in reply to: Ver 7 Compatability ver 5 CMD 0 posted by Jeff A on 06 September 2010 at 16:07:30.
A rev. 7 field device will always send the full rev. 7 data (22 bytes). The rule is that a HART device must accept, but ignore, any data bytes beyond those it understands. From rev. 5 onwards, the data in any command is never changed in meaning (well, hardly ever: see, for example, cmd. 0 bytes 1 and 2 now don't directly indicate the manufacturer!). New data is always added at the end.
This rule ensures complete compatibility between masters and slaves of any revision.
Just occasionally, for commands with more data in a new revision, a slave does have to determine the revision of the master, and behave differently for an older revision master. For example in cmd. 6, a rev. 5 master only sends one data byte (the polling address) and this also controls the slave's loop current mode, whereas in rev. 6 or later, there is a second data byte to control this. The new revision's command spec defines what to do in such cases - basically, the slave must behave in the way the earlier revision device would have done.