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[aprssig] Two-way APRS on the FT51 and TH78!

Robert Bruninga bruninga at usna.edu
Thu Oct 9 23:53:58 UTC 2008


Using the FT51R and TH78 for APRS!

I played with the two-way DTMF messsaging built into the TH-78
(and FT-51R)HT today and this is really something we can exploit
for sending messages and APRS info to these HT's!

Basically it is a simple gateway process added to APRStt that
can monnitor the APRS channel and then send local info over to
the HT51/78 channel for display on these radios.  Here are some
examples.  For full details see:
www.aprs.org/aprstt/FT51-TH78.txt

1) FFFFFF the locally recommended APRS frequency Object
2) CCCCCC the callsign of any LOCAL-DIRECT APRS user in range
3) The CALLSIGN and FREQUENCY of anyone with a TM-D710 radio
4) MESSASGES from APRS users addressed to FT51-TH78
5) Any other message, or special info we care to parse for.

It is assumed that this FT51/TH78 Gateway process will be added
to the general APRStt code for ease of distribution and
maintenance.  To help us refer to this capability, we can call
it the HT51/78 process.   It could even be built into a PIC for
high locations.  Here is how it works.  

First of all, since there are going to be a lot of DTMF tones
flying around, it is receommended that the HT51/78 gateway have
its own local frequency dedicated to these radio users.

The DTMF format used by these radios is:

NNN#MMMMMMMMMMMM#

Where NNN is a DTMF GROUP code that all 51/78 users monitor and
where MMMMMMMMMMM is up to 6 bytes of text entered using the
two-key method of these radios.  That is, press the desired
letter  key, ABC, DEF, GHI, etc and then press the "A" key if
the letter was  the first one listed on the key.  Or press "B"
key if it was the second letter, or press the "C" key if it was
the third or press the "D" key if it was the numeral for that
key.

Actually, for individuals, the NNN can be the individuals own
DTMF code and then only HIS messages will go to him, not just
the general GROUP messages..

APRS PARSING:

The HT51/78 processor code looks on the APRS channels for the
following kinds of packets and then transmits the 6 bytes over
on the HT51/78 channel as indicated.

1) Any packet heard DIRECT:  It sends the CALLSIGN
2) Any FFF.FFF OBJECT:  It sends the FFFFFF
3) Any user with FFF.FFFMHz in his text: Sends CALLSIGN and
FFFFFF
4) Any message sent to "FT51-TH78" it parses into approximate 6
byte
   chunks (looking for SPACEs where it can find them) and sends
all
   of these chunks on the HT51-78 frequency.

This allows these radios to be used to monitor for APRS activity
and to be paged or sent messages from the APRS system!

In fact, this gateway can be implementd so simply, that it could
be put on a PIC chip and placed on a mountain top for full
coverage wide area limited APRS operation with these 1995's
vintage radios.

Bob, Wb4APR





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