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2 Protocol description
2.7 Transmission format (integer, float and text values)
Integer values Integer values are transmitted via the Modbus in the following format:
The high byte first, followed by the low byte.
Example Request of the integer value of address 0x0021, if value "4" (word value
0x0004) is written under this address.
Request: 01 03 0021 0001 (+ 2 bytes CRC16)
Response: 01 03 02 0004 (+ 2 bytes CRC16)
Float values In the case of float values, the Modbus operates with the IEEE-754 standard
format (32bits), the only difference being that byte 1 and 2 are changed over
with byte 3 and 4.
Single-float format (32bit) as per IEEE 754standard
S - sign bit
E - exponent (two's complement)
M - 23bits normalized mantissa
Modbus float format
Example Request of the float value of address 0x0035, if value "550.0" (0x44098000 in
IEEE-754 format) is written under this address.
Request: 01 03 0035 0002 (+ 2 bytes CRC16)
Response: 01 03 04 8000 4409 (+ 2 bytes CRC16)
Once transmission from the device is completed, the bytes of the float value
need to be changed over accordingly.
Float value
SEEEEEEE EMMMMMMM MMMMMMMM MMMMMMMM
Byte 1 Byte 2 Byte 3 Byte 4
Modbus address x Modbus address x+1
MMMMMMMM MMMMMMMM SEEEEEEE EMMMMMMM
Byte 3 Byte 4 Byte 1 Byte 2
A large number of compilers (e.g. Microsoft Visual C++) store the float
values in the following order:
Address x Address x+1 Address x+2 Address x+3
MMMMMMMM MMMMMMMM EMMMMMMM SEEEEEEE
Byte 4Byte 3Byte 2Byte 1
Please find out the way float values are stored in your application.
After the request, it might be necessary to change the bytes over in
the interface program you are using.
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