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Weyalan 21-03-2006 17:13

Repeater & laptop from 1 gps, no?
I am trying to get a decent nav package happening, but on a small budget. Apart from the inherent contradiction in the above statement, here is the current scenario; I have

1 x GPS (Garmin GPS-III) (2nd hand, purchased ultra cheap from E-bay)
1 x GPS repeater (NASA Clipper) (ex-factory demo - cheap from E-bay)
1 x Laptop + chart software (Toshiba Satellite + CMaps ECS)(second hand, cheap from E-bay)

The GPS and laptop will be located at the nav station. The repeater will be located on the instrument bank on the cabin top.

I know that the GPS can interface with the laptop (via USB or serial ports). I know that the GPS can interface with the repeater (NMEA).

The question is: Can I interface my gps to both the repeater and the laptop simultaneaously? Is so, how? If not, do you think that my best bet is to buy another cheap 2nd hand GPS from E-bay and use 1 to run the repeater and 1 to run the laptop chart software?

GordMay 21-03-2006 17:51

I donít know much about modern electronics, but my (mis?)understanding is that the GPS should be able to drive the PC & Repeater simultaneously, and a Double Throw Selector Switch should allow one or the other to feed the GPS.

Pblais 22-03-2006 15:22

My Garmin 128 claims to support three receivers at one time. I do it all the time. The GPS connects to a bus and that feeds the AutoPilot, the radar display and the laptop. All works slick just two wires a signal and a ground. You can feed a number of "Readers" from one "Talker" depending on signal strength. When you get into multiple talkers and multiple readers you need a mutiplexor (some money).

My next phase is to use the GPS direct to the laptop then output from the laptop to the bus. This will let me display more way points than the Garmin can deal with plus make course changes on the fly a lot easier. Now I upload the Garmin with the course from the laptop then can basically drive it all from the Garmin with the laptop just tracking the course. The laptop just adds most fancy things that the Garmin can't do.

So I guess my answer would be why the repeater at all?

If you use USB connections be sure to use ONLY a "U232-P9" adapter a Gogle search will bring it up. These little guys work well and many cheap ones won't and will drive you nuts trying. So don't waste time. They sell for about $24.

I would wire it to one GPS and still buy a second as backup. Get it all working then disconnect the backup and store in a water tight cointainer. I found a eBay backup for pretty cheap that is identical to mine. I justid a firmware upgrade and tested it and stashed it.

Weyalan 22-03-2006 18:45

Thanks for the info guys.


So I guess my answer would be why the repeater at all?
I am trying to rig up a safe and functional navigation system on an absolute shoestring budget. It is not intended to be a long term arrangement, but will allow me to sail as safely as possible using the equipment that I have right now (i.e. the aforementioned GPS, laptop and repeater).

The currnet plan is that the lapto and GPS will stay on the chart table at the nav station. Planning of routes and inputting of waypoints will be done there. Using CMpas ECS charts and software, the laptop will display the chart and the vessel heading, position, speed etc.

The GPS repeater will be mounted on the cabin top (along side the wind instrument, log, speed, depth displays, etc) and will display whatever is displayed on the GPS down below - this will allow me to input a waypoint downstairs and then steer a course to that waypoint using the on-deck GPS repeater.

Pblais 23-03-2006 04:35

Ok, that makes much more sense to me now. I do something similar in that I have a radar display that takes the NEMA input instead of your repeater. It displays it on the radar screen. I can leave the radar in stand By and thus only display the GPS data or turn on the radar for both. The display is on a swing arm that is above the nav station and swings into the compaion way so it can face the helm. So I guess we do about the same thing.

If you find the mid point between the three devices yuo can connect the GPS wires to a bus strip for each wire. Then make connector for each wire lead to each device and connect them all up to the bus strip. You may also be able to add another connection if you have a DCS enabled VHF radio too.

You can get a Garmin plug and an RS232 cable for those two devices and perhaps use the rest of the wire from the RS232 cable to run to the repeater. I would keep the wires as short as possible and put the bus strip some place handy. I added mine inside the nav station since it was close to the GPS and the computer. It's also dry.

This setup can work fine. It should still work if the computer is off and still display on the repeater. It's nice that it works that way as you might not want to run the laptop all the time. Ialso got a 12 volt adapter for my laptop and you vcan find those on the web easy. They are a far better choice than using a inverter to use 12 volts to make 120 volts to them make 12 volts all over again. You'll use far less power this way.

I would consider a backup GPS and set it up so you can replace the current GPS with the back up should it fail. Norramlly you won't need the backup so it would be insurance for the whole setup. You don't need a fancy GPS as your computer software will do more than most GPS systems.

Weyalan 23-03-2006 15:51

Thanks very much for the useful information Paul. Funnily enough, I just got myself a 12v adapter for the laptop off Ebay, super-cheap. I already have a cable that has the relevant Garmin plug on one end and a standard serial port plug on the other. It will just be a matter of working out which wire does what going into the GPS repeater, but I still have the manual for the GPS and I'm pretty sure that has a diagram showing which wires carry which signals...hopefully the repeater will have a similar one.

I do want a back-up GPS, and Ebay will deliver sooner or later.... ;)

Anyway, thanks again for the advice

GordMay 24-03-2006 01:09

Garmin Pin 1. Red = +Power (Fused)
Garmin Pin 2. Black = Ground -to- PC Serial Pin Sg
Garmin Pin 3. Brown = Data Out -to- PC Serial Pin 2 Rx Receive
Garmin Pin 4. White = Data In -to- PC Serial Pin 3 Tx Transmit

Weyalan 26-03-2006 13:00

Thanks Gord 8)

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