A note about the Raymarine AIS250:
It's expensive, but for the money you get a built-in antenna splitter and built-in NMEA multiplexing, which can be important. More on that below. On the down side, the otherwise well-designed AIS250 is only a single-channel receiver. That is, it scans between the two AIS channels but can only take data from one at a time. Surprising that they got this detail wrong...because otherwise it would be a top pick.
Now a note on NMEA multiplexing. Hold on to your hat, Harvey. My Raymarine C-series chartplotter has one NMEA port on the back, and two SeaTalk ports. Here's how they're allocated now:
SeaTalk1: GPS
SeaTalk2: Autopilot
NMEA: VHF radio with DSC, which is both an NMEA "talker" (DSC messages) and a "listener" (GPS coordinates).
Now I COULD use a multiplexer, which costs around $300. An NMEA multiplexer works like this: In one direction, it collects high-speed (AIS) and low-speed NMEA input, aggregates 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 rate.
But guess what: I don't have to use a multiplexer. How? It came to me this morning, and I confirmed with the helpful folks at The Offshore Store today that it'll work. They install plenty of these, and one guy there said he's never had to use a multiplexer. Here's how it will work:
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. 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. 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. 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.
So I'm going to add AIS reception. For $200 plus some specialized cables. That's the theory, anyway! Wish me luck.
It's expensive, but for the money you get a built-in antenna splitter and built-in NMEA multiplexing, which can be important. More on that below. On the down side, the otherwise well-designed AIS250 is only a single-channel receiver. That is, it scans between the two AIS channels but can only take data from one at a time. Surprising that they got this detail wrong...because otherwise it would be a top pick.
Now a note on NMEA multiplexing. Hold on to your hat, Harvey. My Raymarine C-series chartplotter has one NMEA port on the back, and two SeaTalk ports. Here's how they're allocated now:
SeaTalk1: GPS
SeaTalk2: Autopilot
NMEA: VHF radio with DSC, which is both an NMEA "talker" (DSC messages) and a "listener" (GPS coordinates).
Now I COULD use a multiplexer, which costs around $300. An NMEA multiplexer works like this: In one direction, it collects high-speed (AIS) and low-speed NMEA input, aggregates 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 rate.
But guess what: I don't have to use a multiplexer. How? It came to me this morning, and I confirmed with the helpful folks at The Offshore Store today that it'll work. They install plenty of these, and one guy there said he's never had to use a multiplexer. Here's how it will work:
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. 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. 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. 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.
So I'm going to add AIS reception. For $200 plus some specialized cables. That's the theory, anyway! Wish me luck.
