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NMEA 2000: Behind the scenes
4/29/2019

Most boaters have heard the term NMEA 2000, especially if you’ve been shopping for onboard electronics. To their credit, some can even tell you what it is and does. 

There’s little doubt that the NMEA 2000 network interface standard—often simply called N2K--has provided a slew of benefits for the operation and advancement of marine electronics. That didn’t happen overnight or without some thorny challenges that had to be overcome.

What follows is the first of several blogs we’ll run from time to time that will dig into various aspects of N2K—how it has played a central role in allowing your onboard electronics to talk to each other and also bumps along the way that had to be overcome. The article is excerpted from a more detailed report written by marine electronics technical dealer John Barry.

We begin with a brief history and general snapshot of the standard going back nearly two decades. We’ll follow up in future blogs with the rest of the story.

By John Barry

When the National Marine Electronics Association (NMEA) was formed in 1957, boats had a sparse set of marine electronics.  If you had a VHF radio and an RDF (Radio Direction Finder), this was a really complete setup.  As technology advanced and prices dropped though the 1960s and ‘70s, more and more devices found their way on to boats.  With all that equipment on board, the need for interfacing between systems grew. The NMEA’s first interfacing standard, NMEA 0180, was a simple serial interface between the Loran C and the autopilot.  

In the ‘90s, technology exploded and a more complete interfacing standard--NMEA 0183--was introduced. As interfacing became a part of every marine electronics installation, it became clear that a faster, better solution was needed.

Development during the early days of the NMEA 2000 standard was done by volunteers from industry, government and academia. Since NMEA 2000 is based on CAN bus--a standard that already existed--development of components like the "physical” and much of the "application layers” came together relatively quickly.  Development of the PGNs--Parameter Group Numbers--library took time and testing to complete. (PGNs essentially contain information that allow devices to operate and communicate on the network.) During this stage of NMEA 2000 history, some of the standard protocols for data handling and data limitations were defined.  

Each device on the network must be configured manually for the dedicated network.  What the volunteers did in the ‘90s was to make NMEA 2000 use what is called the "Address Claim Process” to automatically assign unique identifiers to all devices, allowing for network changes to be automatically recognized.  Making the CAN bus network configure itself automatically was just the medicine that the marine marketplace needed. NMEA 2000 officially launched in October 2001.

Acceptance

Aboard commercial vessels, use of NMEA 2000 for the purpose of interfacing marine electronics seems inevitable. In fact there are currently a few devices on the market that utilize NMEA 2000, mostly involving Electronic Chart Display and Information Systems—commonly known as ECDIS. Since redundancy is easily achieved, I believe that NMEA 2000 is ideal for wheelhouse data distribution on those vessels.  Today, commercial vessels use 0183 extensively as well as Ethernet.
 
The physical connectors used for NMEA 2000 are IP67 rated (waterproofing) and extremely robust.  Field-installable connectors allow flexibility and are also IP rated, if properly assembled. Inter-device compatibility is greatly enhanced by a single physical connector standard being adopted by major manufacturers.

The early connectors were compliant with their pin out configurations, but the alignment key for the connector was not standardized.  This caused some "T” connectors along the NMEA 2000 cable backbone to align differently when plugged together.  Although the marketplace is resolving this issue, it does persist today.  There were also several specialized connectors deployed by manufacturers that also still exist in the marketplace. 

It is worth mentioning that the "big four” manufacturers, Furuno, Garmin, Raymarine and Navico, have cooperated in an unprecedented manner.  "Coopetition” is the catch phrase for this type of synergy, enhancing the overall marketplace in a "grow the pie” approach.  Facilitated by companies such as Actisense, Airmar and Maretron, among others, adoption has been widespread.  From the days of academia to a future full of promise, NMEA 2000 stands as an industry accomplishment unrivaled by others.

Growth

As manufacturers increased their offerings, and individual installers were trained to understand the NMEA 2000 network installation parameters, life got much better for marine electronics installers.  In the 2000s, getting data to the VHF and autopilot was still primarily done using NMEA 0183 serial data, but instruments and sensors quickly moved to NMEA 2000.

Marine manufacturers really came through with NMEA 2000-certified products hitting the market regularly.  Recent NMEA 2000 interfaces in AIS (Automatic Identification System) and VHF equipment have made life even easier.  Acceptance by the industry has produced benefits beyond just easy interfacing.  The whole market has been enhanced and enlarged by the introduction of numerous devices with NMEA 2000 interfaces.

 "NMEA 2000 has grown far larger than the forefathers and early adopters ever envisioned back in the early 2000s,” said NMEA President & Executive Director Mark Reedenauer. "At that time, N2K was a means to plug and play GPS, waypoint & route, and depth data—the simple stuff. Now basically the entire vessel can utilize the NMEA 2000 network to monitor and control systems from the MFD, which is becoming the onboard command center. Water makers, lights, thrusters, windlasses, stereos and breakers are just some of the capabilities that continue to expand. We commend the industry for believing in this standard and thank manufacturers for coming to NMEA with proprietary messages that they want to make ‘open’ for the industry to utilize.”

One major benefit of NMEA 2000 is cost savings.  Many devices hook up with a single cable connection to the backbone.  Smart sensors reduce the devices to single boxes. A depth sounder and a transducer or a GPS antenna and receiver can now be a single device.  Sensors have combined these functions into smart sensors that output the measured result as an NMEA 2000 PGN.  Benefits extend beyond marine electronics manufacturers to the installers who benefit from reduced complexity, more capabilities and lower cost to end users. 

NMEA 2000 has been a boon to the recreational marine electronics industry.  Several NMEA member companies have business plans built around the NMEA 2000 ecosystem.   Cross manufacturer interoperability has allowed aftermarket companies to thrive and allowed system manufacturers to focus on their specialties.  Gateways have allowed for complex systems, like outboard motors or breaker panels, to be connected to the backbone.  With a more and more complex set of data, NMEA 2000 is evolving to accommodate today’s complex needs.

Overcoming problems

The inevitable result of a reliable network is that it will be made more complex until it breaks.  NMEA 2000 has physical limitations such as a maximum of 50 physical devices and a backbone length of 100 meters (lite cable), etc.—limitations that should not be violated.  As networks get more complex, bandwidth limitations, physical lengths and connections become more critical and more likely to exhibit a symptom.

The standards for NMEA 2000 are written to provide safety and reliability.  This means that a 250 kilobytes per second (Kbps) bandwidth must be maintained throughout the network.  Networks that use less bandwidth may appear to work in a substandard installation, but problems can show up at the worst possible moment.  This is why the physical installation—the basics—will always be critical.

Proper installs are key

Problems with NMEA 2000 are usually caused by substandard installations.  Poorly installed field connectors or using terminal strips without shielding are common installation errors.  Occasionally the physical limits of 78 meters of combined drop length or 100 meter backbone length may be approached.  These problems can be overcome by system design, etc.  Rarely is there an actual network conflict.  The capabilities of NMEA 2000 are proving to be more than ample to handle data sharing on boats. 

Data conflicts on NMEA 2000 networks are rare.  When different manufacturers design their equipment, they follow a standardized protocol to produce a compliant device.  The parameters set forth by the NMEA 2000 standard are versatile.  Some manufacturers use different parts of the standard to enable features and functions within their operating system.  Getting the data from a specific sensor to be displayed on a certain screen can be challenging.  This process is known as data mapping.  Differences in this process, such as using labels or instancing to map the data, may make it difficult or impossible to configure a device or map its data without special tools.  Typically this is simple within a manufacturer’s products, but across brands devices may not be configurable.  

Next time we’ll drill deeper into the benefits, challenges, solutions—and future—of NMEA 2000.

About the author
John Barry owns and operates Technical Marine Support Inc. in Pleasant Prairie, WI. He is a CMET and holds a GMDSS Radio Operator/Maintainer license along with an ABYC electrical certification. Barry is an NMEA Director and instructs Marine Electronics Installer and NMEA 2000 training courses.



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