I’ve written about this before, here and here. Talking with folks from Raymarine and The Offshore Store during the Seattle Boat Show, I have confirmed that yes, my Ray54 DSC VHF uses NMEA for both input and output. Currently, my VHF is receiving NMEA messages just fine, but no NMEA data seems to be getting from the VHF to the C-series chartplotter. Before you ask, yes I’ve set the “DSC Messages” option on the chartplotter to “On”.

The most likely reason VHF-to-chartplotter NMEA isn’t working is that the C-series software didn’t support incoming DSC messages back in 2004, when mine was installed, so it’s POSSIBLE that those wires just weren’t connected. The fellow from Raymarine suggested this possibility, and my buddies at The Offshore Store indicated that they’ve seen this as well.

All that’s just background to my current conundrum, however…the problem is that I’m adding AIS, and I need to connect it to my chartplotter. My Raymarine C-series chartplotter has one NMEA port on the back, and two SeaTalk ports. [Slight correction: It has a SeaTalk port and a SeaTalk2 port.] Here’s how they’re allocated now:

• SeaTalk (2 above): GPS and Autopilot
• SeaTalk2 (1 above): vacant
• NMEA (6 above): VHF radio with DSC, which is both an NMEA “talker” (DSC messages, currently nonfunctional) and a “listener” (GPS coordinates).

Now I COULD use an NMEA multiplexer, which costs around $300. An NMEA multiplexer works like this: In one direction, it collects high-speed (AIS) and low-speed NMEA input, consolidates it into a single high-speed NMEA datastream, and sends it on to the chartplotter. In the other direction, it takes high-speed NMEA output from the chartplotter and sends it onward to all the connected devices at the appropriate baud rates.

But guess what: I don’t have to use a multiplexer. How? It came to me this morning, and I confirmed with Kyle Holloway at The Offshore Store today that it’ll work. They install plenty of AIS units on Raymarine C- and E-series units, and Brian Rickard once told me he’s never had to use a multiplexer.

The problem is the VHF radio, which is DSC enabled and therefore requires a constant stream of GPS data; it’s an NMEA listener. When the VHF receives a DSC call with position data, it can send that data to the chartplotter to be displayed on the chart; so it’s also an NMEA talker. In order to maintain both without a multiplexer, I’m going to split them.

The radio’s NMEA output (”talk”) destined for the chartplotter will go INTO the AIS receiver. Virtually all of the AIS receivers, even the <$200 MillTech Marine unit, will take incoming low-speed NMEA data and insert it into the high-speed datastream going to the chartplotter. It’s not a multiplexer proper, but it is an NMEA “consolidator”, in one direction. So that problem is taken care of.

Now the tricky part. The VHF needs GPS coordinates. And it turns out I already have a device that can supply GPS coordinates in low-speed NMEA. No, not my GPS receiver…it’s SeaTalk only. As discussed several months ago, If you have an NMEA GPS, you CAN split the signal and send it two places, according to The Offshore Store, so you could theoretically send GPS data to both your chartplotter and directly to the VHF.

That isn’t possible in my case, so I’ll use the NMEA output from my autopilot. That’s the clever bit I came up with. The autopilot, in essence, already has a SeaTalk/NMEA converter onboard, and it has terminals for NMEA in and NMEA out which are currently unused.

Oh, and just in case you were about to suggest that I connect both input and output from the VHF through my autopilot, I’ll note that some of the Raymarine reference materials for the the autopilot seem to indicate that it won’t forward EVERY kind of NMEA message. I’m not really sure this is true; does it really filter some out and let others through? Why bother? It probably just passes them all through. But in case it does not, I’m only going to rely on it to provide GPS position data to the VHF, which is one of the NMEA messages the manuals say it will forward.

So at long last, I’m going to add AIS reception to my C-Dory. For $200 plus some specialized cables and an antenna splitter. That’s the theory, anyway! Wish me luck.