HART Protocol

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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.

The protocol was developed by Rosemount Inc. 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.

[edit] Analog/digital mode

Here the digital signals are overlayed on the 4-20 mA loop current. Both the 4-20 mA current and the digital signal are valid output values from the instrument. The polling address of the instrument is set to "0". Only one instrument can be put on each instrument cable signal pair.

[edit] Multidrop mode

In this mode only the digital signals are used. The analog loop current is fixed at 4 mA. In multidrop mode it is possible to have up to 15 instruments on one signal cable. The polling addresses of the instruments will be in the range 1-15. Each meter needs to have a unique address.