[aprssig] Having trouble with APRS setup

John Denison kd5you at houston.rr.com
Mon Dec 27 19:28:54 EST 2004

Thank you all for your replies. I went to Radio Shack and bought a null 
modem adapter, plugged it in, and now the TNC is transmitting. I changed 
the rate from 1 minute to 5. Everything seems to be working ok except 
that the digipeater in downtown Houston is not hearing my signal. I'm 
able to hit repeaters in downtown from here with no problems. Does 
anyone in this area know a specific path I should use?

@ Keith VE7GDH

I don't know if the GPS requires the GPSINIT line, but it is working ok 
with it still in. I guess either it needs the line or it disregards it.


Stephen H. Smith wrote:

>John Denison wrote on 12/26/2004, 4:36 PM:
> > The GPS is working fine because I connected it to my pc and could see
> > the data displayed in hyperterminal every 2 seconds. The cable that
> > came with the GPS has a 9-pin female connector, and the TNC has a
> > 25-pin female connector. I used a 25-pin male to 9-pin female adapter,
> > and a 9-pin male to male gender changer to get the GPS and TNC connected.
> >
> > I configured the TNC exactly the same as the example shown in the
> > manual for GPS use. The baud rate is set to 4800 for both the GPS and
> > TNC. The GPS is set to output NMEA type data. The TNC's receive light
> > comes on as it normally would when a signal is heard. The TNC will key
> > up the radio in TERMINAL mode, but not in GPS mode. I used the
> > following parameters to set up my TNC:
> >
> > LTP 1 GPS via DIGI
> > BLT 1 EVERY 00:01:00
> > ABAUD 4800
>The TNC is intended to be connected "straight through" to the PC.
>The GPS is intended to be connected "streight through" to the PC.
>Therefore connecting the GPS to the TNC will require a CROSS-OVER cable 
>or adapter (i.e. a "null modem") to work.   Specifically a DB-9 male to 
>DB-9 female where pin 5 goes straight through, pin 3 goes to pin 2, and 
>pin 2 goes to pin 3.
>Assuming you get the TNC responding to the GPS, it is now discouraged to 
>do this.  Transmitting RAW NMEA strings over the air (which is what this 
>setup does) is terribly inefficient.   Almost one hundred characters 
>have to  be sent over the air to do the same thing that the standard 
>APRS format can do in about 30 or that Mic-E format can do in about 12.
>The shorter packet bursts of APRS format or Mic-E are far preferrable 
>because they:
>1)  Reduce on-air transmit time and channel congestion.
>2)  Dramatically increase the chance of successfully being received 
>since the much shorter bursts are much less likely to get stepped on by 
>another station, or get hit by mobile flutter or noise.
>3)  Give you the flexibility of using the entire APRS symbol set.
>At the once-per-minute transmit rate you are proposing, you would be 
>occupying almost 10 percent of the entire available channel capacity on 
>144.39 in your local area. A raw NMEA packet is about 6 seconds long.  6 
>out of every 60 = 10%.   By contrast a Mic-E packet is only about 
>ond-half to two-thirds of a second, meaning a channel usage of about 1%.
>Instead of tying up an expensive TNC as a dumb mobile tracker, consider 
>using a $30 Byonics TinyTrak (which is capable of encoding the efficient 
>APRS or Mic-E formats) to mate your GPS to the radio.
>Stephen H. Smith                wa8lmf (at) aol.com
>Home Page:                      http://wa8lmf.com
>New/Updated Symbols for         http://members.aol.com/wa8lmf/ham
>UI-View and APRSplus:
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