Re: stream device string/hex parsing

[Dirk Zimoch <dirk.zimoch@psi.ch>]

Gerrit Kühn wrote:
> On Tue, 31 Jul 2012 16:30:05 +0200 Dirk Zimoch <dirk.zimoch@psi.ch> wrote
> about Re: stream device string/hex parsing:
>
> DZ> > The documentation says the packet LRC is the two's complement of the
> DZ> > sum over the packet's parameter and command bytes. Each binary data
> DZ> > byte is transmitted as two hex digits which have to be transformed
> DZ> > into binary form before adding them. So, looking at the available
> DZ> > checksum algorithms, this calls for something like %<neghexsum8>,
> DZ> > which does not exist...
>
> DZ> %0<negsum>
>
> Hm... not quite:
>
> 2012/08/01 14:33:43.162 ADC16_W Input byte 0xE7 does not match checksum
> 0xFC
> 2012/08/01 14:33:43.162 ADC16_S Input byte 0xEC does not match
> checksum 0x01
> 2012/08/01 14:33:41.162 ADC16_C Input byte 0xD7 does not match checksum
> 0xFB
>
>
> The description for <negsum> is
>
> ---
> One byte. The negative of the sum of all characters modulo 2^8
> ---
>
> However, I do not need to sum over all characters, I need to interpret
> every two characters as hex value, sum over these, invert and compare the
> result with the hex value represented by the two checksum characters. Or
> am I misunderstanding something here?
>
>
> cu
>   Gerrit

Hi Gerrit,

I misunderstood "two hex digits which have to be transformed into 
binary". I understood that for example the hex digits "5a" have to be 
transformed to 0x35 + 0x61 = 0x91, that would be the normal <sum>. Or 
that they have to be transformed to 0x05 + 0x0a = 0x0f, which is 
<hexsum>. But it seems you need 0x5a as the result. At the moment, I 
don't have a checksum function for that. You can write one and add it to 
ChecksumConverter.cc. I usually implement the positive sums as a 
function. You get the negative sum with init=0xff and xorout=0xff (see 
checksumMap).

Good luck,
Dirk