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

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

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 (Start byte)
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.

Preamble is: "ff" "ff" "ff" "ff" "ff" (5 times ff)

Start delimiter

This byte contains the Master number and specifies that 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

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

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. (Note the list below is non-exhaustive and deprecated (Dated Nov. 15th 1996), some of the corporations listed are now defunct and the list is constantly in flux with new manufacturers entering the market. Also, in the current version of the standard devices report ID codes in hexadecimal instead of decimal values.)[2]

Manufacturer codes[3]
ID Manufacturer ID Manufacturer ID Manufacturer
1 Acromag 37 73
2 Allen-Bradley 38 Rosemount 74
3 Ametek 39 75
4 Analog Devices 40 Schlumberger 76
5 41 77
6 Beckman 42 Siemens 78
7 43 79
8 Bourns 44 Toshiba 80
9 45 81
10 Brooks Instrument 46 82 Draeger
11 Chessell 47 Valmet 83
12 Combustion Engineering 48 84
13 49 85 BTG
14 Delta 50 86
15 51 Weed 87
16 52 Westinghouse 88
17 Endress+Hauser 53 89
18 54 90
19 Fisher Controls 55 Yokogawa 91
20 Foxboro 56 92
21 Fuji 57 93 Pepperl+Fuchs
22 ABB - Hartmann and Braun 58 94
23 Honeywell 59 95
24 60 96
25 61 97
26 Kent 62 98 VEGA
27 63 99
28 Leslie 64 100
29 65 101
30 Measurex 66 Samson 102 Besta
31 67 103
32 Moore Industries 68 188
33 69 250 not used
34 70 251 none
35 71 Druck 252 unknown
36 72 SOR 253 special
60C2 24807


  1. ^ Emerson, rev. 2009-09-09
  2. ^ FieldComm Group, retrieved 2020-05-04
  3. ^ HART - SMART Communications Protocol, Common Tables Document, Revision: 9.0, Version:A Nov/15/96 (HCF_SPEC-183)

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