Rockwell Automation 1797-OE8H FLEX Ex HART Analog Modules User Manual - Manual (Page 151)

Publication 1797-6.5.3 - March 2006
Additional HART Protocol Information C-3
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 controller to do this.
Start Character
The start character of a HART message indicates the frame’s format, the
source of the message, and if it is using burst mode.
HART Address
The Smart Transmitter Interface addresses HART field devices using either a
short or long frame address format, as specified by the HART delimiter byte.
A short frame address is one byte long. A long frame address is five bytes long
and includes a unique 32-bit identifier encoded within each field device by the
manufacturer.
HART field device addressing is device dependent. Some devices do not
support long frame addressing while others only recognize short frame
addressing for HART Command 0. In this situation, use HART Command 0
to determine the long frame address, and then use long frame addressing for al
other HART commands. Consult the documentation provided with your field
device for details about the addressing formats it supports.
Table C.1 Start Character Byte Definitions
Frame Type Short Frame Addressing
(hex) Long Frame Addressing
(hex)
Master to slave 02 82
Slave to master 06 86
Burst mode from slave 01 81