JUMO mTRON T Manual page 38

3 Modbus protocol description
3.7.2 Float values
For float values, Modbus operates with the IEEE 754 standard format (32 bit), but with the dif-
ference that byte 1 and 2 are changed over with byte 3 and 4.
Single-float format (32 bit) according to standard IEEE 754
SEEEEEEE
Byte 1
S - sign bit
E - exponent (two's complement)
M - 23 bits normalized mantissa
Modbus float format
MMMMMMMM
Byte 3
Example
In this example, the value of analog variable 1 is to be read out at address 0x1071 of the central
processing unit. The value here is to be 550.0 (0x44098000 in the IEEE 754 format).
Data request:
01
Slave
Response (values in the Modbus float format):
01
Slave
Once transmission from the central processing unit is completed, the bytes of the float value
need to be changed over accordingly. A large number of compilers (e.g. Microsoft Visual C++)
file the float values in the following order:
Float value
Address x
MMMMMMMM
Byte 4
38
EMMMMMMM
Byte 2
Modbus address x
MMMMMMMM
Byte 4
03
Function Address
of the 1st word
03
Function
Address x+1
MMMMMMMM
Byte 3
MMMMMMMM
Byte 3
SEEEEEEE
Byte 1
10 71 00 02
Number of
words
04 80 00
Bytes Float value
read
Address x+2
EMMMMMMM
Byte 2
MMMMMMMM
Byte 4
Modbus address x+1
EMMMMMMM
Byte 2
05 D4
CRC
44 09
Address x+3
SEEEEEEE
Byte 1
F5 20
CRC