Dan Mulally <PICLIST@MITVMA.MIT.EDU> wrote:

> > I would be more inclined to XOR all the numbers in the frame together
> > but otherwise I have used a 16bit version of what you show above.
>
> It seems like adding would be better than XORing because I'm
> "spreading" the information within the 8 bits by the carries

    I've been debating whether or not to answer the recent questions
    regarding error-control coding... I've decided not to, because I
    just don't have the time right now to do the subject justice
    here on teh PICLIST.  Maybe I'll put something up on my web page
    in a week or so.

    In the meantime, however, I can touch very briefly on the
    addition-modulo-256 (ADD) vs. addition-modulo-2 (XOR) issue.

    Dan:  You're correct when you say that adding "spreads the
    information".  Unfortunately, it only spreads it LEFTWARD (i.e.,
    toward the MSB)... Assuming that your checksum is no wider than
    each of your information words, the most-significant bit of each
    information word only affects (at best) the most-significant bit
    of the checksum.

    Unless you know that the errors are more likely to occur in the
    least-significant bits of your information words, there's no
    real advantage to ADDing over XORing.

    Also, while both techniques will always detect a single error
    and both can be fooled by two-bit errors, only the XOR method
    will always detect all 3-bit errors (in fact, it'll detect ALL
    errors that involve an odd number of bits).

    -Andy

=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering, Vista, California
=== http://www.geocities.com/SiliconValley/2499