Revisions of the HART protocol -------------------------------------------------------------------------------- The current revision of the HART protocol specification is 5.3. All new field devices and hosts should follow this revision. But what came earlier? And what should you do about it? Introduction Major revisions Expanded device type code Long frame address format Command #0 Commands #4 and #5 What must you do to be compatible? Field device designers Host designers Host software - how to begin communication Retries Other compatibility issues Future extensions HART Revision 2, 3 and 4 field devices Introduction In the mid-1980s, Rosemount Inc. developed a proprietary digital communication protocol for their smart field instruments. This soon evolved into HART, and was made an open protocol with Revision 2.1 in 1986. Since then, the capabilities of the protocol have been enhanced by successive revisions to the specification, including new commands, additional manufacturer and engineering units codes, protection against cross-reception of messages and improved reporting of command errors. This note summarises the major changes that have been made, and suggests how instrument and host designers should treat them. Users may also be interested to ensure that the host equipment they use can operate properly with the field devices they have. As always, refer to the full specifications from the HART Communication Foundation, for more details. Major revisions The main changes were: Revision Date introduced Features 2 1986 First public specification. Commands #0 to #6, #33 to #48. 3 1987 New command #49 4 1988 Improved support for multiple variables. Write-protect status. Optional type-code expansion. New commands #50 to #56. 5.0 1989 Long frame format, unique identifier. Burst mode. Block commands #4 and #5 replaced by new commands #12 to #18. New commands #11 to #19, #57 to #59, #108 to #112. Improved data link and physical layer specifications. 5.1 1990 Support for multiple analogue outputs and non-current analogue outputs. New commands #60 to #70, #107. 5.2 1993 Physical layer specification revision 7.2 5.3 1994 Minor update to tables With each revision, many of the common tables were extended, for example to accommodate more codes for manufacturers, units and materials. Revisions 5.0, 5.1 and 5.2 included better specifications for the HART physical layer, to assure better reliability and compatibility. From Revision 5.0 onwards, changes have maintained compatibility with older devices. New commands have been added (never removed) and data fields of existing commands have been extended (never shortened). But this was not always so. In particular, Revision 4 introduced the manufacturer identification code, and the step from Revision 4 to Revision 5 brought significant changes in device addressing. These changes are described below. Expanded device type code In HART Revision 4, an option was introduced to allow manufacturer and device type to be coded separately, instead of as a combined 8-bit number. To indicate that this expanded form is used, a field device inserts "254" (hex FE) as the first data byte in its response to command #0. Then the next two bytes contain the manufacturer and device type codes. The expanded form continues in HART Revision 5, and is now mandatory. The long frame address format HART Revision 5 introduced the long frame format, including a "Unique Identifier" (Unique ID) as part of the device address in each message. This Unique ID is formed from a concatenation of the manufacturer code, the device type code, and a device ID number. The device manufacturer must ensure that the device ID number is unique, for each device having a particular device type code. It is rather like a serial number, but doesnot have to be the same as the serial number on the label. The complete Unique ID is