Spectrum Controls 1756sc-OF8H Series A - Manual (Page 99)

Chapter 7: HART Channel Configuration, Data, and Status 85
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Attention: If the HART message being sent or Attention: If the HART message being sent or Attention: If the HART message being sent or Attention: If the HART message being sent or Attention: If the HART message being sent or
received using the pass-through command contains received using the pass-through command contains received using the pass-through command contains received using the pass-through command contains received using the pass-through command contains
floating point values, the order of the bytes must be
floating point values, the order of the bytes must be floating point values, the order of the bytes must be floating point values, the order of the bytes must be floating point values, the order of the bytes must be
r e v e r s e d .
r e v e r s e d . r e v e r s e d . r e v e r s e d . r e v e r s e d .
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 "big-endian". Big-
endian is an order in which the "big end" (most 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 "little-endian" Refer to chapter 8 for a ladder sample
demonstrating the process of swapping the order of the bytes.
Table 7.12 (HART Pass-Through Command Request)
HART pass through command request – command message packet structure
Field
Value Definition
Unconnected Message Header
HART Channel Number
0x00 – 0x07 (1 byte) Module input channel
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)
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