User Manual

Chapter 7: Enabling and Using HART on the 1769sc-IF4IH0
U ser 's M a n u a l 0 300 21 5- 03 R ev. A
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message response. Once the handle is received, the user may issue a Get Command
Query to obtain the status of the pass-through command and the pass-through command
response data, if it is available.
There is a handle timeout associated with the final reply message. After the HART
module obtains the requested information from the HART device, it will start a handle
timeout timer. Refer to Chapter 6 for information regarding how to set the handle
timeout. The reply message will be kept persistent d urin g t he handle timeout period.
When the handle timeout timer expires the reply message will be discarded, and another
pass through message will be serviced without being rejected. The user defined handle
timeout is in the range of 1 to 255 seconds.
!
Attention
If the HART message being sent or received using the pass-through
command contains floating point values, the order of the bytes must be
reversed.
Depending on the HART command, the data contained within the HART message may
include floating point numbers or double integers. If a floating point or double integer is
contained within the HART message, the user must be aware that the order of the bytes
that make up the float or double will need to be reversed. The reason for this is related to
how the bytes are stored in the Controllogix processor .
The Controllo gix pro cessor sto r es the bytes in memory in a format referred to as " little-
endian". Little-endian is an order in which the "little end" (least significant value in the
sequence) is stored first (at the lowest storage address). However, HART devices
transmit the byte data in the reverse ord e r or as you may have guessed "big-endian" .
Refer to Chapter 8 for a ladder sample demonstrating the process of swapping the order
of the bytes.
Table 7-14 (HART Pass -Through Request Command)
HART pass through c ommand request – command message packet structure
Field Value Definition
HART Channel Number 0x00 – 0x03 (1 byte) Module input
channe l number for
HART command
Command Number 0x01 (1 byte) The command
number to issue a
HART pass-through
command.
HART Command N bytes
N = Length of message – 2
Contents are as follows:
Start or Delimiter (1 byte): 0x82
Long form Address (5 bytes)
HART Command number (1 byte)
Request Data Count (1 byte)
Data (“Request Data Count” bytes)
Checksum (XOR of all bytes from delimiter on.
Delimiter is included )
The actual HART
command PDU