Rockwell Automation 1797-OE8H FLEX Ex HART Analog Modules User Manual - Manual (Page 150)

Publication 1797-6.5.3 - March 2006
C-2 Additional HART Protocol Information
Transaction Procedure
HART is a half-duplex protocol; after completion of each message, the FSK
carrier signal must be switched off, to allow the other station to transmit. The
carrier control timing rules state that the carrier should be turned on not more
than 5 bit times before the start of the message (that is, the preamble) and
turned off not more than 5 bit times after the end of the last byte of the
message (the checksum).
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 Ex HART I/O module and the
field device follows the standard HART messaging protocol.
Preamble Start
character
Address Command Byte
count
[Response
code] Data Checksum