Fox Thermal FT3 - Page 98

Floating point data layout
Each 32-bit oating point value uses two consecutive Modbus registers. The most signicant byte of
the lower numbered register holds the least signicant byte of the signicand. The least signicant
byte of the lower numbered register holds the next most signicant byte of the signicand. The most
signicant byte of the higher numbered register holds the sign bit and most signicant 7 bits of the
exponent. The least signicant byte of the higher numbered register holds the least signicant bit of
the exponent and the most signicant 7 bits of the signicand.
In the following tables:
S0 – S23 are the signicand bits from least to most signicant.
E0 – E7 are the exponent bits from least to most signicant.
Sign is 1 if the number is negative, and 0 if the number if positive.
Lower numbered register
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5 S4 S3 S2 S1 S0
Higher numbered register
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Sign E7 E6 E5 E4 E3 E2 E1 E0 S22 S21 S20 S19 S18 S17 S16
Since the Modbus register data is sent most signicant byte rst and the registers are sent lowest
numbered rst, a oating point value will look like this in the data stream:
First byte (MSB of lower register)
Data bit 7 6 5 4 3 2 1 0
Value bit S15 S14 S13 S12 S11 S10 S9 S8
Second byte (LSB of lower register)
Data bit 7 6 5 4 3 2 1 0
Value bit S7 S6 S5 S4 S3 S2 S1 S0
Third byte (MSB of higher register)
Data bit 7 6 5 4 3 2 1 0
Value bit Sign E7 E6 E5 E4 E3 E2 E1
Fourth byte (LSB of higher register)
Data bit 7 6 5 4 3 2 1 0
Value bit E0 S22 S21 S20 S19 S18 S17 S16
Communications | 98
Model FT3
Communications