The Full Form of HEAT is Highway Addressable Remote Transducer.

The Highway Addressable Remote Transducer protocol (HART) is an early version of the fieldbus, a digital, industrial automation protocol. The largest advantage is the fact that it is based on a conventional 4 to 20 mA standard which serves the transmission of analog signals. The existing wiring of the old system can still be used, and parallel operation of both is possible.
Because of the basis of the 4 to 20 mA standard, the Highway Addressable Remote Transducer protocol is one of the most widely distributed industry protocols. Data are transmitted according to the Bell 202 standard using frequency shift keying technology. The Highway Addressable Remote Transducer protocol offers very good user friendliness for users using the conventional 4 to 20 mA protocol who want to use a “smarter” system with a wider scope of services.

The protocol was developed by Rosemount Inc. in the middle of the 1980s as a proprietary communication protocol which was to be used in the company’s own intelligent fieldbus devices. This soon developed into the Highway Addressable Remote Transducer protocol. In 1986, it was changed to an open protocol. Since then, the abilities of the protocol have been expanded by successive changes to the specifications.

There are two modes of operation for Highway Addressable Remote Transducer protocol devices: an analog/digital mode and a multi-drop mode.

In point-to-point operation (analog/digital), the digital signals are applied to the 4 to 20 mA system. As such, the 4 to 20 mA current and the digital signal, are valid output values of the device. The polling address of the device is set to “0”. Only one device can be used per signal cable pair. With this, a signal defined by the user can be sent as a 4 to 20 mA signal. Other signals are applied digitally to the 4/20 mA signal with the Highway Addressable Remote Transducer protocol. For example, a pressure indication can be sent as a 4 to 20 mA signal, but a range of pressures or temperatures can also be sent over the same lines in a digital form.

Only the digital signals are used in multi-drop operation (digital). With this, the analog loop current is fixed to 4 mA. In multi-drop mode, more than one device can be operated per signal cable pair. Revisions 3 to 5 of the Highway Addressable Remote Transducer protocol permit polling addresses in the range from 1 to 15. HART 6 (and above) permits an address up to 63. Each device must have a unique own address.