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[aprssig] THD7A GPS Question

Stephen H. Smith wa8lmf2 at aol.com
Sat Feb 23 20:45:57 UTC 2008

Sam Guccione wrote:
> Thanks to everyone who responded to the problem. To make the record complete
> here is what I found out and my temporary solution.
> I had the cable to the D7A wired correctly and all settings were correct.
> Apparently all of the GPS receivers that I have including my eTrex were not
> able to supply the appropriate signal for the D7A to work that is 8 volts on
> the tip of the D7 plug. So I connected a 9 v battery with a switch to "fool"
> the D7 into thinking it is getting valid data from the GPS receiver. Works
> great just a pain to have to press the switch to get update on GPS location.
> My final question, what GPS models will work with the D7 so I can buy one
> that will work without any work around like my 9 volt battery solution
> above.

You don't need the battery to resolve this problem!

This problem is caused by the power conservation features of the GPS 
device.   (The plus-DC-to-minus-DC power converter required for a serial 
port to produce a true RS-232 signal is one of the major power consumers 
in small-box battery-powered gadgets.)    To save power, the GPS turns 
off the power to the serial port  until a device is actually connected 
to the serial port.  It senses this by looking for a NON-ZERO voltage 
from the TXD pin of the device at the other end.  If either a) you use a 
two-wire RX-data-only cable, or b)the device at the far end sets it's 
TXD line to 0 volts instead of +/- volts when it isn't sending data,  
then the GPS serial port will never see a voltage,  "wake up", and start 

You are "fooling" the GPS by causing it's DATA-IN line to see non-zero 
voltage by connecting the battery.

The TH-D7 puts out the required positive voltage to wake up the GPS.  
The requirement is to have a full three-wire serial cable between the 
GPS and the radio.   (Even through you might think you only need a 
two-wire GPS-to-radio-only cable..)   

The trouble arises when you use a TinyTrak (older revision firmware) 
rather than a Kenwood radio,  since the TT sets it's outgoing data line 
to 0 volts instead of +5 when idle. Thus the GPS never gets a non-zero 
wakeup voltage.  

Details on this issue and a review of a GPS that DOES work with the 
Kenwoods (and may be the ultimate companion for the TH-D7) are here on 
my website:


Some of the other GPS devices I have tested with the TH-D7 and D700 are 
the Garmin GPS-18, Rand McNally, Pharos iGPS-180 (This is bundled as the 
"Microsoft GPS Locator" with Streets & Trips or MapPoint), Delorme 
Earthmate II (Not the original version!) various Holux units. Virtually 
any unit that can output standard NMEA-format data through a real RS-232 
*SERIAL* should work.   

By the way, if you don't want to upload waypoints from the Kenwood Radio 
TO the GPS ,  you can just pull the GPS receive data line up permanently 
to either +5 or +12 VDC through a 4.7K resistor connected to the power 
source used to run the GPS.   This eliminates the switch and having to 
do anything at power up.   

(The Kenwoods can upload received APRS posits TO the GPS for plotting on 
maps of display GPS units.  For an example of this see the 
yellow-highlighted box in the ForeTrex review linked above)


Stephen H. Smith    wa8lmf (at) aol.com
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