3 byte float value to python float conversion

Question

I used the search function without success.

This I the code I want to translate to python3 code.

From my pv inverter I receive the 3 byte data.

The following c program is used to convert the data into a c float value. This code works correct. I have verified that the output is correct.

    // filename: test.c

    #include <stdio.h>
    
    // example data byte
    // byte 24 = 194
    // byte 25 = 112
    // byte 26 = 134
    // result is 
      
    int main(void)
    {   
            int iacpower = ((134 << 8 | 194) << 8 | 112) << 7 ;
            // other example int iacpower = ((135 << 8 | 230) << 8 | 165) << 7 ;

            float facpower = *((float*)&iacpower);  //convert to float

            printf("iacpower = %d\n", iacpower);
            printf("facpower = %.2f\n", facpower);
            return 0;
    }

$ gcc -o test test.c
$ ./test
iacpower = 1130444800
facpower = 225.22

What I'm asking vor is how to convert the three byte to float using python.

I tried to unpack the data like the following python lines:

    facpower = struct.unpack('f', in_data[24,27])

and also

    iacpower = ((in_data[26] << 8 | in_data[24]) << 8 | in_data[25]) << 7
    facpower = struct.unpack('f', iacpower)

Any suggest please?

Thank you!

gog · Accepted Answer · 2022-06-26 09:19:35Z

2

After you've computed iacpower you need to repack it, that is, pack int to bytes and unpack as a float:

>>> iacpower = 1130444800
>>> facpower, = struct.unpack('f', struct.pack('I', iacpower))
>>> facpower
225.21875

answered Jun 26, 2022 at 9:19

gog

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