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4-20 mA

(4 to 20 MilliAmp) A point-to-point or multi-drop circuit mainly used in the process automation field to transmit signals from instruments and sensors in the field to a controller. It sends an analog signal from 4 to 20 mA that represents 0 to 100% of some process variable. As a current loop signal, 4-20 mA also powers the sensor transmitter on the same wire pair, and 4-20mA provides more resistance to interference than a voltage-based line. See HART.



HART

(Highway Addressable Remote Transducer) A widely used wired sensor network protocol that dates back to the Bell 202 standard in the 1980s. An extension to the 4-20 mA analog signal, HART superimposes a 1,200 bps signal onto the line that provides bi-directional communications with instruments in the field. See 4-20 mA and WirelessHART.



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