Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 86)

Chapter 4
C Communicating with the Smart
Transmitter Interface
4-21
The field device response may not be available for several hundred
milliseconds, so if you wish an immediate response to a BTR, set bit 0 of
the Smart Transmitter Interface Parameter field. The Smart Transmitter
Interface will then respond immediately to a BTR with either a HART field
device response, if available, or a Smart Transmitter Interface Error Code
hexadecimal 03. This error code indicates that the Smart Transmitter
Interface is still attempting to obtain the field device response. When the
programmable controller receives a Smart Transmitter Interface Error
Code hexadecimal 03, it should retry the BTR until either the field device
response is available or a communications failure is reported.
Your host computer cannot use bit 0 of the Smart Transmitter Interface
Parameter field in the above fashion. Rather, the host computer should wait
until either the field device responds or a communications failure occurs.
In either case, the Smart Transmitter Interface will send a response
message to the host computer as specified by the DF1 protocol.
Read Burst Data (hexadecimal command 11)
Use this command to read HART burst data from the Burst Data table. This
table contains the latest data available from the bursting device on the
specified channel. Send the command via a BTW or DF1 packet. The data
is returned in the next BTR or DF1 packet.
The Smart Transmitter Interface Data field in the response will contain a
HART Burst Data packet. If the Smart Transmitter Interface fails to receive
data from a bursting device it returns Smart Transmitter Interface Error
code hexadecimal 20 along with the response. If the data is not available in
the channel table because burst data has never been received for this
channel, then Smart Transmitter Interface returns Error code hexadecimal
22 in the response.
Set Number of Preambles (hexadecimal command 20)
Use this command to set the number of preambles transmitted by the Smart
Transmitter Interface before sending a HART poll to a field device. The
default number of preambles is 10, but some devices require fewer and
others require additional preambles to operate reliably. Use as few as
possible: the communications bandwidth to a field device increases if you
reduce this number. The number of preambles from (2 to 32) for the
specified channel is defined in the Smart Transmitter Interface Parameter
field. If the specified channel is hexadecimal FF, then the number of
preambles for all 32 channels is changed to the number in the Smart
Transmitter Interface Parameter field.