Spectrum Controls 1769sc-HART Modules - Manual (Page 97)

Chapter 7: Enabling and Using HART on the 1769sc-OF4IH
User’s Manual 0300217-03 Rev. A
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timeout timer. Refer to Chapter 6 for information regarding how to set the handle
timeout. The reply message will be kept persistent during the 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.
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 Controllogix processor stores 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 order 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.
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Attention