The National Marine Electronics Association (NMEA) 2000 Marine Network Standard is a big enabling factor driving the ongoing trend toward onboard networking of electronic systems; in fact it is the international standard for marine electronics interfacing adopted by the International Electrotechnical Commission (IEC 61162-3) and is the protocol of choice for AIS products. While NMEA 2000 has been around for a while, chances are good that it may be used on board your yacht right now or soon. If you’re not familiar with NMEA 2000, read on as we review some of its technical details.
NMEA 2000 is a single-bus architecture that uses differential signaling and three independent cable shields in an attempt to improve noise immunity in the nautical environment. NMEA 2000 is a true bi-directional network, unlike the previous NMEA 0183 which was just a single-talker, multiple-listener serial interface. The increased data throughput of 250 kilobits/sec makes it 50 times faster than the 0183 standard. This data rate also allows for multiple transmitters and multiple receivers that exchange eight-byte data messages. Error checking is also built in. This shared data integrates engine parameters, navigation information, AIS, electrical systems, tank level status, temperature, depth, speed, and heading just to mention a few and transmits this from the individual devices to the trunk or backbone via a drop line where the data is available to other devices on the network through their drop lines.
The simplicity of its topology makes it easy to install, and since it is a true plug-and-play network with no master mode or device addresses or polling rates to configure and since it supports hot swappable electronic devices, NMEA 2000 is very easy to troubleshoot and expand. One writer likens installing this network and adding new devices to working with tinker toys, how simple is that?
The basic components of the most complex NMEA 2000 network consists of the trunk line that is terminated at both ends with termination resistors, drop tees, drop lines, multi-port junction boxes and the individual electronic devices. The cabling consists of five conductors, two of which provide 12 volts DC power to the electronic devices, two which are the signaling conductors, and one known as the bare drain wire or (shield). The pinout for these cables is:
Pin #1 = Bare drain wire (shield)
Pin #2 = Red net-S power wire
Pin #3 = Black net-C ground wire
Pin #4 = White net-H signal wire
Pin #5 = Blue net-L signal wire
When checking power pins the voltage between pin #2 & 3 should read between 9.0 Vdc and 16.0 Vdc.
Because cabling problems are especially troubling for bus or linear topology networks such as NMEA 2000, the cabling should be checked first if any malfunctions start to manifest. This is because if there is a short or open in the trunk or drop lines it can affect the entire network, downgrading overall performance or even taking down the network. The problem, however, could be as simple as a missing/damaged terminator or loose/dirty connectors. If cleaning and reseating connectors or terminators cannot solve a problem, then by all means seek the professional help of an NMEA-certified electronics technician. Follow this link to find the closest CET to your homeport: http://www.nmea.org/content/nmeatrained/cmet.asp