Glossary
Signals and PLC Terms
Signal and PLC terms: 4 to 20 mA, HART, Modbus, RS485, fieldbus, isolation, grounding, scaling, scan time and digital I/O.
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All Signals and PLC Terms
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Formula4 to 20 mA Current LoopA 4 to 20 mA current loop sends a measurement as a current between 4 and 20 mA, which is not…FormulaADC ResolutionADC resolution is the number of bits an analog to digital converter uses, which sets the…FormulaAnalog ScalingAnalog scaling converts the raw value from a PLC analog input into engineering units such as…BCDBinary Coded DecimalBinary coded decimal stores each decimal digit as its own 4 bit binary group, used by…TermCable Shield GroundingCable shield grounding connects the screen of an instrument cable to earth, normally at one end…TermDry ContactA dry contact is a switch or relay contact with no voltage of its own; the receiving circuit…EMIElectromagnetic InterferenceElectromagnetic interference is unwanted electrical noise coupled into signal circuits from…FormulaEncoderAn encoder converts shaft rotation into pulses or a digital position, used to measure speed and…
TermFieldbusFieldbus is a digital network that connects field instruments to the control system, carrying…TermGalvanic IsolationGalvanic isolation separates two circuits so no current can flow directly between them, while…FormulaGray CodeGray code is a binary code in which only one bit changes between consecutive values, avoiding…TermGround LoopA ground loop occurs when a circuit is earthed at two points with different potentials, letting…TermHART ProtocolThe HART protocol superimposes a small digital signal on the 4 to 20 mA loop, allowing…TermInterposing RelayAn interposing relay is a relay driven by a PLC output that switches a load the PLC cannot…TermLive ZeroLive zero means the lowest signal value (4 mA) is above zero, so a broken loop (0 mA) can be…FormulaLoop ResistanceLoop resistance is the total resistance in a 4 to 20 mA loop, from cable, input resistors and…
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TermMarshalling CabinetA marshalling cabinet is where field cables are terminated and cross wired to the correct…TermModbus RTUModbus RTU is a simple master slave serial protocol, usually on RS485, that reads and writes…TermModbus TCPModbus TCP carries Modbus messages over Ethernet and TCP/IP, normally on port 502.TermNAMUR NE43NAMUR NE43 is a recommendation that defines 4 to 20 mA signal levels for measurement and for…TermNAMUR SensorA NAMUR sensor is a two wire proximity sensor that signals by changing its current, used with…TermPLC RedundancyPLC redundancy uses duplicate controllers, power supplies or networks so the system keeps…FormulaPLC Scan TimePLC scan time is the time a PLC takes to read inputs, execute the program and update outputs once.FormulaPulse OutputA pulse output sends one pulse per fixed quantity of flow, or a frequency proportional to flow,…
TermRS485RS485 is a differential serial interface that lets many devices share one twisted pair over…FormulaSignal DampingSignal damping is a filter applied in a transmitter or PLC that smooths a noisy measurement by…TermSignal IsolatorA signal isolator copies an analog signal from input to output with galvanic isolation, often…TermSinking and SourcingSinking and sourcing describe the direction of current in digital I/O: a sourcing device…TermTwo Wire TransmitterA two wire transmitter is powered by the same 4 to 20 mA loop it signals on, so it needs only…
Frequently Asked Questions
What does the signals and plc glossary cover?
Signal and PLC terms: 4 to 20 mA, HART, Modbus, RS485, fieldbus, isolation, grounding, scaling, scan time and digital I/O.
Are the formulas checked?
Yes. Formulas follow the relevant standards such as ISO 5167, and every worked example is calculated and checked.
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