Rockwell Automation 1794-OF8IH FLEX Isolated Input/Output HART Analog Modules - Manual (Page 101)

Publication 1794-UM065B-EN-E - September 2010
Configure Module Messaging 89
specification since the module will take care of the extra details like adding the
message start delimiter, the device's address, and the checksum of the message
for you. The module also supports sending the fully formatted HART message
if you need the extra security of knowing that the fully formatted message
contains a specific device's address and is only accepted by that device. In
general the EASY-HART method is meant for users who need to send a
message using Ladder to a device but do not want to spend time learning the
details of the HART protocol.
Note that native HART data returned by devices 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 multi-byte value
are in the opposite order so they must be reversed before use. Little Endian
means that the least significant byte of a number is stored at the lowest address
(array index).
Also be aware that Logix aligns multi-byte 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 able to handle these data
layout issues.
Format a HART Pass through Init Request Message
A HART Pass through Init request consists of setting up a MSG instruction
with a Service type of "Set Attribute Single" which allows you to write data
into the module. This data is formatted to include information about the
HART command that you want to send to the HART device.
Message Setup Parameters
Field Value Definition
Message Type "CIP Generic"
Service Type Set Attribute Single Write to Module
Service Code 0x10
Class Name 0x7D FLEX Module Object
Instance 1…8 (Module next to Adapter = 1) Module Location
Object Attribute 0x81…0x86 Select smallest buffer size that
will hold message request
Reply Size 0
Request Size 8-48 bytes