HART Protocol

Infobox Fieldbus Protocol
Name = HART
Type_of_Network = Device Bus (Process Automation)
Physical_Media = Legacy 4-20 mA analog instrumentation wiring or 2..4GHz Wireless
Network_Topology = One-on-One, Multidrop, Wireless Mesh
Maximum_Devices = 64 in multidrop
Maximum_Speed = Depends on Physical Layer employed
Device_Addressing = Hardware/Software
Governing_Body = HART Communication Foundation
URL = [http://www.hartcomm.org/ www.hartcomm.org]

The HART Communications Protocol ("Highway Addressable Remote Transducer" Protocol) is an early implementation of Fieldbus, a digital industrial automation protocol. Its claim to fame is that it can communicate over legacy 4-20 mA analog instrumentation wiring, sharing the pair of wires used by the older system. According to some, due to the huge installed base of 4-20 mA systems throughout the world, the HART Protocol is one of the most popular industrial protocols today.

The protocol was developed by Rosemount Inc., built off the Bell 202 early communications standard,in the mid-1980s as proprietary digital communication protocol for their smart field instruments. Soon it evolved into HART. In 1986, it was made an open protocol. Since then, the capabilities of the protocol have been enhanced by successive revisions to the specification.

There are two main operational modes of HART instruments: analog/digital mode, and multidrop mode.

Peer-to-Peer mode (analog/digital)

Here the digital signals are overlayed on the 4-20 mA loop current.Both the 4-20 mA current and the digital signal are valid output values from the instrument. The polling address of the instrument is set to "0". Only one instrument can be put on each instrument cable signal pair.

Multi-drop mode (digital)

In this mode only the digital signals are used. The analog loop current is fixed at 4 mA. In multidrop mode it is possible to have up to 15 instruments on one signal cable. The polling addresses of the instruments will be in the range 1-15. Each meter needs to have a unique address.

Packet Structure

The HART Packet has the following structure

Preamble

Currently the all newer devices implement 5 byte preamble, since anything greater reduces the communication speed. However, masters are responsible for backwards support.

Master communication to a new devices starts with the maximum preamble length (20 bytes) and is later reduced once the preamble size for the current device is determined.

tart Delimiter

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

Address

Specifies the destination address as implemented in one of the HART schemes. The original addressing scheme used only 4 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 1 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

tatus

The status field is absent for the master and is 2 bytes for the slave. Thisfield is used by the slave to inform the master whether it completed the taskand what its current health status is.

Data

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

Checksum

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.

External links

* [http://www.thehartbook.com The HART Book] directory of products and suppliers
* [http://www.hartcomm.org HART Communications Foundation]
* [http://www.hart-profibus.com HART Software Development and Consulting]
* [http://www.analogservices.com/about_part0 Detailed HART protocol description; waveforms, message structure, etc]