Automatic data stream switching

This is a feature of Brookhouse AIS and AISC multiplexers. It is also described under "Baudrate Conversion", but the importance of the feature justifies a separate discussion.

In integrated systems which include a chartplotter,  computer or both, there are often NMEA listeners that are dependent on NMEA output of the chartplotter or computer. The best example is an auto pilot, connected to receive NMEA sentences for steering to a waypoint. There may also be DSC radio connected, for receiving lat/lon input,  or a repeater instrument.

If hardwired to the NMEA out port,  the chartplotter or computer  always has to be switched on to provide the NMEA output. This can be a problem, if circumstances require that  the vessel is navigated with minimum electronics, to save DC power or if the chartplotter or computer breaks down.

The DSC radio should receive lat/lon input at all times for safety. If the autopilot on a sailing vessel is used to steer in "wind mode", NMEA input from the wind instrument is required. Also, autopilots need speed input for the algorithms of all steering modes, including compass (auto) mode.   

If the dependent NMEA listeners are connected to the chartplotter or computer through a Brookhouse NMEA multiplexer model AIS or AISC, the automatic data stream switching capability can feed NMEA data from  different source if the chartplotter stops producing output.

Output from one or more NMEA talkers, connected to the multiplexer standard NMEA inputs can be copied to the multiplexer's 4800 NMEA output port to replace the data feed from the chartplotter or computer. This also applies to NMEA data from the Seatalk conversion, if the multiplexer has the Seatalk option installed.

Automatic data stream switching is activated by selecting option 6 (baudrate conversion option 2) from the multiplexer setup menu of models AIS and AISC. A channel block mask has to be specified. This allows selective inclusion of NMEA data.







Consider the following example:

A sailing vessel is has the following instruments and navigation equipment:

  • Seatalk ST60 instruments (wind, depth, speed/log)
  • Raystar Seatalk GPS
  • B&G auto pilot
  • Standard Horizon DSC VHF radio.
  • Raymarine C80 classic  chart plotter
  • Comar CSB200 AIS transponder
  • Brookhouse AISC NMEA multiplexer with Seatak option.


  • Seatalk > Multiplexer Seatalk bus connection.
  • DSC radio NMEA OUT > Multiplexer input port 2
  • AP NMEA out (heading) > Multiplexer input port 3
  • AIS transponder NMEA out > Multiplexer input port 4 (38400 bps)
  • C80  NMEA in < Multiplexer NMEA out 38400 bps.
  • C80  NMEA out > Multiplexer baudrate conversion input 38400 bps.
  • Auto pilot and DSC radio NMEA in < Multiplexer NMEA out 4800 bps

With the chart plotter switched on, the C80 receives DSC radio output, fluxgate compass heading, AIS data from the multiplexer and outputs NMEA GPS data (GLL, GGA, RMC) and NMEA wind data to feed auto pilot and DC radio.

Multiplexer baudrate conversion option 2 (menu option 6) has been selected in the setup menu with  channel block mask value E (blocks all except ch 1).

If the chartplotter is switched off, the multiplexer copies NMEA data from the Seatalk conversion directly  to the NMEA 4800 output port. This feeds the auto pilot with wind data (from ST60 wind instr.) for steering in wind mode, water speed (from ST60 speed/log) and lat/lon from Raystar GPS (RMC sentence) as input for  the DSC radio.