NMEA Multiplexer


Flexible multiplexing with preconfigured

versions for Autoswitching or AIS operation...




Why would I want or need a 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 NDC-4 can solve all these problems and more with its simple, easy and flexible designs.

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

Yes, is the simple answer. The Actisense NMEA Autoswitch (or an NDC-4 in 'Autoswitch mode') has been specifically designed to work together with the NMEA Multiplexer / Combiner NDC-4 (in 'Combine Mode') to create a complete NMEA system solution.

The normal NMEA system would have an Actisense NMEA Autoswitch connected to two (or more) GPS / Depth sounder units. The NMEA 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 Multiplexer's 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 can the NMEA Data Multiplexer reduce the congestion on my NMEA network?

Some NMEA instruments output a large number of NMEA sentences every period (usually 1-3 seconds), some of which are of no use to the average boat owner, and just clog up the NMEA network. GPS units are especially guilty of this and not all allow the user to reduce the number of unwanted sentences that are transmitted. Of those that do, the procedure can often be fiddly and time consuming.

The Actisense NDC-4 can filter the incoming NMEA data to remove any sentences that are deemed unnecessary or problematic by the user. Each input port has its own 'Inclusion List' which it uses to stop any NMEA sentences that are not on the list from appearing on the combined NMEA output port.

For example, most GPS units automatically output the "GSV" sentences (normally 2 but could be as many as 4), which give details of all the satellites that are currently in view. All of this information is of no use to 99% of users, but this alone can use up to 5% of the total available NMEA bandwidth/capacity.

The Actisense NDC-4 'Inclusion Lists' default is to allow all NMEA 0183 version 3.xx sentences through. The user can modify these lists using a single "tick the box" panel within the easy to use NDC Control Centre software.

How do I give my GPS unit priority on the NMEA Data Multiplexer network?

The Actisense NDC-4 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).

The Actisense NDC-4 'Port Priority' defaults follow the logical port numbering, i.e. port 1 is the highest priority and port 4 the lowest. These priorities can be re-configured as required by using the NDC Control Centre PC software - please view the downloads page to get your copy of the NDC Control Centre.

My GPS unit can output its data at a baud rate of 9600bps. How can do I configure the NDC to accept this baud rate?

By default, the Actisense NDC-4 uses the NMEA 0183 default communication speed of 4800 Baud (bits per second) for all of its NMEA input ports.

However, if the users GPS unit allows the data to be output at 9600 Baud / BPS, then the Actisense NDC-4 can be configured (using the Control Centre PC software) to accept this as an input. This will reduce the time taken for the GPS unit to transfer the data to the Actisense NDC and so allow more time to handle the data.

All the NMEA input ports (except for input 4) can have their Baud rates set to any standard rate between 4800 and 38400 (for NMEA high-speed v3.0 support). They also offer the option to set the Baud rate to 57600 - allowing for future support of very high speed AIS transponders.

Please note that the Baud rate of opto input 1 is shared with the 'NMEA Data Combined' output (ISO-Drive output 1), so the required output Baud rate must be considered when changing input Baud rate of input 1.

I do not have a serial port on my laptop computer. How can I connect the NMEA Data Multiplexer?

If you have already purchased an NDC-4, Actisense offers a 'USB kit' that can convert the standard NDC-4 into an NDC-4-USB. Please contact Actisense or your nearest distributor for pricing and availability of the 'USB kit'.

Alternatively, If you have not already purchased an NDC, the new NDC-4-USB has a USB port built in for easy connection to a PC.

Simple Plug'n'Play installation and operation makes either of these options a perfect solution.


end faq

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