[aprssig] THD7A GPS Question

Stephen H. Smith wa8lmf2 at aol.com
Sun Feb 17 01:32:40 CST 2008

Sam Guccione wrote:
> After a long lapse I am trying to get my THD7A setup with a GPS. However,
> something doesnt seem to be working. I am using a Garmin ETREX set to NMEA
> output. I can see the GPS sentences when I plug it into the serial port on
> my laptop. I connected the ETREX to a cable with a subminiature plug then
> into the GPS jack on the THD7A. I expected to see the POS have the GPS lat
> and long but all I get are all zeros. Manual does not say much of anything
> about the GPS port. Anyone have suggestion on what I could try to get it
> working. What am I missing? Something stupid?
> Sam K3BY
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> aprssig at lists.tapr.org
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Are you using a 2 or 3 conductor cable?    You need a full-three-wire 
serial cable. 

The Etrex, like many battery-powered gadgets conserves battery power by 
shutting down it's serial port when nothing is connected.   (The DC-DC 
power converter required to produce the negative half of a true RS-232 
signal is one of largest power consumers in low-power battery gadgets.) 

These devices don't power up the outgoing side of the serial port until 
they see a NON-ZERO voltage coming into the receive side of their port, 
indicating that the GPS has actually been connected to another device.   
When the GPS is connected to a PC with a full bidirectional three-wire 
cable, the quiescent state of the PC's outgoing serial port is -8VDC 
which is sensed by the GPS data in. This makes the the GPS serial TXD 
wake up and start talking.   

The TH-D7 leaves the outgoing side (TXD) of it's serial port resting at 
+8VDC when it is not talking, but the GPS won't see this if you use a 
simple two-wire RXD-only connection. 


Stephen H. Smith    wa8lmf (at) aol.com
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