Basic NMEA 2000 network on board

Back in the day, each electronics unit aboard your boat did what it did, and never the twain did meet. Your depthsounder told you the depth, your radar showed what was around, your GPS told where you were, and so on. I remember my first onboard interface: my new radar accepted input from my GPS, so nav data showed up on my radar screen. Neat! It was like having a second GPS display, plus my waypoint appeared on my radar screen and confirmed I wasn’t headed for the rocks.

Today, of course, most electronics can be connected to onboard networks. The latest data protocol, NMEA 2000 (promulgated by the National Marine Electronics Association), is touted as the Holy Grail of onboard network protocols, and supersedes its predecessor, NMEA 0183. NMEA 2000 is 26 times faster than NMEA 0183, with a throughput speed of 250 kilobits per second.

This chartplotter has interface connections for both NMEA 2000 and NMEA 0183  NMEA 2000 isn’t just a data network, but a 12-volt power network as well. Each device tees into a “backbone” via pre-made “drop cables” that have strong quick-connect waterproof connectors, and these cables carry both power and data. The 12-volt connection to ship’s power is just another drop cable, usually yellow instead of black. The backbone must have a termination resistor to mark each end of the network, like “End of Road” signs.

Ultimately, NMEA 2000 can be used to control and monitor engines, to interface with various computers, and to monitor onboard fire and electrical systems, but for now most of us just want to use it to interconnect our navigation electronics. To add a new device to an NMEA 2000 network, say a black-box AIS transceiver or a new depthsounder transducer, the electrical/data part of the installation should take about seven seconds—it’s literally “plug and play”—and the rest of the network will recognize the device immediately. Or perhaps you want to set up a second nav station, with all your data displayed in a second location—how many cables will you need to run? Answer: just one.

With old NMEA 0183 networks, on the other hand, data connections are routinely made with several teeny-tiny, often untinned wires, which have a short lifespan in the marine environment. I’ve always thought it dodgy to have such tiny wires leading into connections, often in damp places where they are subject to jostling and vibration. In just physical terms, NMEA 2000 is finally a seamanlike solution to onboard data cabling.

Flies in the Ointment

A bundle of NMEA 2000 cabling at the base of a mast. Connections are plug-and-play  Unfortunately, we are now in transition, and NMEA 2000 is fraught with controversy. Its name implies that the standard was adopted in the year 2000, but it wasn’t until years after that NMEA 2000 Certified electronics first hit the market, and they’re still slow in coming. Many brand-new devices still only speak NMEA 0183, and some only speak proprietary languages created by their manufacturers. To cover all the bases, some products now have both NMEA 2000 and NMEA 0183 interfaces.

For a device to be NMEA 2000 Certified, the manufacturer must purchase the standard and license each individual product. The cost is negligible, but there have been some obstacles in bringing certified products to market. These problems don’t have to do with the NMEA 2000 standard itself, but are instead due to marketing, competition and vested interests in existing technology.

Still, NMEA 2000 may not be all it’s cracked up to be. One experienced electronics installer (who prefers to remain anonymous lest the NMEA send its goons after him) lamented to me, “It’s still a data network. How many times have you connected to a data network and had it work the first time? It’s buggy and complicated. My partners and I miss good old NMEA 0183, where you could just connect everything up to a single bus and be done with it.”

He went on to show me drawer after drawer of NMEA 2000 parts and complained that what was supposed to be so simple really involves lots of bits and bobs—bits and bobs that vary slightly from one manufacturer to another.

“And it’s expensive,” he added. “Here’s a T-fitting for $14 and an end fitting for $7.50. Just to connect a GPS to an antenna you’d need two of each, plus the drop cables and backbone. That’s probably close to a hundred bucks just in cabling. With NMEA 0183 you can set up a whole data network for a couple bucks in wire.”

Still, what new technology didn’t have some complaints and issues to resolve? Progress is progress, and NMEA 2000 will undoubtedly be the marine data network we use for the next few decades. But if you’re like me, you’re not about to toss out all your electronics just because they aren’t cutting edge. All these devices are still useful and still have a place aboard our boats.

An Actisense “gateway” for converting 0183 data to NMEA 2000  The question then becomes how to connect various devices together if they don’t speak the same language. If you’ve got a mix of equipment on your boat, you can purchase what are variously called adapters, bridges or gateways to connect NMEA 2000 devices to NMEA 0183 devices. These adapters, produced by manufacturers like Simrad, Actisense and Maretron, translate NMEA 0183 data into NMEA 2000 data so the two networks can talk to each other. They cost between $90 and $150. Whether or not this seems expensive depends on circumstances. One sailor I know replaced all his old malfunctioning electronics with new NMEA 2000 Certified stuff, but had a $4,000 autopilot that only spoke NMEA 0183 and still worked flawlessly. Buying an adapter was an obvious solution and a small price to pay to keep from replacing a reliable autopilot.

In some cases a device that has interfaces for both NMEA 2000 and NMEA 0183 can act as an adapter of sorts. Another acquaintance of mine has an NMEA 2000 network throughout his boat, but also has a black box AIS transceiver that only speaks NMEA 0183. His chartplotter has both interfaces on the back, so he plugs his AIS into the NMEA 0183 plug and the rest of the network into the NMEA 2000 plug, essentially running two “networks” without an adapter.

Reality Check

While NMEA 2000 is wonderful, and NMEA 2000 networks are very stable once installed, they imply interdependence among devices. In many systems I’ve installed, the whole system is based around a single display, which really cleans up a nav station. The various image overlays (chart, radar, depth, AIS, weather) provide a powerful cross-referencing tool for a navigator, but what happens if that single display gets smashed by a winch handle? If I were to take off cruising again, I’d want multiple displays, and I’d be sure I could connect each device, on its own, to a display.

Garmin’s NMEA 2000 “starter pack” with cables and connectors  In some cases it may be critical that devices exchange data, but in many cases it’s just a matter of convenience. There is something to be said for each device being able to stand on its own two feet. The curmudgeonly opinion (and this, strangely, comes more often from marine electronics experts and installers who’ve been around the block a few times) is that each device should stand alone, with its own display. If you lose your radar, you just lose your radar and still have everything else.

The reality is that for the foreseeable future most boats will carry a mish-mash of NMEA 2000, NMEA 0183, manufacturers’ proprietary networks (like Raymarine’s SeaTalk), various Ethernets, and the occasional old depthsounder that’s worked for 25 years and has never let you down. Compatibility will vary with product and manufacturer, but theoretically any device that has both interfaces (proprietary and NMEA 0183/2000) should translate data without any problems.

Some proprietary networks are essentially duplicates of NMEA 2000 (Navico/Simrad’s SimNet is one example), but some manufacturers’ NMEA-based networks don’t interface perfectly with straight-up NMEA equipment without some tricks and fixes. Where things really go haywire is with Ethernet connections, for which there is no standard. As a result, each manufacturer’s version of this technology really is proprietary and can’t talk to that of others. Unfortunately, Ethernet, or something like it, is a necessary evil, because it’s much faster than NMEA 2000 and is needed for transmitting graphic data like radar, weather maps and complex cartography.

When building a complex network your options are therefore somewhat limited, as it normally makes sense to build around your radar/display interface. You can bet on any of the newer radomes and displays in Navico’s suite of brands (Lowrance, B&G, Simrad) all talking to each other through Navico’s Ethernet, so you can safely mix and match equipment from these manufacturers. Otherwise it’s probably best to stick with the same radar and display manufacturer.

The bottom line is that manufacturers so far have only made half-hearted efforts at standardization and inter-operability. Compatibility between brands is getting better, but you still need to be careful when building a hybrid network. You can’t take compatibility for granted, and often you’ll need to determine whether (and how) one piece of equipment can talk to another on a case-by-case basis.

Images (top) by Clark Beek; (middle) courtesy of Actisense; (bottom) courtesy of Garmin
Illustration ® 2012 Mirto Art Studio