NMEA 2000, smart mixing

While the extreme N2K cable mixing and daisy chaining shown yesterday did seem to work OK in lab conditions, here’s a smarter approach for a serious boat system (bigger here). The main change is that every cable section possible is NMEA “approved,” which means it’s built to the ODVA DeviceNet standard (that NMEA borrowed). While every N2K cable and connector seems vastly superior to the fine-gauge 0183 wires, crimp connectors, terminal blocks etc. they replace, DeviceNet seems the best of the lot—particularly in terms of interference protection—and I don’t see that it’s significantly more expensive or harder to work with (especially given the growing number of install doodads, even breakout boxes).

And if DeviceNet’s rigorous spec prevents a hard-to-run-down EMI problem sometime in a boat’s life, then it’s probably the least expensive cabling system. This stuff gets really techy, but here’s are some tidbits from a DeviceNet fan:

Do you know why there is 120 ohm resistor at the end of the NMEA 2000 trunk line? As the electrical wave propagates down the trunk line and comes to the end or terminator, the signal is absorbed into the resistor and does not reflect back down the line. This assumes that the characteristic impedance of the data pair is 120 ohms, otherwise there will be a reflection which causes ringing and the potential for an  indistinguishable signal. Many of these cheap imitation cables coming on the market don’t have a characteristic impedance of 120 ohms, thus the waveforms are dubious. Some of the cables don’t have a shield around the power and ground which allows noise to creep into circuits through conduction on the supply lines. Some don’t have shields around the data so they emit or pick up energy from switching noise on the power lines or from external sources. Granted, it all must be taken in context, a 22’ day sailor without single side band, radar and bundles of wire may never have a problem, however vessels with high power RF or bundles of wires are a whole different story.

It’s my understanding that Molex and Maretron have already gotten NMEA approval, and that three other companies—Belden, Turck and Lumberg—have DeviceNet cabling that could easily get approval. Now, if I were doing an all Raymarine or all Simrad boat, I wouldn’t hesitate to use their proprietary cabling. SimNet has been around for years now, and I haven’t heard of any problems, and SeaTalkNG does have its pluses. But personally I’d be unlikely to do an all anything system, and I’d just use those made up patch cables seen above or a breakout box to maximize DeviceNet. (As for smaller boats and systems, most anything seems to work, and, if there are problems, plug-n-play makes them easy to diagnose.)
   However, NMEA’s rule against daisy chaining devices along a drop line, as seen with the two SimNet IS20s above, may well be overly restrictive. I think all the devices that currently support chaining—like the Raymarine ST70, many SimNet products, and Furuno’s FI50 instruments—pass data through whether they’re on or off, or even broken. The rule may assure that any single device can be removed from a network without cutting off any other device, but the install efficiency of daisy chaining may trump that easily-solved issue. For instance, if you had to pop out one of the FI50s below, you can make the network whole again by just connecting the male and female DeviceNet cables you unscrew. I’m hoping to receive samples of this Furuno gear tomorrow, by the way, and we’ll be looking at how they compare to all the other N2K instruments. Good times!