Highway Addressable Remote Transducer Protocol

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Protocol Information
Type of NetworkDevice (Process Automation)
Physical Media4–20 mA analog instrumentation wiring or 2.4 GHz wireless
Network TopologyPoint-to-point, multidrop, wireless mesh
Maximum Devices15 in multidrop
Maximum SpeedDepends on physical layer employed
Device AddressingHardware/software
Governing BodyFieldComm Group

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.

According to Emerson,[1] 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.

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.


Example of current loops used for sensing and control transmission. The HART protocol can be overlaid on the 4–20 mA loops.

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

Point to point[edit]

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.


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 allowed addresses 1 to 63; HART revision 7 allows addresses 0 to 63. Each instrument must have a unique address.

Packet structure[edit]

The request HART packet has the following structure:

Field Name Length (in bytes) Purpose
Preamble 5–20 Synchronization and Carrier Detect
Start byte 1 Specifies Master Number
Address 1–5 Specifies slave, Specifies Master and Indicates Burst Mode
Expansion 0–3 This field is 0–3 bytes long and its length is indicated in the Delimiter
Command 1 Numerical Value for the command to be executed
Number of data bytes 1 Indicates the size of the Data Field
Data 0–255 Data associated with the command. BACK and ACK must contain at least two data bytes.
Checksum 1 XOR of all bytes from Start Byte to Last Byte of Data


Currently all the newer devices implement five byte preamble, since anything greater reduces the communication speed. However, masters are responsible for backwards support. Master communication to a new device starts with the maximum preamble length (20 bytes) and is later reduced once the preamble size for the current device is determined.

Start delimiter[edit]

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


Specifies the destination address as implemented in one of the HART schemes. The original addressing scheme used only four bits to specify the device address, which limited the number of devices to 16 including the master.

The newer scheme utilizes 38 bits to specify the device address. This address is requested from the device using either Command 0, or Command 11.


This is a one byte numerical value representing which command is to be executed. Command 0 and Command 11 are used to request the device number.

Number of data bytes[edit]

Specifies the number of communication data bytes to follow.


The status field is absent for the master and is two bytes for the slave. This field is used by the slave to inform the master whether it completed the task and what its current health status is.


Data contained in this field depends on the command to be executed.


Checksum is composed of an XOR of all the bytes starting from the start byte and ending with the last byte of the data field, including those bytes.

Manufacturer codes[edit]

Each manufacturer that participates in the HART convention is assigned an identification number. This number is communicated as part of the basic device identification command used when first connecting to a device.

Manufacturer codes[2]
ID Manufacturer ID Manufacturer ID Manufacturer
1 Acromag 37 Ronan 73 Elcon Instruments
2 Allen-Bradley 38 Rosemount 74 EMCO
3 Ametek 39 Peek Measurement 75 Termiflex
4 Analog Devices 40 Schlumberger 76 VAF Instruments
5 Bailey 41 Sensall 77 Westlock Controls
6 Beckman 42 Siemens 78 Drexelbrook
7 Bell Microsensor 43 Camille Bauer 79
8 Bourns 44 Toshiba 80 K TEK
9 Bristol Babcock 45 Transmation 81 Flowdata
10 Brooks Instrument 46 Rosemount Analytical 82 Draeger
11 Chessell 47 Valmet 83 Raytek
12 Combustion Engineering 48 Valtek 84 Meridian Instruments
13 Daniel Industries 49 Varec 85 BTG
14 Delta 50 Viatran 86 Magnetrol
15 Dieterich Standard 51 Weed 87 Neles Jamesbury
16 Dohrmann 52 Westinghouse 88 Milltronics
17 Endress+Hauser 53 Xomox 89 HELIOS
18 Fischer and Porter 54 Yamatake 90 Anderson Instrument Company
19 Fisher Controls 55 Yokogawa 91 INOR
20 Foxboro 56 Nuovo Pignone 92 ROBERTSHAW
21 Fuji 57 Promac 93 Pepperl+Fuchs
22 Hartmann and Braun 58 Exac Corporation 94 ACCUTECH
23 Honeywell 59 KDG Mobrey 95 Flow Measurement
24 ITT Barton 60 Acrom Control System 96 KAMSTRUP
25 KayRay Sensall 61 Princo 97 Knick
26 Kent 62 Smar 98 VEGA
27 Leeds and Northrup 63 Eckardt 99 MTS SYS CORPS SENSORS
28 Leslie 64 Measurement Technology 100 Oval
29 M System Co 65 Applied System Technologies 101 Masoneilan DRESSER
30 Measurex 66 Samson 102 Besta
31 Micro Motion 67 Sparling Instruments 103 Ohmart
32 Moore Industries 68 Fireye 188 Aplisens
33 Moore Products 69 Krohne 250 not used
34 Ohkura Electric 70 Betz Equipment 251 none
35 Paine 71 Druck[disambiguation needed] 252 unknown
36 Rochester Instrument Systems 72 SOR[disambiguation needed] 253 special
60C2 Rhosonics Analytical B.V. 24807 Barksdale Control Products


  1. ^ Emerson http://www.automation.com/content/emerson-proves-advancements-in-eddl-electronic-device-description-language-technology, rev. 2009-09-09
  2. ^ HART - SMART Communications Protocol, Common Tables Document, Revision: 9.0, Version:A Nov/15/96 (HCF_SPEC-183)

External links[edit]