Allen-Bradley 1794-IE8H, User Manual, Page: 102 / 146

Publication 1794-UM063A-EN-P - March 2006
C-4 Additional HART Protocol Information
HART Command
This one-byte field specifies the HART command that is to be sent by the
Smart Transmitter Interface to the field device. Many commands are device
dependent. Consult the documentation provided with your field device for
details about the commands supported. Set this field to a device-recognizable
command before sending the packet to the Smart Transmitter Interface.
Byte Count
This one-byte field indicates the number of bytes to follow this field excluding
the check byte. Valid values are 0 to 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 from
0 to 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).
Table C.2 Representative of HART Universal Commands
Universal Command
(dec) Description Expected Response
0 Read unique indentifier Unique 32-bit device indentifier,
revision levels, number of
preambles required
1 Read primary variable Primary variable in floating point
(IEEE 754 format)
2 Read primary variable
current and percent of range Primary variable in milliamperes
and percents
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