NMEA Buffer

The NBF-1 is no longer in production. Please see the current NBF-3 model.





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.

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.

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.

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.

1 in and 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

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.


end faq

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