How to completely integrate an hybrid NMEA System

Faced with a large array of electronics, a yacht owner needs to interconnect all the units to each other. New, networked equipment still connects to NMEA 0183 outputs, and so do today's VHF radios, etc. I was faced with this problem on Frog's Leap, basically having to interface everything to everything.

While the

  • PC with Maxsea Time Zero Software capable of controlling the Autopilot
  • Garmin Plotter and repeater unit capable of controlling Autopilot
  • Raymarine Instruments integrated on proprietary bus with SeatalkTM
  • DSC in/out on VHF radio
  • DSC in on SSB radio and Pactor Modem
  • PC interface to the Garmin Plotter capable of various functions
  • AIS receiver
  • Spare inputs for PC to receive other sensor information (e.g. fuel, fuel flow, etc as needed in the future)

    The Integration of all the equipment was possible using a Brookhouse NMEA multiplexer with AIS input. The mux receives AIS and outputs it on a NMEA output interface at 38.4 kb/s. The mux receives all the Raymarine instruments data and combines these sentences with those received from other devices.

    I added a spare GPS with a toggle switch control In case of failures of the primary GPS, or the mux, the GPS outputs still connect directly to the plotter and the VHF radios. I probably should have added a buffer to ensure no more than three NMEA receivers are driven by a single output, but cable runs are short and the receivers exhibit high impedance, so I am able to place five devices on the output.

    Finally, the PC connects to the mux through a USB to serial port expander, and the RS-232 outputs of the port expander connect to the plotter and the NMEA mux. The PC controls the Autopilot through the port expander, as does the plotter.

    I suspect this type of interfacing problem is common. The boat was retrofitted with different equipment at different times, and the choice of a fully integrated system may not meet everyone's budget or desires. The use of a third party mux allowed me to integrate all the units into a single system. The only thing I have not done yet is to transfer routes and waypoints to and from the Garmin directly from the MaxSea program.

    Finally, there are two USB ports on my PC, and one is dedicated to the Pactor Modem. So, in principle, I can send automated position reports to the world as well as DSC emergency reports, using the PC or manually sending the reports from the radios.

    After completing the project, I realized that I can monitor all the boat functions remotely if I can send the NMEA data over a HSDPA, eVDO, or LTE cellular, or satellite link. This means that any friend could see where I am, what ships are around me, etc. More importantly, a tablet computer can be connected via a USB wireless expander, and this waterproof tablet can stay with the skipper.


  • The Brookhouse Multiplexer is the heart of the sytem. It is available in several options. Seatalk capability, AIS capability, and USB. This particular unit is configured for AIS, Seatalk, and a separate, filtered, NMEA output dedicated to the autopilot.

    Ideally, I should have designed a PC Board breadboarded for this interface effort, but screw terminals did the trick and offer some advantages as well. The control board allows access to all the wires from DSC radios, AIS, Seatalk network, NMEA GPS inputs, PC connections, and serves as the wiring center for all connections. Screw terminals allow me to quickly rewire essential functions in case of the catastrophic failure of one of the systems. Note the RS-232 industrial interface box with its own 5 volt dedicated switching supply. This supply can also feed a USB port expander in case I want to plug in or charge my blackberry, PACTOR modem, Instruments data, wireless mouse, and charge my headset all at the same time. WHile the system is a bit complex, it works very well. I have the PC and the plotter at the nav station, the plotter in the cockpit, and the the ability to see depth and all other vital information everywhere.