Highway Addressable Remote Transducer Protocol: Difference between revisions

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{{Short description|Industrial automation protocol}}

{{Infobox fieldbus protocol

| Name = HART

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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.

Based on the [[OSI model]], HART resides at Layer 7, the Application Layer. Layers 3–6 are not used.<ref>instrumentationtools ''https://instrumentationtools.com/hart-communication-tutorial-part-3/'' </ref> When sent over [[current loop|4–20&nbsp;mA]] it uses a [[Bell 202 modem|Bell 202]] for layer 1. But it is often converted 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.

==Modes(unknown)==

[[File:Smart current loop positioner.png|thumb|Example of current loops used for sensing and control transmission. The HART protocol can be overlaid on the 4–20&nbsp;mA loops.]]

There are two main operational modes of HART instruments: point-to-point (analog/digital) mode, and multi-drop mode.

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| 5–20

| Synchronization and Carrier Detect

|-

| Start byte

| 1

| Specifies Master Number

|-

| Address

| 1–51 or 5

| defined by bit 7 in Delimiter.
Specifies slave, Specifies Master and Indicates Burst Mode

|-

| Expansion

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===Start delimiter===

This byte contains the Master number and specifies that the communication packet is starting.

{| class="wikitable" style="margin: 0 auto; text-align:center"

|+Delimiter Field

|-

! [[Bit]]

! style="text-align:center;" | 7 !! 6 !! 5 !! 4 !!3 !! 2 !! 1 !! 0

|-

! Desc

| Address size

|colspan="2"| Number of Expansion bytes

|colspan="2"| Physical layer type

|colspan="3"| Frame type

|}

;bit 7, if high use Unique (5 byte) address, else use Polling (1 Byte) addresses.

;bit 6 and 5, Number of Expansion bytes: normally it set if Expansion field is used, normally 0.

;bit 4 and 3, Physical layer type: 0=Asynchronous, <br> 1=Synchronous

;bit 2, 1 and 0, Frame type: 1=BACK Burst Acknowledge send by Burst-mode Device<br> 2=STX Master to Field Devices.<br> 6=Slave Acknowledge to STX frame.

===Address===

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==External links==

* [http://www.FieldCommGroup.org/ FieldComm Group]

* [http://hartprotocollite.codeplex.com/ .NET Open Source project] {{Webarchive|url=https://web.archive.org/web/20171223221824/http://hartprotocollite.codeplex.com/ |date=2017-12-23 }}

{{List of automation protocols}}