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NMEA Connections Question.

Jeff Asbury

Principal Partner
NMEA Connections Question,

I have a Magellan hand help GPS currently connect to my Simrad Tiller Pilot. I have finally purchased a new VHF with the NMEA capability. I believe there are only two wires (IN/OUT) on the GPS. My question is can I just splice together so that the GPS will share data with the VHF & Tiller Pilot or do I need some kind of interface device?

I do know that the Baud Rate, Data Bits, Stop Bits need to be the same for both the Tiller Pilot and VHF. But what if they are different?

Thanks in advance.
 

Shadowfax

Member III
I have a Garmin that is supposed to be able to talk to my Autohelm and Icom VHF via NMEA. I've tried to get it to work a number of times and have not been able to make it happen. According to the paper work there are only 2 wires out of the GPS that hook into the Autohelm to make it work, no other interface is necessary.

Hope you have better luck then I did.
 

windjunkee

Member III
Jeff,

We have our GPS (Garmin) interfaced with our instrument system, computer navigation software (through a serial port), VHF and SSB. Our Autopilot is a Simrad W32, which is connected to the GPS through the instruments (B&G H-1000). The autopilot has no problem steering to GPS waypoints, wind angle or compass heading.
The way the connections work is that we connected the GPS through a buss bar and from there ran wires to the instrument interface, computer and two radios. Since the radios are installed right next to one another, we ran one connection wire to another buss bar next to the radios and then ran wires to each. There is no loss of data. Everything works together flawlessly. We do have multiple wire connectors attached to one contact point on the buss bar, but that hasn't been a problem at all.

Jim McCone
Voice of Reason E-32-2 Hull #134
Redondo Beach, CA
 
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Jeff Asbury

Principal Partner
Ahh Ha, Bus Bar, good idea. I think I have my auto pilot set up to just receive data at this point. I too am able to have it steer to way points. But this is the first time I have wanted to connect another device because the VHF has the distress feature as well as displaying other data from the GPS. I know I should be able to hook my radar up as well but I don't really need to because it will only display data as far as I know.

The Buss Bar makes a lot of sense. I just hope since I have different manufacturers the data transfer rate will not be a problem.

Hey Paul, Have you checked the different settings on your GPS. It should work with your auto pilot. If it's hooked up correctly it's just a matter of having the right setting selected on your GPS. My auto pilots been working fine with my GPS for a couple years now but I remember having to find the right setting.

Good Luck

Thanks for all the help Guys!
 

Shadowfax

Member III
Jeff,
Yeah I tried all the different settings and even reversed the wires. Talked to Garmin, talked to Autohelm, they both said it should work. I do like the buss bar idea, haven't tried that. I suspect that before it's over I will have to seek professional advice, unless Tom Metzger can come down and figure it out

Well time to go out and warm up the snow blower. I think I should take up cross country skiing as a winter sport, then maybe this ridiculous weather wouldn't be so depressing
 

Howard Keiper

Moderator
First, recognize that the NMEA 0183 standard has been around a long time and it works every time, all the time. It's important to know this when you're confronted with a problem that makes you wonder if you're dealing with an untried system.

Keep in mind that devices in the NMEA network either provide data (talkers) or receive data (listeners). Your GPS is a talker...always use NMEA OUT for a talker. Your autopilot is a listener, as is your radar or your chart display and your VHF (although your VHF can, in some instances, be a talker too). Use the NMEA IN when connecting these devices.

I almost always use a barrier strip and crimped ring terminals for my in/out leads from the devices. It's sooo much more convenient to figure out what's going on. You'll only need a three terminal strip. This is where you connect the out from one device to the in of another. Dont ignore the shield, or drain, either, it's the return for the particular device. Shields can always be connected together on the terminal strip. Don't connect the shield to in or out of a device. The conductors are always thin and fragile...be very careful when crimping

Don't monkey with the data rate between devices that seem to be working. Almost all devices in the 0183 net use 4800 bpi. What that rate is is not so important as that they are the same. Find out how to change the rate of the device giving you trouble...if you're reasonably sure that's the problem.

Howard Keiper
Berkeley
 

Tom Metzger

Sustaining Partner
No joy

Sorry Paul - I'm up here with lots of shoveling to do. Wasn't it 80* down there this week. Quitcha bitchin. :cool:
 

Jack Loekken

Junior Member
I belive that the standard requires that one output shall be able to drive 2 inputs (correct me if I am wrong). You may get away with connecting more than 2 inputs to an output, you just have to try.

However, each input can only be connected to one output. So if you need data from both wind instruments and GPS to your autopilot, and the autopilot has only a single NMEA IN, you need to use a multiplexer.
 

Jeff Asbury

Principal Partner
OK,

I installed my new Uniden VHF with all the whistles and bells but I have not hooked up the NMEA connection to my Magellan Meridian Marine hand held GPS yet because I need to determine if it is strictly a listener or talker. That determination came after reading all three manuals for the devices. Currently my GPS is connected to my Simrad Auto Pilot and it works fine. I don't want to screw that up. Simrad's manual said it can only be connected to one talker so I need to determine if the Uniden VHF is only a listener and not a talker. There is no clear information in the manual that states whether it is indeed a listener or a talker.

This concurs with Howard's information. Thank you Howard for the following connection information:

"I almost always use a barrier strip and crimped ring terminals for my in/out leads from the devices. It's sooo much more convenient to figure out what's going on. You'll only need a three terminal strip. This is where you connect the out from one device to the in of another. Dont ignore the shield, or drain, either, it's the return for the particular device. Shields can always be connected together on the terminal strip. Don't connect the shield to in or out of a device. The conductors are always thin and fragile...be very careful when crimping

Don't monkey with the data rate between devices that seem to be working. Almost all devices in the 0183 net use 4800 bpi. What that rate is is not so important as that they are the same. Find out how to change the rate of the device giving you trouble...if you're reasonably sure that's the problem."

I have read a little about the multiplexers Jack, are they expensive? Where can I get one?

I guess I will be placing a call to Uniden. Thanks for all the help everyone. I will let you know how it turns out.
 
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Jeff Asbury

Principal Partner
I got my answer!

According to Uniden my unit has both listening and talking capabilities. The connections listed are GPS Data IN (-) and GPS Data IN (+). These are the connections they said to hook it up to. The unit also has NMEA Out (-) and NMEA Out (+). They told me I do not, and should not connect these. The guy I talked to did not know about multiplexers for NMEA connections and assured me that what he recommended would work. Once again I will let you know if it works correctly.

Thanks Again All!
 

Howard Keiper

Moderator
I assume the UNIDEN is your VHF. When I mentioned earlier that in some cases the VHF could be both a listener (accepting GPS data, presumably for reporting your position to whomever has a use for it...a nice DSC feature), and a talker, this is what I had in mind. Certain radios, not all, have the ability to query, or poll other DSC radios, selectively of course, and report those received data via NMEA OUT to your chart display or possibly your radar.
The (+) is always the signal conductor, the (-) is it's return, usually the shield, if coax.
All DSC radios have the NMEA In feature, not all have NMEA OUT.

Your Magellan is strictly a talker, and has no NMEA IN at all.

You almost always use a multiplexer in situations where you want to see two or more sets of information, GPS coordinates for instance, on one display. For instance, you might want to see your GPS position on your chart display, AND you want to see the GPS position of the DSC radio on your buddy's boat on there too, at the same time. Your chart display is a listener only. If it has only one NMEA IN port, you'll need to split thr inputs, and that's what a multiplexer does.
Howard



Howard
 

Howard Keiper

Moderator
Jeff...I think my computer missent the last message..here is what I wanted to go out...

Your Magellan, NMEA OUT (+) can talk to both your Simrad NMEA IN (-) and your Uniden NMEA IN (-). Just crimp ring terminals on the conductors and connect the proper outs to the respective ins, crimp and connect the shields together, and PRESTO!
 

Jeff Asbury

Principal Partner
Success!

Success!

I now know that the rule of thumb when it comes to connecting NMEA devices from different manufactures is that you can only have one talker like a GPS, Chart Plotter, Computer connected to listening devices like Auto Pilot, VHF and Radar.

I also learned that you need to make sure the baud rate is the same on any of the listeners and to select it in the talker (in my case, my GPS). I had to experiment with the three different NMEA settings in my GPS to find the one that worked for both the listeners I wanted to connect it to. In my case that was the Auto Pilot and VHF.

I may go wild at some point and see if I can connect my Radar as well, but it would only give me readings on the Radar screen like position, speed, heading and time which will already be on the GPS and VHF.

I did set up a designated bus bar for the GPS NMEA Out + and - connections and I have more posts I can add to now.

Hope this helps others in the future.

I love this sport because I am learning all the time.
 

Mikebat

Member III
NMEA 0183 is nothing more than a RS-422 serial interface at 4800 baud. RS-422 is electrically compatible with RS-232, which is used in the PC's serial port that everyone is familiar with. But NMEA does not use any of the out of band signalling leads such as DTE or DCE that a modem might use. It only uses RX, TX and of course GRND.

This compatibility allows you to pipe NMEA signals over anything that is RS-232 compatible, and that includes the Bluetooth serial profile. ShipModul makes a NMEA multiplexer with a Bluetooth interface included. I plan to incorporate this into my navigation electronics. It will allow me to use a handheld PDA to run a moving map display, or to connect my Mac Mini to the navigation system wirelessly. I'm already using TackTick wireless instruments, so Bluetooth fits right in.

It's also possible, if you have headaches making wiring runs, to pair two Bluetooth serial adapters together in place of a wire run, and make any NMEA connection wirelessly. I've found Bluetooth to be as reliable as any radio device, including my TackTick instruments, but I think certain connections ought to be wired, like from your GPS receiver to the autopilot. :)
 
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