Solution for Garmin, and probably other incompatable GPS units

July 22nd, 2007, 15:01 Posted By: Art

Hi Guys,
Here is a tested solution to get the Garmin GPS18 PC
to work with the PSP and MapThis! program.

The problem with my GPS18 is that the output is closer
to RS232 standard than desired for this application,
and the GPS transmit line must be inverted.

Lucky only one directional comms is used so a single
transistor circuit is needed that will fit into a DB9 backshell,
or hidden inside an XLR connector.

One of the 1K resistors in the circuit is the usual
resistor in series with the PSP serial Rx line that is used in most
homebrew cables for the MapThis! application.
You can try higher values to reduce the current as
much as can be tolerated before your setup fails,
then lower the value a little to ensure reliable communication.

The 4x AA battery pack is required to power the GPS unit since it
has no compartment for batteries of it's own.
The same battery pack powers this circuit, and since it has a biult
in switch, that can be used to power the GPS mouse on and off.

The GPS18 is a high end unit priced at over $200 in Australia,
and I don't know how many other brand units will have
the same characteristic which is desirable in some applications..

Cheers, Art.

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