Rockwell Automation 1794-OF8IH FLEX Isolated Input/Output HART Analog Modules - Manual (Page 116)

Publication 1794-UM065B-EN-E - September 2010
104 Additional HART Protocol Information
Byte Count
This one-byte field indicates the number of bytes to follow this field excluding
the check byte. Valid values are 0 …113. Insert the number of bytes required
for this packet before transmitting it.
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 0…113.
Only use this field when 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).
Response Code
This two-byte code contains the HART field device status as sent by that
device. Field devices detecting a communications error set the most significant
bit, bit 7, of the first byte and identify the error in the other seven bits. If the
last message is received without error, the field device clears bit 7 and returns a
device-dependent response in the other seven bits.
3 Read dynamic variables and
primary variable current Primary variable and up to 4
predefined dynamic variables
6 Write polling address Assigned polling address - short
form
11 Read unique identifier
associated with tag Unique 32-bit device identifier,
revision levels, number of
preambles required
Representative of HART Universal Commands
Universal Command
(dec) Description Expected Response