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Modbus Double Register Format
IEEE Floating Point Data Format
10 HC900 Hybrid Controller Communications User Guide Revision 4
9/03
3. Modbus Double Register Format
Data that is 32 bits requires 2 sequential registers (4 bytes) to transfer its data. Data of this type includes IEEE 32-
bit floating point, 32-bit signed integer and 32-bit unsigned integer. The stuffing order of the bytes into the two
registers differs among Modbus/TCP hosts. To provide compatibility, the double register format for the HC900
controller is configurable.
To set the controller’s double register byte order, go to the “Set Controller Network Parameters ” wizard in the
"Controller Utilities Function" section of the Utilities Tab on the Hybrid Control Designer and configure “Modbus
Double Register Format”. This can be done in the RUN mode.
The selections are:
Table 3-1 Modbus Double Register Format Selections
Selection Description Byte order
(See Figure
2-1)
Notes
FP B Floating Point Big Endian Format 4, 3, 2, 1 HC900 default
FP BB Floating Point Big Endian with
byte-swapped
3, 4, 1, 2
FP L Floating Point Little Endian Format 1, 2, 3, 4
FP LB Floating Point Little Endian with
byte-swapped
2, 1, 4, 3 Modicon and
Wonderware
standard
See IEEE Formats on page IEEE Floating Point Formats on page 11 and 32-bit integer formats on page 13.
NOTE: Byte Swapping only applies to Function Codes 3, 4, and 16.