in reply to: HART protocol bytes posted by Kalle Happonen on 19 December 2002 at 12:34:54.
Here's how the start byte is encoded:
In HART Revision 5:
Start characters (all in hex) ...
Message type Short frame Long frame
Master to slave 02 82
Slave to master 06 86
Burst message from slave 01 81
These characters can be fully identified by the content of bits 0, 1, 2 and 7, thus:
Bit 7 indicates the Address Type (1 byte or 5 bytes),
Bits 2, 1 and 0 indicate the Frame Type, with values 1 (burst frame), 2 (master to slave), or 6 (slave to master).
In HART Revision 6, in addition to the above:
Bits 6 and 5 indicate the number of Expansion Bytes (0 to 3 bytes, between the Address and Command fields),
Bits 4 and 3 indicate the Physical Layer Type (0 for normal FSK HART).
The use of Expansion Bytes is controlled by the HCF. As far as I know, no uses have yet been specified.