HART^® (Highway Addressable Remote Transducer) is a digital communication protocol that is based upon the Bell 202 telephone communication standard. By using FSK (frequency shift keying) to superimpose low level digital signals on top of an analog 4-20 mA signal, two simultaneous communications channels exist. The 4-20 mA signal communicates the primary measured value, and the digital signal communicates additional device information to the plant control system.