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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.
The '1 in, 6 out' buffer version is typically capable of driving up to
36 instruments - 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. To avoid this, the NMEA output from the supplying
instrument must be isolated from that of the receiving
instrument inputs.
The Actisense NMEA Buffer module will isolate the receving device from
all the output devices using full galvanic Opto-isolation circuitry.
As all NMEA instruments are required by the NMEA 0183 specification to
have Opto-isolated inputs, the Actisense NMEA Buffer outputs are not required
to be Opto-isolated to completely remove the common ground issues.
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How many NMEA devices can a single output drive?
The exact number of NMEA 0183 devices / instruments that the NMEA Buffer
module is capable of driving will depend on the cable length to all the
devices supplied, the quality of the cable used, and the device input circuitry.
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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An RS422 output is typically capable of driving up to
8 devices / instruments, and even more under ideal conditions.
An RS232 output can typically drive 2 devices (limited
by the RS232 specification).
This gives a total device drive count of typically 36, and even more under
ideal conditions.
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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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The 3 output grommets can be used directly with any mixture of 3 outputs
(RS422 or RS232).
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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Why are 2 outputs RS232 instead of the normal NMEA
RS422?
There are two good reasons why the Actisense NMEA Buffer module has two
RS232 outputs available to the user:
A) Version 1 of the NMEA 0183 specification used RS232
signal levels, so these outputs are ideal for converting new NMEA 0183
data signal levels to those required by older instruments.
B) Computer communication ports are RS232 based, so these
outputs will convert from the NMEA specification to that required by a
computer - allowing PC programs access to the NMEA data stream.
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These RS232 signal levels are importantly still compatible with
any NMEA 0183 input, so these outputs can also be used to supply
NMEA 0183 data to any NMEA device.
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