HART is an acronym for “Highway Addressable Remote Transducer”. The HART protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA as shown in fig. This enables two-way field communication to take place and makes it possible for additional information (tag numbers, measured variables, range & span data) beyond just the normal process variable to be communicated to/from a smart field instrument.
The HART protocol communicates at 1200 bps without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a field device. As the digital FSK signal is phase continuous, there is no interference with the 4-20mA signal.