4-20 mA Analog Signal
4-20 mA is the dominant analog signaling standard in industrial controls. A two-wire field instrument modulates the loop current between 4 mA, zero scale and 20 mA, full scale to communicate a process variable to the control system. Below 4 mA or above 20 mA indicates a fault, per NAMUR NE 43.
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The 4-20 mA current loop has been the default analog signal in process instrumentation since the 1970s, and it survives in current greenfield projects for a reason that has nothing to do with bandwidth. It is the most troubleshootable signal in the plant. Because information is carried as current rather than voltage, the same value reads correctly at the end of a long cable run, where a voltage signal would have drifted. Because the live zero sits at 4 mA rather than 0, a broken wire or a dead transmitter falls to 0 mA and is distinguishable from a legitimate bottom-of-scale measurement. A two-wire transmitter draws its own power from the same pair that carries the signal, so one cable does both jobs, and every analog input card on every controller accepts it, so a technician can clamp a meter on the loop and read the process variable directly. The standard that turns the live zero into a fault-detection scheme is NAMUR NE 43, which defines currents below 3.6 mA and above 21 mA as instrument faults rather than measurements. Digital systems layer on top rather than replacing it. HART rides a digital signal on the same 4-20 mA pair to expose calibration, diagnostics, and secondary variables, while Foundation Fieldbus and Profibus PA replace the loop with a digital bus where the segment economics justify it. On an I/O list every analog instrument carries its engineering range alongside the signal class, because that range is what the controller uses to scale the raw current back into process units.
Where 4-20 mA wins.
It is loop-powered, the field device draws its own power from the same two wires that carry the signal, it tolerates long cable runs, and it is impedance-tolerant. Voltage-based signaling, 0-10 V drifts over distance. 4-20 mA does not because current is conserved. Any 1980s-era field instrument talks 4-20 mA. Any modern AI card on any PLC accepts it. It is the universal solvent of process measurement.
Modern variants and overlays.
HART, Highway Addressable Remote Transducer overlays a digital signal on the same 4-20 mA pair, letting smart instruments report calibration data, secondary variables, and diagnostics without re-cabling. Foundation Fieldbus and Profibus-PA replace 4-20 mA entirely with digital bus topology. Both add value, both have not displaced 4-20 mA in greenfield petrochemical work because the troubleshooting and replacement workflow with 4-20 mA is so well understood.
Frequently asked.
What does NAMUR NE 43 specify.
NE 43 defines fault indication. 3.6 mA, or below means transmitter under-range, fault, 21 mA, or above means transmitter over-range, fault. PLCs configured to NAMUR detect these out-of-band currents and raise an alarm rather than treat them as legitimate measurements.
Can a 4-20 mA loop be powered from the PLC.
Yes. Loop-powered transmitters draw current via the AI card. 4-wire transmitters get separate AC or DC power and use the 4-20 mA pair only for signal. The I/O list flag tells the panel builder which wiring scheme each tag needs.
How does a 4-20 mA signal map to engineering units in the PLC.
The AI card digitizes the current to a raw count, typically 0-32767 or 0-65535 and the PLC configuration block applies a linear scaling. 4 mA maps to the low process-variable range, 20 mA maps to the high range. The I/O list carries the engineering range, for example, 0-100 kPa for PT-204, and the panel builder or integrator enters those values into the scaling block during commissioning.