[aprssig] APRStt Callsign and Checksum

[Robert Bruninga]

> 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?

>> ... You just add up all the keys in the callsign, 
>> and keep only the units digit of the sum.

We will make it clearer in the docs when we get the chance.

Thanks
Bob

> 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
> 	
> 	
> 
> 
>