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. 4-20 ma is an analog electrical transmission standard for industrial instrumentation. ... A Fieldbus is an industrial network system for real-time distributed control. ... This article does not cite any references or sources. ... 4-20 ma is an analog electrical transmission standard for industrial instrumentation. ...

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.

Analog/digital mode

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. 4-20 ma is an analog electrical transmission standard for industrial instrumentation. ...

Multidrop mode

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.

Start 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

Status

The status field is absent for the master and is 2 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

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

• The HART Book directory of products and suppliers
• HART Communications Foundation
• HART Software Development and Consulting
• Detailed HART protocol description; waveforms, message structure, etc