On 10/10/05 11:30, Karsten Hilbert <[EMAIL PROTECTED]> wrote:
> On Mon, Oct 10, 2005 at 10:20:18PM +0100, Chris Ridd wrote:
>
>>> I suggest XORing the data with/without the control characters.
>>
>> That occurred to me too, but recall the result is 16-bits wide. No amount of
>> XORing 8-bit values will produce a result with stuff in the top byte.
>>
>> I tried doing a 16-bit sum too. No luck.
>>
>> I'd expect the algorithm being used to be either very simple, or easily
>> implemented in extremely cheap hardware (or both). But not as simple as
>> addition or XORing :-)
>
> Well :-)
>
> Using XON/XOFF flow control requires the checksum to be sent
> not as binary but rather as an ASCII interpretation of the
> binary value. Usually what's done is converting an 8 bit
> binary value into it's two byte hex representation, eg.
> value 255 becomes "FF".
>
> That should help...
Ah ha!
If I add together all the bytes into an unsigned 8-bit variable, then
convert that to hex, the result matches the meter's checksum. In C:
int verify_checksum(const unsigned char *s, const char *hex)
{
unsigned char calculated = 0;
while (*s) {
calculated += *s++;
}
unsigned char purported;
purported = strtol(hex, NULL, 16);
if (purported == calculated)
return 1;
return 0;
}
The bytes of 's' include the leading "1" and the trailing 0x03, e.g.:
120051010\x0918:56\x09XC0824-0109 133-113-04 \x0d\x03
Thanks for the major clue :-)
Cheers,
Chris
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