Combining two 16 bit registers into a 32 bit float

Question

So I have a project where I am reading data from MODBUS registers.

Each register is 16 bits, and each value is 32 bits. So I have to read two registers at a time to read each value.

So what I'm doing now is reading both registers and storing them in a regs array and getting their bits like so:

bits = (regs[0] << 16) + regs[1]

then I try converting the bits to a float like this:

s = struct.pack('>l', bits)
final = struct.unpack('>f', s)[0]

And final should be my 32-bit float. This works pretty much all the time, however, in a few cases I get this error:

Read registers failed: 'l' format requires -2147483648 <= number <= 2147483647

And so I went back and added a print statement to see what bits looked like before I turn it into a float, and it was this: 3309382817, which is clearly outside of that range...

So what am I doing wrong that's causing me to get this error? Is there something I am missing?

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Update:

Just to give a bit more info, ran it again while printing the value of regs as well and got this:

[50561, 10024]  # this is the value of regs
3313575720      # this is the value of bits

Answer 1

You are using the signed type l in your struct.pack. Use the unsigned type L instead.

See Struct Format Characters.