on  Wed, 21 May 1997 06:44:13 -0700 Dan Mulally wrote

> I've been seeing the messages on various error checking routines such as
> Hamming and CRC. I had a project that uses 32 bit "packets" to transfer

I have dabbled with Hamming, CRC, and checksums

-SNIP-

> best way but it was easy. I've read that the checksum method is not as
> sensitive to detecting errors as other methods and I have no idea of the
> odds of this method giving a correct checksum with errors in the data.

If you just fed random numbers into your system as described you
would get bad data slipping through every 1 in 256 times. As your
checksum could hold any of 256 combinations and one of them must be
correct. Because of this I have used 16bit checksums to cut down on
the chances of this happening.

>
> Here is the code that does the checksum checking:
>
> ;*****************************************************
> ;Checksum Sub
> ;*****************************************************
> chksum   bcf   flags, chksum_ok     ;clear checksum flag
>          movf  frame0, 0            ;move frame0 to W
>          addwf frame1, 0            ;add frame1 to W
>          addwf frame2, 0            ;add frame2 to W
>          addwf frame3, 0            ;add frame3 to W
>          xorwf frame4, 0            ;XOR checksum frame with W
>          btfsc status, z            ;result = 0?  q=yes
>          bsf   flags, chksum_ok     ;if yes, set checksum flag
>          retlw null                 ;return with null
>
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.

I have also heard that CRC is more robust. It is supposed to be less
susceptible to added leading zeros on your message. Though this would
not apply in your fixed message length case. But as far as hard
facts regarding how sensitive CRC is at detecting errors - I am as
interested as you - If anyone out there knows this I would like to
hear.

TIA

p.s. I have a CRC routine written in 6502 if you are interested!!!

----------------------------------------
Martin Lane
Computer Manager
Metrol Technology Ltd
Tel. +44 1224 772771
Fax. +44 1224 772660
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