Editing Highway Addressable Remote Transducer Protocol (section)

==Modes==
[[File:Locale_RS6_Smart current loop positioner.png|thumb|Example of current loops used for sensing and control transmission. The HART protocol can be overlaid on the 4–20&nbsp;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 [[Current loop#Process-control use|4–20&nbsp;mA]] loop current. Both the 4–20&nbsp;mA current and the digital signal are valid signalling protocols between the controller and measuring instrument or final control element.

The [[polling (computer science)|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&nbsp;mA signal.  Other signals are sent digitally on top of the 4–20&nbsp;mA signal.  For example, pressure can be sent as 4–20&nbsp;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]].

===Multi-drop===
In multi-drop mode the analog loop current is fixed at 4&nbsp;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.