Prevac MG15 - Float IEEE-754 Format in Modbus Table; Example Loat IEEE-754 Value in Modbus Table

MULTI GAUGE CONTROLLER COMMUNICATION
6.4.7.1 ENUMERATED, BOOL AND INTEGER PARAMETERS
These variables are always occupying one word in the memory. For variables smaller than: 2 bytes,
the upper byte is illed with 0x00 value.
These variables are read via 0x03 function, and they can be recorded via 0x06 or 0x10 function.
Remember to ill with 0x00 the upper byte (MSB) of variables smaller than 2 bytes during writing.
6.4.7.2 FLOAT TYPE
The format is compatible with the IEEE-754 standard and is used in almost every programming lan-
guage of higher level such as C, C++. In addition, most of automatics equipment or built-in systems
allows for their automatic decoding.
Variables written in this format are organised as 4-byte data in two consecutive table word in the
“MSB irst” order. Writing and reading of these variables is possible only via 0x03 and 0x10 com-
mands. An attempt to write or read such variable in half will be discarded and an error message
will be generated.
Note that in practice, when using C, IEEE loats may usually be decoded by
placing the values returned over comms into memory and ‘casting’ the region
as a loat, although some compilers may require that the area be byte swapped
high to low before casting (e.g. MSB B2 B1 LSB →LSB B1 B2 MSB).
MODBUS ADDRESS MODBUS ADDRESS + 1
MSB LSB MSB LSB
Bits 31 - 24 Bits 23 - 16 Bits 15 - 8 Bits 7 - 0
Table 6.10: Float IEEE-754 format in Modbus table
For example, to transfer the value 1.001, the following values are transmitted (hexadecimal):
MODBUS ADDRESS MODBUS ADDRESS + 1
MSB LSB MSB LSB
0x3F 0x80 0x20 0xC5
Table 6.11: Example loat IEEE-754 value in Modbus table
User manual rev. 03 Page 99