NMEA Multiplexer

The NDC-3 is no longer in production. Please see the current NDC-4 model.





Why would I want or need an NMEA Data Multiplexer?

The NMEA 0183 standard allows any suitably designed marine electronic device to share its gathered information with any other device on a vessel. Unfortunately, there is one very large drawback with this standard - only one device on a connected network can actually send data (a single 'talker' device), with multiple devices (determined by the current limit of the sending unit) listening to that data (multiple listeners).

If the vessel owner has an instrument that ideally requires the data output of two or more devices, for example a chart plotter, then the owner has no alternative but to settle on connecting only the most important device, usually the GPS.

What happens if the owner prefers the vessels gyro compass output to that of the GPS, or requires that the current depth be displayed on the plotted chart to help avoid the possible case of running the vessel aground on a shifting sand bank? The NMEA 0183 standard cannot supply an answer to those questions: "one transmitting / talking device only".

The Actisense NMEA Data Multiplexer's NDC-2 and NDC-3 can solve all these problems and more with their simple, easy and flexible designs.


Can I use an NMEA Autoswitch with an NMEA Data Multiplexer?

Yes, is the simple answer. The Actisense NMEA Autoswitch has been specifically designed to work together with the NMEA Multiplexer / Combiner to create a complete NMEA system solution.

The normal NMEA system would have an Actisense Autoswitch connected to two (or more) GPS / Depth sounder units. The Autoswitch adds a special NMEA tag to the data that passes through it to help identify which of these GPS / Depth sounder units supplied the NMEA data.

The autoswitched NMEA output is connected to one of the NMEA Data Multiplexers inputs. In this way the highest priority device's NMEA data is autoswitched into the complete NMEA system and the actual device supplying the NMEA  data can be determined (and displayed if required and possible) to help the user.

How do I give my GPS unit priority on the NMEA data Mulitplexer network?

The Actisense  NDC-2 and NDC-3 have many useful features, among which is the ability to prioritise their input ports. The usefulness of this feature becomes apparent when the NMEA network is running over 50% loading/capacity (most setups). At this point a standard NMEA Multiplexer could start to delay the important information by an unacceptable amount and may even lose it altogether.

The Actisense NDC however, will always transfer the highest priority input's NMEA sentences first, keeping the delay to the minimum possible. The remainder of the inputs will have their NMEA sentences transferred in turn, until all data is either transferred or no more time / bandwidth is available in the NMEA output.

For example, the most relied upon unit e.g. GPS, should be attached to input port 1 (the highest priority). It is possible to change the port priority order from within the Control Centre software if you do not want to manually reconnect the cables to the Actisense NDC. However, it may be simpler to the keep the port priority in a logical order (1 highest - 4 lowest).

The Actisense NDC-2 'Port Priority' default is the same as the port number, i.e. port 1 is the highest priority and port 4 the lowest.

end faq

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