Help with Garmin NMEA issue

[helm]

Our system is 3 garmin 8208 MFDS, a garmin 8212 MFD, gps 19 and gps 24 xd gps antennas, GMR 24 HD radom, GMI 20, card reader, GRA 10 RAI, GSD 24, and VHF 210. *We suddenly had issues with loss of GPS signal on the devices and no displays of the rudder angle on one 8208. *The long and short of it is that removing the 8208 that did not display position data and rudder angle from the NMEA 2000 network restored the gps data to all the other units. *Putting it back on the network we loose all the gps data. *Doesn’t matter what antenna we pick. *Moving NMEA connections from socket to socket made no difference.
I tried a factory reset of the suspect unit, put it back on the network and again lost all the net work gps data. *Took it off and all the gps data is back on the network. *
Anyone have any ideas? *I’m pretty stumped but will have to contact garmin next week.
Thanks

 

[GingerMd]

The N2K bus is a CAN bus at the physical layer, so if the instrument is holding either the D+ or D- signals, no traffic can pass. Do you know if other N2K data is still working on the bus ? Also if you have a N2K monitor like the Actisense NGT-1, you can monitor what is happening.

 

[twistedtree]

Sounds like a bum N2K interface on the Garmin.

 

[Iggy]

I would match up the N2k settings with the other units.

After that, I would make sure that all the units have the newest firmware. Than as a last resort, do a factory reset.

When you say moving the NEMA connection? You mean you took the GPS and moved it using another tee and drop cable, yes? If so than the tee and the drop cable are good. If no, it's too easy to try new ones.

 

[O C Diver]

You may lack sufficient power to the network or the way it's laid out. Simply, each and every item connected to the network draws power (measured in LEN). The power supply needs to be about in the middle of the network with about half the LEN draw on either side of the tee. If the network has to much draw, 2 or more power tees need to be used. Between the 2 power tees there needs to be a power isolation tee. My boat is setup this way with 2 power tees and an isolation tee.

This may sound mind boggling, but Garmin has a very good online document on setting up NEMA 2000 network, determining LEN values, and and other setup guidelines for the network.

I understand that it was working and now it's not. You may have been on the edge of not enough power, and something is using slightly more.

Ted

 

[helm]

Thanks for the replies
I am going to try a new drop cable. The power supply is an interesting idea. I powered up the system with everything on the NMEA system except I left the GMI 20 powered off, (its powered from the NMEA network) and still no bueno.
I can always leave that display off the NMEA network and just use it for depth and radar
Again thanks for all the thoughtful comments!
Eric

 

[murofrelwart]

You may lack sufficient power to the network or the way it's laid out. Simply, each and every item connected to the network draws power (measured in LEN). The power supply needs to be about in the middle of the network with about half the LEN draw on either side of the tee. If the network has to much draw, 2 or more power tees need to be used. Between the 2 power tees there needs to be a power isolation tee. My boat is setup this way with 2 power tees and an isolation tee.

This may sound mind boggling, but Garmin has a very good online document on setting up NEMA 2000 network, determining LEN values, and and other setup guidelines for the network.

I understand that it was working and now it's not. You may have been on the edge of not enough power, and something is using slightly more.

Ted

This is good advice. I suggest getting the Garmin document, looking up your LEN values, confirming where power is supplied, sketching the network and making sure it "should" work.

 

[Iggy]

If it is power, its an easy thing to test. Try to remove 2 devices and there tees. Does the problem go away. But grab your meter and do a voltage check first. In a prefect world, should the tee suppling power be in the middle of the back bone, yes. But its not mandatory. It can be at the end or anywhere else as long as the voltage drop is less than 1.5 volts on the opposite end/s of the network.

Specs on NEMA2k https://www.maretron.com/products/pdf/Network Installation Guide.pdf

But I would go over the specs of NMEA 2000. Something may come to light for you. As an example, No drop cable should be over 20' off the backbone. Or total of the whole network or cumulative drop is no more than 256'.

 

[Barking Sands]

Well...if its a suspected power issue and NOT a suspected unit (the one that did not display rudder angle) then simply swap the suspected unit in and take another out. if any combination of two units works but NOT three.. then its likely a power issue. If the same unit kills GPS data when installed even when installed in as a group of two or even that unit alone then look to that unit further.

 

[wkearney99]

Maretron's N2K programs are good for mocking up an N2K network. Well worth using to make sure you've created a compliant setup.

Also note that N2K devices each have their own instance ID. When all you've got is one of any particular kind of device it doesn't really matter. But when you've got more than one... it'll confuse the other devices if they can't tell which is which. I think I used Maretron's DSM410 4" display to set the instance of my two GPS units, weather and heading sensors. This cleared up some random "loss of GPS" and heading angle issues I'd been having. The Airmar weather unit can provide some of the same data as the heading unit (but with less precision). This was leading to my (then) NavNet3D chartplotters thinking my boat heading was 90° off. Once I'd changed the instance IDs on everything I went into the plotters and made sure they were configured to use the specific ones I wanted.

 

[wkearney99]

It's also important to configure your displays on what THEY put back OUT onto the network. Some MFDs have their own internal GPS. You don't want to have that squawked back onto the N2K bus if there's other devices already handling it. I don't know the specifics of the ones you've got, or how to configure them.

A similar problem can happen when devices have more than one bus. I seem to recall someone else complaining about certain Icom radios being a problem echoing GPS that came in on one port, right back out the other. I think it was 0183, not N2K, so it's not directly relevant, but it does illustrate the larger issue. Make sure you don't have devices passing data back out onto the bus if they're not supposed to be doing so.

And also note that some chartplotters shared data via Ethernet, and that you COULD NOT connect more than one to the same N2K bus. The idea was the first MFD would handle picking up N2K data and re-transmit that to the others via Ethernet. This required that MFD to be running any time the others needed the N2K bus data (but this was usually the 'master' MFD for charts and such). And here again if you had other data on the Ethernet (AIS, for example) you'd have to make sure the MFD doing the bridging was configured to pass it properly.

 

[scurvydog_pnw]

This is were getting a N2K gateway really helps in the troubleshooting. I have both an Actisense and Maretron. I mainly use the Maretron with their PC software and it gives me a nice view of data on the network. You could actually see if your MFD were sending out either conflicting device ID OR if say you were getting multiple sources for PGN data.

You also may have more than one issue going on. Have you looked at the MFD themselves and see if you can see an option to select which GPS source to take? I have Raymarine system and you must set the device ID to be unique and tell the MFD which of multiple internal/external GPS to make primary. You could also have a bad cable/tee/switch that when you connect the 3rd MFD shortens the network. Did you make sure to include a terminator on each end of the N2K network? with some basic swapping of cables/parts you can rule out a bad component. After that I would either get a marine electronics person to come out OR buy a N2k gateway. Last do a google search but, someone made a N2K terminator last year that will report out bus power and other info.

Good luck.

 

[helm]

Thanks again for the help. I’m going to try a new drop cable. I did notice that the mfd 8208 on eBay has a bad nmea output so maybe not a rare problem with those devices. The system was very stable for years, and the problem occurred suddenly on power up. The rest of the nmea 2000 system works fine with the offending mfd 8208 off the the nmea network and we have three more displays and can mirror to a tablet for gps dependent data. The remaining unit is fine for depth, radar, video etc. I understand second hand that garmin won’t service the 8208’s so we can make the current system work
Eric