Rockwell Automation 1756-OF8H ControlLogix HART Analog I/O Modules - Manual (Page 131)

Rockwell Automation Publication 1756-UM533C-EN-P - February 2011 131
Getting HART Data By Using CIP MSG Chapter 8
CIP Services to Pass-
through a HART Message to
the HART Field Device The HART object supports these CIP messages for HART pass-through
messaging: Pass-through Init, Pass-through Query, Flush Queue (rarely needed).
With these three CIP messages, your Logix controller can format the individual
bytes of a HART command, send it to a HART field device, and retrieve the
response in HART format.
Note that native HART data is in a different format than used by Logix
controllers. HART uses the Big Endian format and Logix uses Little Endian
format. This means the order of the bytes in a number are in the opposite order so
they must be reversed before use. Logix Little Endian means that the least
significant byte of a number is stored at the lowest address (array index).
Logix also aligns data on boundaries that permit fast access and HART packs
them into the smallest space. HART encodes text strings using 6 bits per letter
into a format called Packed ASCII. When using pass-through messaging, your
Logix program must be aware of these data layout issues.
The pass-through message CIP services supported by the HART object are
somewhat simplified. The module provides the 5-byte address usually required by
HART messages and the Checksum is calculated automatically for you.
Follow these steps to send a HART pass-through message.
1. Send a CIP message to tell the 1756-IF8H or 1756-OF8H module to send
a message to a HART field device (Init).
2. Send a CIP message to retrieve the HART Reply from the 1756-IF8H or
1756-OF8H module (Query).
Pa
ss-t
hro
ug
h Init
Pa
ss-th
ro
ug
h Qu
er
y HART Reply Bytes
Handle
Destination
MSG
Source
Source
Handle
Destination
HART Command Bytes
MSG
Service 16#4F
Service 16#4E
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