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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.
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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-1, NDC-2 & NDC-3)
can solve all these problems and more with their simple, easy and flexible
designs.
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Can I use an NMEA Autoswitch with my NMEA Data
Multiplexer?
Yes, is the simple answer. The Actisense™ NMEA
Autoswitch has been specifically designed to work together with the
NMEA Multiplexer / Combiner NDC-1
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.
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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.
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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-1 &
NDC-2 can
filter the incoming NMEA data to remove any sentences that are deemed unnecessary
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. If you do not
require the ability to filter out unwanted NMEA data, then please view
the Actisense NDC-3.
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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-1 &
NDC-2 ' 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-2 Control Centre
software.
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How do I give my GPS unit priority on the NMEA
Data Multiplexer network?
The Actisense NDC-1, NDC-2 & 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.
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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-1 & 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.
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All the NMEA input ports can have their Baud rates set to any standard
rate between 4800 and 38400 (for NMEA High Speed v3.0 support).
Please note that the Baud rate of port 1 is shared with the NMEA
data combined output, so the required output Baud rate must take precedence
over the input Baud rate of input 1.
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Simple Plug'n'Play installation and operation makes either of these options
a perfect solution.
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