[OZAPRS] Suitable GPS for HF APRS & SCS DSP Tracker ??

Dave Ingram bulk at ingramtech.com
Tue Sep 23 21:15:27 EST 2008

Andrew Errington wrote:
> Can you wire up the Garmin so that the trigger pin is held 'high' (or the
> appropriate state) to fake it as if it were connected to a PC?  Are you
> sure it is a power-saving feature, and not simply hardware flow-control? 
> I suppose it doesn't matter either way.
> Aside from that, I can confirm that the Garmin Geko 201 "just works".  I
> have one, and I have set the output mode to NMEA.  When I turn it on stuff
> appears on the serial port.  When I turn it off, it stops.  (Yes, I know
> you asked about the 301).
> The Geko 201 is probably the most basic-but-useful GPS receivers you can
> get.  The 101 is definitely not worth buying (and should never have been
> made IMHO), and the 301 is not worth the extra money (it has a built-in
> electronic compass and barometer, independent of the GPS-derived direction
> and elevation calculations).


Your Geko 201 must be wired differently to mine. I have one that is around 18
months old and there is no printed geko figure on the front. It will not
transmit serial data unless there is a valid RS232 signal (mark or space) on
its receive pin. I found this out the hard way when I tried to use it with my

My solution was some SMD surgery to tie the appropriate pin of the RS232 line
driver to ground so the device stayed on the whole time. Unfortunately the GPS
now uses the batteries faster.

An easier option (if you don't need bi-directional comms) is to tie a 9V
battery through a 100k resistor to the RxD pin of the serial port. I was using
the GPS to transmit positions and also to display received locations as
waypoints so this wasn't an option. The Opentracker serial port was a 0/5V one
rather than a +5/-5.


Another thing to try is get a MAX233 or similar and wire it back to back so
the serial port goes through a TTL stage. The resulting voltages from the
drivers on the MAX233 will be within spec and will activate the line driver in
the GPS. If you're clever you should be able to mount the whole thing inside a
gender conversion backshell or similar and give it 5V from somewhere.

David Ingram (VK4TDI)
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Brisbane, Queensland, Australia
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