Highway Addressable Remote Transducer Protocol


The HART Communication Protocol 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, 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.

Modes

There are two main operational modes of HART instruments: point-to-point mode, and multi-drop mode.

Point to point

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.

Multi-drop

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

The request HART packet has the following structure:
Field NameLength Purpose
Preamble5–20Synchronization and Carrier Detect
Start byte1Specifies Master Number
Address1–5Specifies slave, Specifies Master and Indicates Burst Mode
Expansion0–3This field is 0–3 bytes long and its length is indicated in the Delimiter
Command1Numerical Value for the command to be executed
Number of data bytes1Indicates the size of the Data Field
Data0–255Data associated with the command. BACK and ACK must contain at least two data bytes.
Checksum1XOR of all bytes from Start Byte to Last Byte of Data

Preamble

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 and is later reduced once the preamble size for the current device is determined.
Preamble is: "ff" "ff" "ff" "ff" "ff"

Start delimiter

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

Address

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.

Command

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

Specifies the number of communication data bytes to follow.

Status

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

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

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.
IDManufacturerIDManufacturerIDManufacturer
1Acromag37Ronan73Elcon Instruments
2Allen-Bradley38Rosemount74EMCO
3Ametek39Peek Measurement75Termiflex
4Analog Devices40Schlumberger76VAF Instruments
5Bailey41Sensall77Westlock Controls
6Beckman42Siemens78Drexelbrook
7Bell Microsensor43Camille Bauer79
8Bourns44Toshiba80K TEK
9Bristol Babcock45Transmation81Flowdata
10Brooks Instrument46Rosemount Analytical82Draeger
11Chessell47Valmet83Raytek
12Combustion Engineering48Valtek84Meridian Instruments
13Daniel Industries49Varec85BTG
14Delta50Viatran86Magnetrol
15Dieterich Standard51Weed87Neles Jamesbury
16Dohrmann52Westinghouse88Milltronics
17Endress+Hauser53Xomox89HELIOS
18Fischer and Porter54Yamatake90Anderson Instrument Company
19Fisher Controls55Yokogawa91INOR
20Foxboro56Nuovo Pignone92ROBERTSHAW
21Fuji57Promac93Pepperl+Fuchs
22Hartmann and Braun58Exac Corporation94ACCUTECH
23Honeywell59KDG Mobrey95Flow Measurement
24ITT Barton60Acrom Control System96KAMSTRUP
25KayRay Sensall61Princo97Knick
26Kent62Smar98VEGA
27Leeds and Northrup63Eckardt99MTS SYS CORPS SENSORS
28Leslie64Measurement Technology100Oval
29M System Co65Applied System Technologies101Masoneilan DRESSER
30Measurex66Samson102Besta
31Micro Motion67Sparling Instruments103Ohmart
32Moore Industries68Fireye188Aplisens
33Moore Products69Krohne250not used
34Ohkura Electric70Betz Equipment251none
35Paine71Druck252unknown
36Rochester Instrument Systems72SOR253special
60C2Rhosonics Analytical B.V.24807Barksdale Control Products