In two-wire 4-20mA control loops, we use 2-wire transmitters to convert various process signals representing flow,
speed, position, level, temperature, pressure, strain, pH, etc., to 4-20mA DC for the purpose of transmitting the signal over some distance with little or no loss of signal. This paper reviews the operation of this transmission standard and its advantages, in particular as it relates to two-wire transmitters and the associated 4-20mA current loop.