Rockwell Automation 1794-OF8IH FLEX Isolated Input/Output HART Analog Modules - Manual (Page 114)

Publication 1794-UM065B-EN-E - September 2010
102 Additional HART Protocol Information
The master is responsible for controlling message transactions. If there is no
reply to a command within the expected time, the master should retry the
message. 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…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