NMEA 2000 Network Not Making Sense - Working Without Terminators

[TCL]

Bottom line question: My NMEA 2000 network has been working. Past couple of days, I took the time to map out the network and discovered it has no terminators. I traced everything and didn't find any in-line terminators either. My understanding is this is not supposed to work properly. But everything appears to be working (except as noted below). I've attached a diagram. I'm baffled how this has been working if, as I've read everywhere, terminators are required.

Details:
The network was partly installed by professionals and partly by me. As noted above, it has been working for a long time. Last year, I had to replace the old B&G mast-top wind transducer because of corrosion at the connection up there; I did so with a new B&G WS310, and it has been working. A couple of days ago, sitting in the harbor in high winds, I turned on the navigation system to get a wind reading, and it worked for a while then it stopped. At the main display, it shows the WS310 but no data. Importantly, everything else is working--depth, compass, etc. There are no errors showing up in the system. I'm emphasizing that because of my primary puzzlement over how it is working without terminators.

As I began trying to troubleshoot the wind 'ducer problem by swapping out Ts or drop cables, I noticed a drop cable with an open end (just dust cover). Again, this has been in place for a while. I researched whether that could somehow be a problem (I now know it is not a problem to have a "blank" spot, and often desirable to have one for diagnostics.) As a result, I learned that all networks are supposed to have two terminators, one at either end. I didn't see any, so I mapped out the whole network. It has none. My understanding is that a device or display is not going to function as a terminator.

Anybody have any thought on how it could be that the system has been working -- is working, except for the wind 'ducer (which could be the instrument itself; I haven't gone up the mast yet to check) -- without the terminators? I'm baffled and any input would be welcome.

Thanks,
TCL

[SailingHarmonie]

The CAN buss is actually pretty robust. Terminators are not usually required for it to work, but they are certainly required for it to work reliably. Unconnected drop cables are not a problem, and will cause no issues. Non-standard topologies can sometimes work (sort of), some time not at all, and sometimes work sometimes.

That said, if drawn accurately, the topography of your system is hopelessly messed up. You either have a lot of reading and studying to do, or you hire someone who actually KNOWS what they are doing to straighten it out. It's either a lot of work, or a lot of money.

Last time I sorted out a client's system, which like yours was partly "professional" and partly homebrew, it took 3 full days.

[jhall767]

Many mast top nmea 2000 sensors have built in terminators. You should check the owners manual of the WS310. Also you can check the operation of a CAN bus with a volt/ohm meter. Functionally it is the same bus as in a car or a truck. NMEA2000 essentially defines the language they speak but electrically they are the same.

Terminators are supposed to be at the ends for best reliablity - but they will work if placed in the middle

Plenty of videos on line that will get you started.

And yes a lot of time will be required.

[smac999]

generally it will work with 1 or 3. but not with 0. some wind devices have them built in. so do the old simnet power cables.

shut power off to the network, and take an ohm reading across the 2 can cables. then you'll know for sure. the easist way is with a spare drop cable and just cut then end off so you can access bare wires to measure. otherwise you'll need the connector pin out and some papter clips or something to fit in the holes.

your drawing makes no sense. you should have a single backbone. with single drops to each unit.

[stevead]

Your current network is totally wrong.

Suggest you have a read of some of the manuals produced by the vendors.

I was doing some drawings for others, so here's a suggestion for you.

A couple of points.
Check whether your wind transducer includes the terminator and is wired as part of the backbone. Most do because the length of the cable exceeds the maximum length of a drop cable. Other may be analog, with an interface box at the base of the mast.

Check both the voltage and resistance on your network. It should measure 60 ohms (two 120 ohm terminators) resistance across the data pairs; CAN-H (white) and CAN-L (blue) and between 9 to 14 volts across the power pairs (Red, Black)

For dense cable areas (eg. multiple displays in the cockpit), consider using a multidrop. Actisense make one, as do Maretron

[wholybee]

Yikes. If drawn accurately, well I don't want to just repeat what others have said.

One check to make, is to measure resistance on the backbone, between pins 4 and 5 (NET-H and NET-L). If there are 2 terminators, it will read 60ohms. One terminator will read 120ohms. No terminators will read very high, many k ohms. This test won't check that they are in the correct place, only that they are there. So check to see if you have 60 ohms. You can check this at any tee connector.

Without terminators, what actually happens is that the signal hits the end of the backbone (where a terminator should be) and is reflected back and interferes with the next signal on the line. Obviously, as in your case, the network might still work, but with much less reliability, and likely with many unseen errors.

Now, looking closer, it *might* not be that bad. What I see in your drawing is one main backbone (that you labeled "aft", "nav station", "backbone", and "saloon sole" . At one end of this is the GPS antenna, and the other end the wind instrument. The wind instrument probably has a terminator in it. The GPS might. If it does, then your backbone is terminated correctly. The issue then is the 2 spurs with multiple devices on it. It is bad practice to have multiple devices on one spur, but it isn't electrically wrong. It will still work just fine as long as the spur cable is fairly short.