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Automatic Position Reporting System (APRS) 

"Using the Garmin GPS-18PC with the Kenwood TM D-700A Radio"
          by Chuck Welman KG6NJP 


The Garmin StreetPilot 2610/2620 GPS cannot connect to the Kenwood D-700A or D-7A Radios. Unlike the StreetPilot III, the SP-2610/20s do not have NEMA-0183 Data input/output! I really like my SP 2610, so the reasonable solution was to add a "dedicated" GPS receiver to supply the required NEMA-0183 data.

Garmin makes various "OEM" units (e.g., Jerry WA0GLD uses a GPS-35). I chose the OEM GPS-18 series (there are three models in this series). All three models are 61mm (2.4") in diameter x 19.5mm (0.76") high (about the size of a hockey puck), are waterproof per IEC 60259 IPX7 and have a magnetic base with provision for "safety" screw. Forget the GPS-18USB - it does not output NEMA-0183 data. The GPS-18LVC is a "bare-wire" model and discussed briefly later. The GPS-18PC includes a single cable assembly leading from the Receiver that splits into a Power cable with Cigarette Lighter Plug/Adapter (more on the Adapter later) and a Data cable with a female DB-9 connector. It also includes a suction-cup windshield mount and the tiny safety screw. The latter model is also available as the GPS-18PC Deluxe, adding nRoute software and City Select 6 Map data. This is the model I bought, only because no one had the "plain vanilla" GPS-18PC in stock! The software is not used with the TM-D700A
None of the models in the GPS-18 series have a display or buttons. Each are tested and/or configured by using software downloaded from the Garmin < > Support page. The GPS-18PC DB-9 connector is factory-wired as follows:
          Function         Wire Color         DB-9F

          RCV              Green              pin 3
          TXD              White              pin 2
          Ground           Black              pin 5
          Drain            Blk/Shield         pin 5 
Radio cable

The D-700A has a 2.5mm (3/32") Stereo Jack labeled "GPS". I bought a 6 ft long cable with molded right-angle 2.5mm Stereo Plug on one end and bare wires on the other. This cable is available from Byonics < > - look on the " Pocket Tracker" page.
I installed a male DB-9 connector on the bare end of the Byonics cable, wired as follows: 
          Stereo Plug      Wire Color         DB-9M

          Tip              White              pin 3
          Ring             Red                pin 2
          Barrel           Shield             pin 5
Check the wire colors on the cable you use - they may differ from mine...!
Note: the photo(s) show female-to-female DB-9s with gender changer. That was my prototype since I only had a DB-9 female in the junk box.. I have since replaced the DB-9F on the radio cable with a DB-9M and eliminated the gender changer. The connectors are located under the rear seat in a "baggie". See photo(s). 


My D-700A is mounted vertically in the "recess" in the interior wall of the GL-1800's right saddlebag. It's a very tight fit, and the Serial (for a Laptop to program the radio or connect to the TNC), Mic, Control-head and GPS cables have only about 5/8" of clearance between the radio and the floor of the saddlebag. Hence, the necessity for the right-angle Stereo Plug on the GPS cable!
I originally planned on mounting the GPS-18PC in the space between the trunk lid and the its inner liner. But I wanted the GPS-18PC to have a 360 degree "clear view" of the sky, down to the horizon line if possible. The shape of the GL-1800's trunk lid and the rear seat back blocked the forward 180 degrees of view - unacceptable! Instead, I decided to use the Windshield bracket (without the suction cups). That allowed me to use the safety screw without worrying about tearing the brass insert out of the base of the GPS. I bent the suction-cup end of the bracket at an angle to fit between the forward rail of the trunk rack and the seat back. (see photos) and mounted it with two nylon clamps. Mounted essentially level with the top of the rear seat back, the Receiver has a clear view of most of the sky down to the horizon, except for the small portion blocked by the Rider! The Windshield bracket was originally installed as a "temporary" mount, until I could fabricate a better one. Nah, it works fine and I'm keeping it!
Power Cable Adapter

This is the main reason I chose the GPS18PC instead of the GPS-18LVC Receiver. The Receiver itself is identical, but... connecting either Receiver directly to 12VDC will "fry" it! The GPS-18PC's cigarette lighter adapter has a small internal circuit board that steps down the 8-30VDC to the proper voltage (5VDC plus/minus 10%) for the Receiver. The GPS-18LVC requires you to build or buy a suitable power supply. The GPS-18LVC is only $10 (street) cheaper than the GPS-18PC - it's not worth the hassle!
Hard-wiring the Adapter to 12VDC power

After I checked out the GPS-18PC to make sure it was working correctly, I was ready to tear into the Adapter! I originally thought that I would have to cut open the Adapter, remove the power supply intact (hopefully) and mount it in a suitable enclosure.
I unscrewed the plastic ring from the nose of the Adapter and removed the tip and fuse. Looking at the two halves of the Adapter, I noticed that they were not glued/fused together. I took a small straight-blade screwdriver and carefully tried to pry off the chrome collar at the fuse end. It moved! With a little more "coaxing", the, collar popped right off of the Adapter. As I slowly pulled the halves of the Adapter apart at the fuse end, the body opened up like a clamshell. When the "upper" half (the half without the LED) was up about 45 degrees, it cleared a couple of tabs and separated from the lower half (the half with the LED)! See photo.
The lower half holds the cable strain-relief, the SMD circuit board (PCB), the fuse spring and the two plates that comprise the negative connection for the cigarette lighter adapter plug. This entire assembly lifts right out of the lower half. Unsolder the spring (+)and the two black "plate" wires (-) from the PCB and you're ready to solder on some wires to connect to switched 12VDC power. Use a "blade"-type fuse holder in the positive leg (Bussmann ("BUSS") model HHM mini-ATC Inline Fuse Holder with waterproof cover -- about $2 at Kragen Auto Parts < > and a 2A mini-ATC fuse (grey body). Current draw is only 55mA at 12.8VDC. Garmin recommends a 1A fuse, but 2A is the lowest-amperage mini-ATC fuse.   
I put 4 globs of hot glue around the base of the Inductor to "harden" the PCB from shock & vibration. The rest of the components were rigid enough. I installed 18AWG stranded hook-up wire from RS and protected it with a flexible plastic loom. I routed the new wiring out the hole in the nose of the Adapter and replaced the chrome collar and the threaded plastic ring to hold the whole thing together. I used several sizes of shrink tubing on the plastic loom, to essentially seal the nose of the Adapter to the cable assembly. The Adapter fits perfectly along the left upper frame rail under the rear seat of the GL-1800 and the LED pilot light is visible with the seat off. See photos.
Follow the Kenwood manual for setting up the APRS function in the D-700A. The "[Marc] BASIC SETUP FOR APRS ON TM-D700A" email dated 11/28/04 from Ted KB6CUS provided an excellent starting point for the settings. The D-700A will flash the "GPS" symbol on its display as soon as the GPS outputs valid data.
< Email me > if you have questions!


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