The HART Communication Protocol (Highway Addressable Remote Transducer) is a hybrid analog+digital industrial automation protocol. Its most notable advantage is that it can communicate over legacy 4–20 mA analog instrumentation current loops, sharing the pair of wires used by the analog only host systems.
According to Emerson, 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. HART protocol has made a good transition protocol for users who wished to use the legacy 4–20 mA signals, but wanted to implement a “smart” protocol.
The protocol was developed by Rosemount Inc., built off the Bell 202 early communications standard in the mid-1980s as a proprietary digital communication protocol for their smart field instruments. Soon it evolved into HART and 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: point to point (analog/digital) mode, and multi-drop mode.
Point to point
In point-to-point mode the digital signals are overlaid on the 4–20 mA loop current. Both the 4–20 mA current and the digital signal are valid signalling protocols between the controller and measuring instrument or final control element.
The polling address of the instrument is set to “0”. Only one instrument can be put on each instrument cable signal pair. One signal, generally specified by the user, is specified to be the 4–20 mA signal. Other signals are sent digitally on top of the 4–20 mA signal. For example, pressure can be sent as 4–20 mA, representing a range of pressures, and temperature can be sent digitally over the same wires. In point-to-point mode, the digital part of the HART protocol can be seen as a kind of digital current loop interface.
Multi-drop
In Multi-drop mode the analog loop current is fixed at 4 mA and it is possible to have more than one instrument on a signal loop.
HART revisions 3 through 5 allowed polling addresses of the instruments to be in the range 1–15. HART revision 6 and later allowed address up to 63. Each instrument needs to have a unique address.