Today I spent a couple of hours trying to map Tahina’s navigation network. I’m doing this in order to determine how best to install the new AIS receiver (it’s a marine transponder system which all ships are required to have). Tahina’s owner’s manual has nice diagrams for all the main systems, EXCEPT the navigation network. Electrical, plumbing (fresh and black), rigging, gas, fuel, etc. all have good diagrams. I wrote St. Francis Marine (Tahina’s builder in South Africa), but they replied with a very basic diagram that was definitely not for our specific configuration.
Tahina’s navigation system is based on Raymarine Marine Electronics (http://raymarine.com). Tahina’s system has a chartplotter, GPS, autopilot, fluxgate compass, lifetag wireless man overboard system, fishfinder, and all the other basic instruments (wind/depth/speed/log/etc.). Also, we have a desktop computer built into the nav station which is plugged into the boat network and has Raymarine’s navigation software. The network is comprised of three different protocols (all with different wiring): Seatalk (the primary instrument network), Seatalk HS (High Speed version), and NMEA (which is an older serial communications system).
Here’s a basic map of a Raymarine network (this is not Tahina’s specific diagram, but it gives you an idea of the complexity – click for larger version):
The problem is that the AIS unit we have uses NMEA and has to plug into the E120 chartplotter. But, the chartplotter already has something plugged into the NMEA port. And, I haven’t determine what is connected to the other end of that cable. None of the other devices seemed to be using that port (GPS, fluxgate compass, autpilot, lifetag, etc. are all either using Seatalk or plugged in elsewhere). BUT, as I was writing this post, I noticed in the diagram above that one possibile use of the NMEA is DSC (which stands for Digital Selective Calling – and is an emergency broadcasting function on the VHF). I checked, and my radio is connected for DSC. Not only that, but the chartplotter is configured in a non-default mode to allow for DSC messages to be displayed! This may be the solution to the mystery… If so, I need to figure out what to do next – either get a multiplexer so both can plug into the same port, or wire the VHF DSC differently.