The HART Communications Protocol (Highway Addressable Remote Transducer Protocol) is an early implementation of Fieldbus, a digital industrial automation protocol. Its claim to fame is that it can communicate over legacy 4-20 mA analog instrumentation wiring, sharing the pair of wires used by the older system. According to some, due to the huge installed base of 4-20 mA systems throughout the world, the HART Protocol is one of the most popular industrial protocols today.