[ubuntu-uk] Reverse engineering data files

[Gareth France]

Now I'm with you! I have to say my programming is all rooted in basic. I 
am stuck at a certain point and struggling to adjust the way I view 
problems and their solutions. Until I started looking at this I had 
never even attempted to input, manipulate or output anything more than 
text files (csv etc). This has been a real eye opener and for me now is 
more about being creative with code than solving any particular issue. I 
have learned a lot this week.

Thanks

On 27/11/14 13:49, Paul Sladen wrote:
> On Thu, 27 Nov 2014, Gareth France wrote:
>> I really don't understand what you have done here.
>>>>>>>>> F3 01 90 B7  - 42.79 - 0x90b7 & 0x7fff => 0x10b7 => 4279 decimal
>>>>>>>>> F3 01 97 78  - 60.08 - 0x9778 & 0x7fff => 0x1778 => 6008 decimal
>
> The two bytes we're interested in (0x90b7) together give us 16 bits of
> information.  This is split into two, 1-bit for one value (true or
> false) and 15-bits for a measurement value (zero to 32k).
>
>    0x90b7
>    1001000010110111 (binary)
>
> When we split it, the two parts are:
>
>    1 and 001000010110111
>
> We can pass these to Python to convert from binary to decimal; the
> '0b' prefix means binary, just like '0x' means hexadecimal:
>
>    python -c 'print 0b1, 0b001000010110111'
>    1 4279
>
> The first value is actually a boolean (True/False) and is inverted,
> and the second is multipled by 100:
>
>    python -c 'print bool(not 0b1), 0.01*0b001000010110111'
>    False 42.79
>
> We don't need to go to the binary representation at all, we can split
> the field using bitwise arithmatic AND (&) and right shift (>>) :
>
>    https://en.wikipedia.org/wiki/Bitwise_operation#AND
>    https://en.wikipedia.org/wiki/Bitwise_operation#Arithmetic_shift
>
>    python -c 'print bool(not 0x90b7 >> 15), 0.01 * (0x90b7 & 0x7fff)'
>    False 42.79
>
> We can test this another result too:
>
>    python -c 'print bool(not 0x9778 >> 15), 0.01 * (0x9778 & 0x7fff)'
>    False 60.08
>
>    python -c 'print bool(not 0x48db >> 15), 0.01 * (0x48db & 0x7fff)'
>    True 186.51
>
> In this case, this is above >99.99, which the new meter shows as
> '99.99' so we need to cap it with the minimum function:
>
>    python -c 'print bool(~0x48db>>15),min(99.99,0.01*(0x48db&0x7fff))'
>    True 99.99
>
> and just to re-check with the others:
>
>    python -c 'print bool(not 0x9778>>15),min(99.99,0.01*(0x9778&0x7fff))'
>    False 60.08
>
> 	-Paul
>
>
>
>