NMEA 2000 Network Bus.Boat Electrics


by Gavin Sorrell*


Instruments – Analogue or Digital?

The advent of digital techniques has revolutionised the way we create our monitoring systems.
The question now is whether to stick with the tried and trusted single function gauges or monitors, change to an integrated digital system or maybe even a combination of both.
Monitoring engine functions and measuring environmental parameters such as wind speed and direction, depth and chart positioning is where digital techniques are beginning to have a profound influence.
I don’t mean that readouts should necessarily be digital. In fact, analogue displays may be preferable.
Very often you do not need an absolute value of any function, but it is more important to be able to ascertain at a glance whether the function is operating within its normal range, with perhaps alarms to indicate critical levels of temperature, oil pressure, depth and other important parameters.
The major difference between analogue and digital technology is in the method of transferring data from sensors to readouts and the speed of data communication between devices.
An analogue meter has a discrete connection back to its appropriate sensor and usually has no connectivity with other monitors.
The latest digital networks rely on a common backbone cable to which spur cables are connected to individual items.
These items may include: wind speed and direction, depth sounders, compass, chart plotters and GPS, VHF radios with DSC, AIS receivers, tank level monitors and engine monitoring. Basically, all these items are able to talk to each other in a digital language known as NMEA 2000.
NMEA 2000 is a data specification for communication between marine electronic items. Its origin is the National Marine Electronics Association. NMEA 2000 supersedes a protocol called NMEA 0183 which was based on a different language format.
Both systems are currently available. NMEA 0183 was originally designed to be a one way system. For example: a GPS sensor transmitting to an LCD display.
However, it is possible to wire up a two way system such as a VHF DSC radio to a chart plotter using two separate input and output cables enabling both devices to talk to each other with a four way connector interface.
If you have electronics based on the 0183 format, they won’t talk on the 2000 bus structure, unless you install a ‘black box’ interface. Similarly, an adapter is required to enable an NMEA 2000 system to talk to a PC.
To further complicate the compatibility issue, some companies have their own proprietary networks such as Raymarine’s SeaTalk. So you are probably already thinking that if network compatibility is an issue with these digital systems, you may as well stick with the traditional analogue technology although there are interfaces available to enable different protocols or digital languages to communicate with each other.
NMEA 2000 is a much faster (up to 20 times) protocol than NMEA 0183, therefore can support more sophisticated electronic talking and listening devices. It also has standardised connectivity using a single backbone cable.
The NMEA 2000 communication hardware consists of a single cable bus which runs fore and aft along the vessel and can be up to 200 metres long. The bus must be terminated at both ends with 120 Ohm resistors.
Connection to each product is typically via a tee connector and a spur cable of not more than six metres length. It is essential to run a correctly terminated and balanced network.
It is worth remembering that the integrity of the system depends on a single backbone. If that goes out, then the whole system can go down. For this reason, the components specified are of a very high quality with a very high ingress protection rating, so that failures are rare.
There is no doubt that digital systems are the future of instrumentation development. There is an elegant simplicity in a system where data from many different sources can be fed into one multifunction display.

Next Month: Multifunction Displays.

*Gavin Sorrell works in collaboration with Aquavolt Electric Boat Parts.
Tel: 02 9417 8455 www.aquavolt.com.au