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Why would I want or need an NMEA Buffer?
There are a number of reasons why an NMEA Buffer is very useful, or absolutely
necessary on any size of vessel:
A) Most NMEA instrument outputs are only capable of driving
one, or possibly two receive inputs of other instruments. If the output
NMEA data is required by more devices, then the NMEA data must be buffered before
splitting it to the individual instruments. If not, the NMEA data can become
corrupted as the signal quality degrades below the required level.
The Actisense NMEA Buffer module will perform this buffering task
simply and efficiently.
Whilst the 6 outputs of the NBF-2 are capable of driving up to
60 NMEA 0183 instruments, to maintain the galvanic isolation between each
of the NMEA devices, it is recommended to use only
one NMEA device per NMEA output.
If all the NMEA 0183 devices connected to a particular
NMEA output are
proven to have a correct NMEA 0183 opto-isolated input,
then it is possible to connect one NBF-2 output to multiple NMEA devices
and still maintain galvanic isolation. However, this
is the only condition where multiple NMEA 0183 devices per NBF-2 output
is recommended.
When using multiple NMEA devices per output, the exact
number possible will depend on the total cable length, cable quality, and
instrument inputs. Longer total cable lengths, poor cable resistance and
capacitance, and device inputs not meeting the NMEA 0183 specifications
(stipulating a maximum 2 mA drain) will reduce the maximum number of devices
drivable.
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B) Ground problems can arise when multiple devices are
wired together, causing interference and/or excessive current drain from
the vessel's batteries (which can result in damage).
To avoid this, each
of the NBF-2 NMEA outputs use the Actisense ISO-Drive design
that will isolate the NMEA device connected each of the NMEA outputs from
the NBF-2 ground, and from the NMEA 0183 device supplying the data.
This keeps all NMEA 0183 devices isolated from each other, and prevents
any ground loops from being made. Without ground loops, all the NMEA 0183
devices are safe from being damaged from ground loops currents.
This allows the installer to connect up the NMEA 0183 system without the
need to worry about what devices are true NMEA 0183 devices (with opto-isolated
inputs) and without worrying about resulting ground loops.
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How many NMEA devices can a single output drive?
Whilst the 6 outputs of the NBF-2 are capable of driving up to
60 NMEA 0183 instruments, to maintain the galvanic isolation between each
of the NMEA devices, it is recommended to use only one NMEA device
per NMEA output.
If all the NMEA 0183 devices connected to a particular NMEA output are proven to
have a correct NMEA 0183 opto-isolated input, then it is possible to connect one NBF-2 output
to multiple NMEA devices and still maintain galvanic isolation. However, this
is the only condition where multiple NMEA 0183 devices per NBF-2 output is recommended.
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When using multiple NMEA devices per output, the exact
number possible will depend on the total cable length, cable quality, and
instrument inputs. Longer total cable lengths, poor cable resistance and
capacitance, and device inputs not meeting the NMEA 0183 specifications
(stipulating a maximum 2 mA drain) will reduce the maximum number of devices
drivable.
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1 in & 6 out: How can there be 6 outputs when
there are only 5 grommets?
To make the Actisense NMEA Buffer module highly cost competitive, the module
case only has 5 'waterproof' grommets. What grommet is used for what purpose
is entirely flexible and up to the user, but it has been designed thus:
1 x battery power input
2-core cable (bottom grommet)
1 x NMEA data input
2-core twisted pair cable (top left grommet)
1-6 x NMEA data outputs
Utilise the 3 remaining grommets as required.
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When more than 3 outputs are required, multi 'twisted pair' cable is required
to share the 3 output grommets between the 6 outputs.
If it is more practical, any of the inputs or outputs could obviously be
combined together in a multi-core cable and share a cable grommet - the
module is totally flexible.
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