I recently spoke with Brian Rickard at The Offshore Store here in Seattle, and he claims to have done several AIS installations on boats with Raymarine chartplotters, and he’s never had to use an NMEA multiplexer. Whuh-huh?!

To bring you up to speed, let me remind you that Raymarine C- and E-series chartplotters have only one NMEA port on the back. If that port is already occupied (as it is on my boat, carrying position info to my DSC-enabled VHF radio), then there’s no place to connect an AIS receiver. At least, not until you buy a $300 NMEA multiplexer.

I explored a few options in an earlier post, and decided that a secondary GPS receiver connected directly to the VHF (isolating the VHF from the chartplotter entirely) would free up my C-80’s NMEA port, incurring only the cost an extra GPS receiver, around $100.

Brian told me about another option that eliminates even the need to buy a second GPS receiver! How does he accomplish this? Well, it turns out he knows a lot more about AIS units than I do. Here’s the key fact he knew that I did not: Some (many? most?) AIS receivers will “mix” the data they generate with whatever data is coming into the unit through the NMEA port. The AIS unit is not a proper multiplexer, but if data comes into the unit at 4800 baud, the unit will insert that data into the AIS data stream, and send it all out over NMEA at 38,400 baud.

In theory, I could reconfigure my GPS receiver to output data in NMEA instead of SeaTalk. Then I could split the signal and send it to both the VHF radio and the AIS receiver. This would make the GPS-to-VHF connection direct, which I like: it means dependency on fewer individual components for DSC to work effectively in an emergency. Meanwhile, the AIS receiver sends both GPS position data and AIS data onward to the chartplotter. No Multiplexer needed!

So which AIS receivers will do this? The ones Brian mentioned specifically were the Smart Radio SR161 and SR162 units, distributed in this area by Milltech Marine. I wrote to Milltech, and heard back right away from Doug Miller, who confirmed Brian’s assertion: Yes, you can use the SR161 or SR162 as a consolidator for inbound NMEA data streams. “That is a key feature with our units. Basically input NMEA data (e.g. GPS) on pin 3 at 4800 baud and output the combined external NMEA and AIS sentences at 38,400 baud on pin 2. We sell a cable to do this or you can build your own using the attached diagram.”

Wow. That could come in handy, you know? Now I’m wondering: how many other AIS receivers will do this?