Highway Addressable Remote Transducer 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.

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.

In point-to-point mode the digital signals are overlaid on the 4–20 mA loop current.

HART revisions 3 through 5 allowed polling addresses of the instruments to be in the range 1–15.

The request HART packet has the following structure: Specifies slave, Specifies Master and Indicates Burst Mode Currently all the newer devices implement five byte preamble, since anything greater reduces the communication speed.

The newer scheme utilizes 38 bits to specify the device address.

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

Each manufacturer that participates in the HART convention is assigned an identification number.

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