The three-wire transmitter uses two wires for power and the third wire is used for the output signal (+) positive terminal. The power (-) negative terminal is used as a common reference for power and the signal (-) negative reference. This allows the best of both transmitter features to be utilized.he four-wire transmitter has two wires for power and two wires for the output signal. The power for a four-wire transmitter can be either AC or DC and the output signal can be either voltage or current.