The HART Communications Protocol (Highway Addressable Remote Transducer Protocol) is an early implementation of Fieldbus, a digital industrial automation protocol. Its most notable advantage is that it can communicate over legacy 4~20mA analog instrumentation wiring, sharing the pair of wires used by the older system. According to Emerson, due to the huge installed base of 4-20 mA systems throughout the world, the HART Protocol was one of the most popular industrial protocols today. HART protocol made a good transition protocol for users who were comfortable using the legacy 4-20 mA signals, but wanted to implement a "smart" protocol. Industries seem to be using Foundation fieldbus (also by Rosemount) more and more as users become familiar with current technology. The protocol was developed by Rosemount Inc., built off the Bell 202 early communications standard, in the mid-1980s as proprietary digital communication protocol for their smart field instruments. Soon it evolved into HART. In 1986, it was made an open protocol. Since then, the capabilities of the protocol have been enhanced by successive revisions to the specification.

There are two main operational modes of HART instruments: analog/digital mode, and multidrop mode.

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