safety Extra-Low Voltage)
An electrical supply system with an output voltage not exceeding 50 V AC or 120 V DC that is electrically separated from earth and higher-voltage grids.
PELV (Protective Extra-Low Voltage)
An extra-low voltage circuit similar to SELV but deliberately referenced or bonded to protective earth at one defined point for safety grounding.
Interposing relay
An intermediate electromechanical or solid-state relay used to isolate a sensitive low-power PLC digital output from a heavy industrial contactor or motor coil.
Solid-state relay (SSR)
An electronic switching device that switches load current using power semiconductors (TRIAC, MOSFET, IGBT) without moving contacts, triggered via optocoupler.
Contact bounce
The repeated mechanical rebound of electrical contacts during closing, causing multiple transient on/off pulses before coming to rest.
Inductive kickback
A high-voltage spike generated across an inductor (L * di/dt) when current flow is abruptly interrupted, capable of damaging transistor outputs if unclamped.
Snubber circuit
An electrical network consisting of a resistor and capacitor (RC) connected across contacts or semiconductor switches to suppress voltage transients and arcing.
Hart protocol
A bi-directional digital communication protocol that modulates 1.2 kHz and 2.2 kHz frequency-shift keyed (FSK) audio tones directly on top of a standard 4–20 mA current loop.
NAMUR
The international user association of automation technology in process industries that publishes influential standards for instrumentation, diagnostics, and explosion safety.
M12 connector
A circular 12 mm screw-locking industrial connector standard (IEC 61076-2-101) utilizing various keyway codings for power, sensors, and fieldbuses.
A-coding
The standard M12 keyway configuration used for DC sensors, actuators, discrete I/O, and low-voltage DC power distribution.
D-coding
A 4-pin M12 keyway configuration dedicated to 100 Mbps industrial Ethernet protocols, such as Profinet, EtherNet/IP, and Modbus TCP.
X-coding
An 8-pin shielded M12 connector keyway designed to support Gigabit (1 Gbps to 10 Gbps) Cat 6A industrial Ethernet in severe vibration environments.
Watchdog timer
A hardware or firmware safety timer that detects software crashes or communication freeze by timing out and forcing outputs to a safe shutdown state unless periodically reset.
Scan time
The total duration required by a PLC processor to read physical inputs, execute the complete user application program, write to outputs, and complete house-keeping tasks.
Optical isolation (Optocoupler)
A semiconductor component that transfers electrical signals between isolated circuits using a light-emitting diode and a photodetector.
Gauge pressure
A pressure measurement referenced relative to ambient local atmospheric pressure (0 bar gauge = 1 atm absolute).
Absolute pressure
A pressure measurement referenced to a perfect vacuum zero (0 bar absolute = full vacuum).
Differential pressure (DP)
The mathematical difference in pressure between two distinct measurement points, widely used to calculate fluid level or orifice flow rate.
Creepage distance
The shortest path along the surface of an insulating material between two conductive parts, critical for electrical isolation safety.
Clearance distance
The shortest distance through air between two conductive parts, determining dielectric breakdown resistance against transient overvoltages.