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

Stephen H. Smith wa8lmf2 at aol.com
Sat Nov 24 04:46:00 EST 2007

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.

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.    


Stephen H. Smith    wa8lmf (at) aol.com
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