PS Advisor July 1, 2003 Issue

PS Advisor: 07/01/03

Data, Displays, and the NMEA
I currently own navigation equipment from three different companies, primarily because I felt they were the best value or provided functions unavailable by purchasing these items from the same company.  I now have the problem of where to mount these devices—below, or at the helm.  I would actually like them mounted below for security reasons as well as keeping them out of the elements when not in use.  Is there a way to network these displays together to one slave display so that I can toggle from one to the other? They all have NMEA output.  I have seen repeaters that give lat/long information, but not for the chartplotter display or sonar display, which is what I would really want at the helm.

-Ray Smith
Via e-mail 

We know what you mean—it would be great to be able to feed information from all your instruments through a single display screen, either mounted on the pedestal or on a cockpit bulkhead where you could reach over and toggle through the displays from the helm. It would also be nice to be able to control individual instrument functions from that display.

Without knowing specifically what you have aboard, we can't be too specific ourselves, but for boats with instruments installed more than about two years ago, this scenario will be difficult to achieve with any off-the-shelf, retrofit equipment.

We're going to assume that your NMEA output is the 0183 version. As many readers know, the NMEA 0183 standard (from the National Marine Electronics Association) defines an electrical language and data protocol for communications between marine instrumentation; between data sources ("talkers") and repeaters ("listeners").

NMEA 0183 data is transmitted at 4800 baud, in the form of "sentences." Examples of theses sentences are: DBT - Depth below transducer, GLL - Geographic position, Latitude and Longitude, HDM - Heading, Magnetic, and MTW - Water temperature, Celsius, to name a few. This language standard does allow a single "talker" and several "listeners" on one circuit. If you're wiring components yourself, the recommended interconnect wiring is a shielded twisted pair, with the shield grounded only at the talker. After you make the wire hookups you need to make sure your data source has its data port turned on and that your repeater is capable of receiving the data you want displayed.

Again, with instruments from several manufacturers, communicating by NMEA 0183 protocol, we know of no store-bought way to "slave" a display from belowdecks or view and control things like sonar, chartplotter, and radar from one display at the helm. Features like this are currently reserved to the new integrated networking systems like Furuno's NavNet and the Raymarine hsb2 network. These systems do offer a much greater amount of data display control and systems function control. They allow your vessel to have sonar, radar, and chartplotter displays in multiple locations, and you can both view information and control functions from two or three systems on one display. It sounds like this is the kind of system you're looking for. Unfortunately, these are proprietary – you can't mix-and-match components from them with other systems.

See the Instrument Systems article in this issue for more on this topic.

When it comes time to install new electronics, especially an integrated system, you may want to contact a certified technical installer or a marine electronics technical dealer for assistance. They can assist with layout, equipment training, and troubleshooting.

By the way, many people pronounce NMEA as "neema." This has caused a lot of confusion, because there are standards set by NEMA, the National Electrical Manufacturers Association, that have nothing to do with marine electronics. Nevertheless, you'll often read and hear the phrase "NEMA 0183." There's no such thing.

The NMEA has put forth a new protocol, NMEA 2000. The standard has been designed to the International Standards Organization Open Systems Interconnect (ISO/OSI) model. In this new communication protocol or language, data is structured and transmitted in much the same way as it is in a computer network you might have at work or home.

The NMEA 2000 standard is currently undergoing beta tests, and was submitted to marine electronics industry leaders last year. Eventually, the NMEA 2000 standards will revolutionize the way systems share and control information on board. The data will flow through a four-conductor shielded cable, as apposed to the NMEA 0183 data on a two-conductor wire. This will allow systems to exchange data very quickly. The intention of the new standard is to offer plug and play compatibility between all electronic systems on board.

We'll discuss NMEA 2000 more in upcoming issues.

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