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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 most notable advantage 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 Emerson,〔Emerson ''http://www.automation.com/content/emerson-proves-advancements-in-eddl-electronic-device-description-language-technology'', rev. 2009-09-09〕 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 were comfortable using the legacy 4–20 mA signals, but wanted to implement a "smart" protocol. Industries seem to be using Profibus DP/PA and Foundation fieldbus (also by Rosemount) more as users become familiar with later technology and look to take advantage of the enhanced diagnostics they can provide. 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. ==Modes== There are two main operational modes of HART instruments: analog/digital mode, and multidrop mode. In point-to-point mode (analog/digital) the digital signals are overlaid 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. 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. The analog loop current is fixed at 4 mA. In multidrop mode it is possible to have more than one instruments on one signal cable. HART revisions 3 through 5 allowed polling addresses of the instruments to be in the range 1–15. HART 6 and later allowed address up to 63. Each instrument needs to have a unique address. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Highway Addressable Remote Transducer Protocol」の詳細全文を読む スポンサード リンク
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