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The '''HART Communication Protocol (Highway Addressable Remote Transducer)''' is a hybrid analog+digital industrial automation open 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. HART is widely used in process and instrumentation systems ranging from small automation applications up to highly sophisticated industrial applications.
HART is a in the [[OSI_model]] a Layer 7, Application. Layer 3-6 is not used. <ref>instrumentationtools ''https://instrumentationtools.com/hart-communication-tutorial-part-3/'' </ref>. When sendt over [[current loop|4–20 mA]] it uses e [[Bell 202 modem|Bell 202]] for layer 1, Physicals. But is often convert to RS485 or RS232.
According to Emerson,<ref>Emerson ''https://web.archive.org/web/20181107104043/https://www.automation.com/automation-news/industry/emerson-proves-advancements-in-eddl-electronic-device-description-language-technology'', rev. 2005-04-14</ref> due to the huge installation 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.
==History==
The protocol was developed by [[Rosemount Inc.]], built off the [[Bell 202 modem|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.
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