[aprssig] APRStt Callsign and Checksum

[Matt Murphy]

Thanks for the info.  I wasn't it reading right the first time.  I have a
further question though.
The total for my callsign is 101 like yours so my checksum is 1.  Where do
we get 1 from 101?
I do not fully understand how to get this yet and I don't see it in the
aprstt literature.
I do understand the importance of it though.

Thanks

73,
Matt Murphy, kc8bew
http://mcares.kc8bew.net
Muskingum Co. ARES EC


On Tue, May 12, 2009 at 10:14, Robert Bruninga <bruninga at usna.edu> wrote:

> It would be nice if someone would write an APRStt CALLSIGN
> GENERATOR routine for everyone to use.  They could just type in
> their callsign, and out comes the DTMF keys to put into memory.
> This automatically generates the 2-key conversion and checksum.
> An ON_LINE generator would even be better!
>
> > I have a question about the APRStt checksum.
> > What is it and what is it for?
>
> Since the callsign is only entered once into a DTMF memory for
> easy transmission, we thought it should have a checksum to
> greatly minimize the chance for errors.  You just add up all the
> keys in the callsign, and keep only the units digit of the sum.
> That then is the checksum digit.  If APRStt reecives a DTMF
> string and the checksum does not match, then there is an error.
>
> The APRStt DTMF string for callsigns is:
>
> AcccccccccccVK#
>
> Where V is a user selected overlay byte (or two byte letter)
> Where K is the checksum (treat ABCD keys as HEX)
> Where each letter of the callsign is encoded using the 2-key
> text system
>
> See www.aprs.org/aprstt/aprstt-user.txt
>
> Bob, WB4aPR
>
>
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