Rockwell Automation 1794-APBDPV1 FLEX I/O PROFIBUS Adapter User Manual - Manual (Page 135)

Publication 1794-UM064B-EN-P - May 2014
122 Additional HART Protocol Information
After a few retries, the master should abort the transaction, since presumably the
slave device or the communication link has failed.
After each transaction is completed, the master should pause for a short time
before sending another command, to allow an opportunity for the other master
to break in if it wishes. This way, two masters (if they are present) take turns at
communicating with the slave devices. Typical message lengths and delays allow
two transactions per second.
Burst Mode (not supported)
To achieve a higher data rate, some field devices implement an optional burst
mode. When switched into this mode, a slave device repeatedly sends a data
message, as though it had received a specific command to do so. Special
commands, 107, 108, and 109, are used to start and stop this mode of operation,
and to choose which command should be assumed. If burst mode is
implemented, commands 1, 2, and 3 must be supported; other commands are
optional. There is a short pause after each burst message to allow a master device
to send a command to stop the burst mode operation, or to initiate any other
single transaction, after which burst messages will continue.
Generally, burst mode is only useful if there is just one field device attached to a
pair of wires, since only one field device on a loop can be in burst mode at any one
time. In burst mode, more than three messages can be transmitted per second.
The actual HART message between the FLEX I/O HART module and the field
device follows the standard HART messaging protocol.
Preamble
The preamble is a number of hexadecimal FF characters that precede all frames
sent to the HART field device. The size depends on the field devices being used,
but it can be from 2 to 32 hexadecimal. The default is 10. The Smart Transmitter
Interface inserts the required preamble before each packet or frame transmission
to the HART device. This is done automatically so you do not have to program
the host processor to do this.
Preamble Start
Character
Address Command Byte
Count
[Response
Code] Data Checksum