Many of us take for granted that certain data is shared among onboard electronics components. To facilitate data sharing, more than 20 years ago the National Marine Electronics Association established a data-sharing standard called NMEA 0180, based on systems used in the automobile industry. This technology evolved to NMEA 0183, from version 1.0 all the way through the most recent 3.01. However, we’ll use the industry standard version 2.0 for comparison.
NMEA 0183-HS (high-speed) version 1.0 was released several years ago to solve the issue of increasing speed and complexity in marine electronics components. While its 38,400-baud capability is a significant improvement over 4,800 baud for v.2.0, NMEA 0183’s short, structured data sentences and limitation of a single talking component continue as major obstacles to enhancing functionality of a marine electronics network. HS v.1.0 has not been widely adopted by industry, and the prevailing v.2.0 standard remains too slow and inflexible for today’s growing need to share data among devices.
With the proliferation of marine electronics capable of sharing and using far more data than NMEA 0183 could support, there was a need for a completely new protocol. As happens in any industry, it took time to arrive at a standard. In the meantime, most major manufacturers built their own proprietary networks that share information at high speed with devices designed for that specific network, and which also support the NMEA 0183 v.2.0 standard.
Simple devices pass small bursts of data using various NMEA protocols. Components passing larger quantities of data use established SeaTalk/HSB/HSB2 (RayTech), NavNet (Furuno), SimNet (Simrad) and other systems. Finally, some marine electronics systems, like Nobeltec, run on conventional personal computers. In general, one manufacturer’s system can exchange data only with that manufacturer’s systems designed for that specific data protocol. The NMEA standards allow data to be shared among different brands of gear.
Single talker, multi-listener
Well aware of the need for something beyond NMEA 0183, the NMEA set forth in the late 1990s to design a new system for sharing data. In order to appreciate the quantum leap forward that NMEA 2000 represents, let’s examine NMEA 0183 in more detail. NMEA 0183, its predecessors and revised editions are all single-talker, multi-listener networks, where one instrument places data on the network and numerous other instruments access the data. NMEA 0183 data is sent in specially formatted ASCII data sentences. Since your personal computer (PC) “speaks” ASCII, you can easily monitor and troubleshoot the NMEA 0183 system.
Seeing the data stream
Connect your PC to the NMEA 0183 network, and when there is activity on the network you can see it on the computer screen, decipher the data sentences, and troubleshoot any problems using your PC’s HyperTerminal program (Start/Programs/Accessories/Communications/HyperTerminal). If you don’t see a data stream, you may need to make a new connection and/or modify the port settings (File/Properties/Connect To/Configure/General and also Advanced Settings). There are several good software programs that make handling NMEA 0183 data on your PC easier. Unlike 0183, however, your PC will not understand CAN-based NMEA 2000 data on its own.
NMEA 0183 can share information — let’s say a specific bit of GPS position data — among devices, but there can be only one GPS on the network. By comparison, NMEA 2000 can accommodate specific data exchanges between individual components, allowing you to address multiple GPS units separately, or share a single GPS antenna with two GPS units.
After much debate, and based on technology available at the time, the NMEA made the excellent choice of a Controller Area Network-based system for NMEA 2000. Chip-maker Intel and the German firm Bosch developed CAN about 20 years ago for automotive applications, and its use has spread. NMEA 2000 data is transferred at 250,000 baud, or 50 times faster than standard NMEA 0183.
CAN do
CAN has demonstrated an ability to handle modest parcels of data reliably and remain completely transparent to the consumer. CAN has very modest requirements for hardware, so network participants can be small, efficient and inexpensive.
NMEA 2000 uses a single trunk cable as a conduit for both low-amperage electrical power and data. Individual components connect to the trunk via a drop cable. NMEA 2000 allows as many as 50 devices to send data (talk), display data (listen) and receive instructions to take a certain action (listen to a message addressed specifically to that device). Components may join the trunk at any location, using standardized connectors and data-sharing protocols. Many manufacturers use their own connectors, so you may need adapters, and in some cases an interface box. NMEA 2000 cables/connectors, come in two diameters, based on the electrical needs of the network.
CAN allows for much more flexibility than NMEA 0183’s ASCII format, in defining how information is packaged and shared, including which critical devices have priority on the network. Each device is automatically self-monitoring; if a device determines it is causing a fault, it removes itself from the network.
Of course, if something goes wrong with the self-maintenance of the system, it is possible for one piece of gear to disable the network completely. Though this is rare, it can be much harder to find the offending device for several reasons. Troubleshooting requires a purpose-built electronic unit and skilled operator.
Data compatibility
NMEA 2000 data sentences are totally different and are not compatible with NMEA 0183. Luckily, many manufacturers designed their NMEA 2000 gear to at least understand NMEA 0183 traffic, so you do not have to replace every NMEA 0183 component with an NMEA 2000 unit.
In some cases you may need a smart box running software to convert data back and forth between NMEA 0183 and NMEA 2000, and manage the NMEA 0183 network so it does not become overloaded. The NDC-2-A (NMEA Data Combiner) by Actisense is one example.
Chris Parker
