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Modbus RTU Message Format

6 Modbus® RTU Serial Communications User Manual 02/13

Reserved Operands

Per the Standard certain exceptional forms of floating-point operands are excluded from the numbering

system. These are as follows:

EXCEPTION EXPONENT MANTISSA

+/- Infinity All 1’s All 0’s

Not-a-Number (NAN) All 1’s Other than 0’s

Denormalized Number All 0’s Other than 0’s

Zero All 0’s All 0’s

Modbus Double Register Format

Each IEEE 32-bit floating point number requires two consecutive registers (four bytes) starting with the

registers differs among Modbus hosts. The selections are:

Selection Description Byte order

(See Figure

2-1)

Notes

FP B Floating Point Big Endian Format 4, 3, 2, 1

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 starting on next page.

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