Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 70)

Chapter 4
C Communicating with the Smart
Transmitter Interface
4-5
The Smart Transmitter Interface transmits HART Poll packets to field
devices using the format illustrated in Figure 4.3 and Figure 4.4. The host
processor provides data for all fields except the preamble. If you are using
a programmable controller as host processor, you provide data for the
HART device via Block Transfer Writes to the Smart Transmitter Interface.
If you are using a computer as host processor, you provide the data by
using the computer’s software to send DF1 commands to the Smart
Transmitter Interface.
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.
HART Delimiter
This is a one-byte field that is used to identify the frame type. Set the
frame type as STX (010) to identify this packet as a HART poll. Also,
indicate whether a short or long frame address follows this byte, by
specifying hexadecimal 02 for a short frame address or hexadecimal 82 for
a long frame address.
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 and contains a polling
address from 0 to 15. A long frame address is five bytes long and includes
a unique 38 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 all other HART commands. Consult the documentation
provided with your field device for details about the addressing formats it
supports.
HART Poll Packets