Loop Powered Detectors are typically operated using two wires connected to
a regulated 24VDC power supply. The two power wires also carry the measurement
See Model TA-2102 Hydrogen Iodide Gas Detector
Four wire Detectors utilize one pair of wires for connection of power (regulated
24VDC) and a separate pair of wires for carrying the 4-20mA measurement signal. Optional Relays, Modbus...
See Model TA-2100 Hydrogen Iodide Gas Detector