How to build a Better Garmin Cable for your laptop PC

Since I'm more of a geek than an outdoorsman, I went searching for a handheld GPS with the thought of hooking it up to my Compaq Armada 7770. I didn't have any idea what I'd do with it, but if I could connect it to my PC, it'd be useful...right? It was no problem shelling out $149.00 American for the Garmin GPS12, but I was floored when I saw the cost of the data cable. Not only was it $35, but I'd have to wait a week for it to come in. How could I stare at that shiny black box for a week without hooking it up to my PC?!?

After an hour of dialing around town to various computer stores, parts houses, and sporting goods stores, it began to look like my only option was to either succumb to poverty and buy the cable or hack my own. Time to hit the web to find someone had done this before. It was then I stumbled onto Larry Berg's Purple Open Project. If you're here, you probably already know all about it, if not, stop right now and visit http://pfranc.com. Not only has this page renewed my faith in the good nature of man, but it provided me with everything I needed...especially the elusive little 4 pin connector that Garmin holds so dear.

I decided I could improvise the GPS connection for a few days until the nearest Pfranc (which is now me, but that's another story) got me a connector, so I started on the power end of the cable. Since my GPS12 likes 5 to 8 VDC, I rummaged through my junk bin for a 7808 regulator but came up empty. What I did come across was an old QuickCam and a flash of inspiration hit me. The QuickCam gets it power from the keyboard connector, so why couldn't the GPS12? It only draws about a watt and that's less than a few hundred mA...plenty of power there. It was time to get to work, so I grabbed the following tools and parts and within an hour, I was hooked up and running.

The first order of business is freeing up the cord from the dead mouse. I opted to actually open up the mouse so I could:

a.) scavenge the innards for other parts,
b.) admire the construction, and
c.) cut the white connector off where it enters the circuit board so I wouldn't have to strip back the insulation later when I mounted the Pfranc connector.

With a clean cord, the next order of business was to 'ring out' the cable to find out which color wire connects to which pin. Actually, we're only concerned with what color pin 4 is, because all we want from the keyboard connector is the +5VDC. All other signals come from the serial port.

The wire will probably be orange or red, but NEVER ASSUME ANYTHING. Even if you just cut the same brand of mouse open yesterday, nothing guarantees the manufacturer used the same color coding between batches. Meter leads with the little clips on them are helpful if you only have two hands.

If you're not too sure about which one is PIN 4, I took the liberty of making a little sketch. Since the wire is molded into the connector, we'll have to settle for the front view.

With whatever you use to strip wire, remove about 1" of the outer jacket approximately 9" back from the keyboard connector. This distance isn't critical, but should be long enough to span the distance between the serial port and the keyboard connector on your machine. Be careful not to nick the wires inside. If you do, you can insulate things later with tape, but it's tough to squeeze a big wad of electrical tape back into the headshell when you want to close things up.

Sorry if this is a little out of focus, but I haven't received that high-res digital camera from you yet. It should be clear enough to get the point across.

Anyway, now we want to clip every wire except for the one that goes to PIN 4 at the connector end of the insulation gap.

If you accidentally clip our PIN 4 wire, it's not the end of the world. Just be sure you insulate it well when you stuff the headshell together and deduct 10 neatness points. Drink another cup of coffee, too. You deserve it. 

Familiarize yourself with the back end of the DB9 (left). If you're going to go out and buy one, try to get one with gold pins with nice deep cups for the wire to lay in. The gold is easier to solder to and deep cups lend for a better mechanical connection.

Next, pick 3 wires out of the bundle and make a little chart to show what color of wire your going to use for which function. I did mine up on the inside cover of my GPS book in case I want to refer to it later. It doesn't matter what color you use, as long as you get the right pin connected to the right pin. Here's what mine looked like. Remember, your colors may vary!

Now that you've decided what color is what, it's time to prep the wires and solder them up. It's not cheating to refer back to the picture of the DB9 above, in fact, I look at it 4 or 5 times. Taking your time here and making sure you don't flow solder across to other pins can save your PC and/or your GPS.

I usually make the ground wire just a hair shorter to provide some strain relief on the other wires, but this really only works well for shielded cable...which your mouse probably doesn't have.

Clip off the excess wires and hopefully, you'll end up with something that looks like the picture off the left.

With a final inspection for shorts and bad solder joints, put the headshell together. Depending on which one of 247 styles of DB9 you bought, this could take a few minutes or a few hours.

This is not a bad time to pull out the multimeter again and check for continuity and for shorts. Once you've finished all this, have another cup of coffee and check your mailbox to see if your connector has arrived from your pfriendly Pfranc. If you already have it, take a few minutes to clean it up with a razor blade, nail file, or preferably, your Leatherman® Tool.

Ready for the grand finale? Good, my fingers are starting to cramp.

The huge purple thing on the right is similar to what you should end up with if you followed Dave Sorenson's diagram on the Pfranc page. Just solder the wires up according to the chart you made earlier and clamp the lid down with the included screw. It's that easy.

I'd give everything a final check with the meter just to be sure. Plug the keyboard & serial connectors in and turn the laptop on. You should see 5 volts on the appropriate pin of the GPS connector if everything went well. Once the PC boots and you get logged in, turn on the GPS and have fun. Here's one final photo of the whole thing with the PC, minus the GPS.

I use my GPS mostly in 'stand alone' mode, or NOT connected to my PC. MapBlast gives decent point to point directions along with GPS track information. With Waypoint+, a nice Windows95 program that speaks Garmin's proprietary language, I dump the track to my GPS12 and off I go. I've only lived in New Jersey for about 7 months, so there's still a lot of places I haven't been yet, or in other words, a lot of chances to play with my Garmin.

I hope this helps if you're looking to do something similar. I make no guarantees that this is the solution for your needs or that this won't fry your equipment. If you want a guarantee, pay Garmin for their cable. If you can solder and know a little about DC electricity, then try this and save a buck.

Let me know if you have any comments, improvements, or suggestions by emailing zim@z-space.net. I'd like to hear how you made out.

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