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

Jack Chomley radio at irock.com.au
Sat Nov 24 16:54:46 EST 2007

>Jack Chomley wrote:
>>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:

>Aha!  This sounds similar to my Garmin Foretrex 201 Wrist 
>GPS.    The serial port interface chip is one of the major power 
>consumers in a GPS, partly because of the DC-to-DC converter needed 
>to produce the negative half of the RS-232 signal swing.   (True 
>RS-232 swings back and forth between -8VDC and +8VDC.)
>To conserve battery power, the Foretrex (and I also suspect the eTrex)
>serial interface doesn't power up and start talking until it sees a 
>NON-ZERO voltage on it's INPUT pin.  (This indicates to the Foretrex 
>that it has actually been connected to an external 
>serially-interfaced device, since a serial port OUTPUT not sending 
>data will normally have a constant -8 VDC on it.)   This issue has 
>come up many times on the TinyTrak mailing list since the TT's 
>OUTPUT line is set to zero volts (rather than -8 VDC)  during normal 
>operation, causing battery-operated GPS units such as the Radio 
>Shack Digitraveller, Foretrex and eTrex units to not turn on when 
>connected to a TT.
>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.



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