4-Wire Current Loop

DC electric current signals may also be used to communicate process measurement information from
transmitters to controllers, indicators, recorders, alarms, and other input devices. The simplest form
of 4-20 mA measurement loop is one where the transmitter has two terminals for the 4-20 mA signal
wires to connect, and two more terminals where a power source connects. These transmitters are
called “4-wire” or self-powered. The current signal from the transmitter connects to the process
variable input terminals of the controller to complete the loop: