[aprssig] NMEA Sentence problems into SCS Tracker....from GarminEtrex

Stephen H. Smith wa8lmf2 at aol.com
Sat Nov 24 17:49:33 EST 2007

Jack Chomley wrote:
>> Jack Chomley wrote:
>>> Update!
>>> Well it looks like the Etrex needs an adapter cable between its 
>>> proprietory Garmin cable and the SCS DSP Tracker. Its something to 
>>> do with the DSR or maybe RTS lines. When I tested with the scope, 
>>> the MAP76s was putting out data fine, with no terminal connected, 
>>> the Etrex was sending NOTHING in the way of a readable level at all. 
>>> As soon as the terminal was hooked up, away it went!  I wonder IF 
>>> the output chip in the Etrex is being powered by the Terminal RS232 
>>> port?.....looks like its a possibility.
> At 07:46 PM 24/11/2007, Stephen H. Smith wrote:
>> If this is the problem, connect a source of +volts through a 4.7K 
>> resistor to the GPS unit's DATA IN pin. [The GPS will turn on with 
>> any non-zero voltage, either positive or negative.] This will make 
>> the port wake up and start sending data.  The voltage is not critical 
>> if you have a resistor in series to limit the current.  You could use 
>> 13 VDC from the primary mobile power source, or 5 VDC if you can 
>> steal it from somewhere inside the TNC.    Note that you will have to 
>> disconnect the TNC data-out line from the pin on the TNC plug before 
>> trying to pull up this line going toward the GPS to +volts.
> My terminal puts -6v on this line, to trigger the GPS port. So your 
> idea is the answer, however I am not sure if my best solution is to 
> make a 3v regulated supply and feed that in via an adapter to the VCC 
> and ground of the GPS, hoping that will trigger the data port, because 
> the unit will be running from external power, not the internal 
> batteries. I sure ain't paying the A$85 for the Garmin combination 
> Power/Data cable, with inbuilt regulator, since I just paid A$35 for 
> the data cable by itself. I might just splice in 2 x 1.5v cells to 
> test, before I make up the regulator circuit.
> 73s
> Jack VK4JRC

You don't need any complex regulator.  Just pull up the DATA IN line of 
the GPS to any convenient source of DC through a 4.7K to10K resistor.   
As long as you have the current limiting of a series resistor, the 
source voltage can be higher than the normal data-in voltage level.  

A simple fix without modifying any existing cables is to use one of the 
male-to-female serial-cable patch blocks used to make custom adapters 
for serial cables.  (These are typically grey plastic boxes with a 
female DB9 or DB25 on one end, a male DB9 or DB25 on the other, and a 
small PC board inside where you solder jumpers and crossover connections 
between the two ends.   They are sometimes referred to as 
"mini-patchers", and are available at almost any store that sells 
computer or electronic supplies, even Radio Shack.  Note that these are 
not the common gender changers with the same sex connector on both ends, 
or "null modems" that have fixed wiring inside.)  

I connect the common and DATA OUT lines (data out relative to the GPS) 
end-to-end straight through to like-numbered terminals.   I then connect 
the resistor and external power lead to the DATA-IN terminal on the end 
facing the GPS but NOT to the end facing the computer or TNC.   Drill a 
small hole in the plastic cover to pull the power lead out. You can then 
just unplug your existing GPS cable from the TNC and sandwich the 
adapter block between the TNC and the existing cable. 


Stephen H. Smith    wa8lmf (at) aol.com
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