Encode floating-point and 32-Bit data in Modbus RTU Messages
In Data Conversion, incoming data manipulation is a necessary step to create outgoing data. This is done by either joining smaller data types to create a larger data type or splitting larger data types to deliver smaller data types.
In Modbus, 16-bit registers are used to transfer a 32-bit floating-point value. Upon receipt of these two registers, the FieldServer needs to join the integers to extract the floating-point value.
Functions Available for Type Casting:
- Join_Float , Split_Float
- Join_Int16, Split_Int16
- Join_Int32, Split_Int32
- Swapped versions of the above (Big Endian vs Little Endian)
- Bit_Extract, Bit_Pack, Bit_Move
The following legacy functions have been replaced by the functions listed above. They are simply presented in the table below for reverse compatibility.
Example 1
Moving a preloaded float;
In the data array offset 90, the value stored is 6584.567.
DA_FLOAT, 6584.567 , float, 90
To move this from the server-side to the User’s end we would use the function 1.float-2.i16-sw which takes one float value and splits it to two unsigned 16-bit integers.
DA_FLOAT , 90 , DA_UINT16 , 90 , 44 , 1.float-2.i16-sw
The results on the user end will be two split 16-bit integers represented in hexadecimal as;
To confirm this is right, Chipkin provides its own calculator IEE 754 Calculator FREE
If the data was served as;
DA_FLOAT , 90 , DA_UINT16 , 90 , 44 , 1.float-2.i16
The split 16-bit integers would be swapped as;
Chipkin offers a free Modbus TRU Parser that can be used to check this.
Example 2
In the data array offset 94, the value stored is 2500.2.
DA_FLOAT , 2500.2 , float , 94
The function 1.float-2.i16-sw below takes one float value and splits it to two unsigned 16-bit integers.
DA_FLOAT , 90 , DA_UINT16 , 90 , 44 , 1.float-2.i16-sw
The results on the user end will be two split 16-bit integers represented in hexadecimal as;
Using Chipkin’s IEEE-754 calculator, the results obtained is similar as;
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