Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 74)

Chapter 4
C Communicating with the Smart
Transmitter Interface
4-9
Data
This field specifies a number of data bytes associated with the command
number given in the command field. Set the number of data bytes to the
appropriate value for the command in question. The valid range is from 0
to 113. Only use this field when you are writing data to the HART device.
Check Byte
The Smart Transmitter Interface calculates the value of this field and
transmits it to the field device as the last byte of a packet. The field device
verifies the integrity of the received data packet by checking this byte.
Since the Smart Transmitter Interface calculates this byte, you can set this
field to a null (00).
Field devices transmit HART Response packets or HART Burst Data
packets to the Smart Transmitter Interface using the format illustrated in
Figure 4.6 and Figure 4.7. The Smart Transmitter Interface removes the
preamble bytes and sends the remaining fields to the host processor. With a
programmable controller use BTRs to accomplish. With a computer use
DF1 Responses to receive data from the Smart Transmitter Interface.
Preamble
All frames transmitted by field devices are preceded by a number of
hexadecimal FF bytes which are called the preamble to the frame. As the
Smart Transmitter Interface handles all preamble bytes, the host processor
does not receive them.
HART Delimiter
This one-byte field encodes the HART frame as either an ACK or Burst
frame. An ACK frame indicates a response packet while a Burst frame
indicates a burst packet from a field device. One bit of this field specifies
whether a short frame or long frame address follows this byte.
In poll and response mode you will receive hexadecimal 06 for a short
frame address or hexadecimal 86 for a long frame address. In burst mode,
when the host processor is receiving data from the Smart Transmitter
Interface’s Burst Data table, this field will contain hexadecimal 01 for a
short frame address or hexadecimal 81 for a long frame address.
HART Response and Burst
Data Packets