Networking the boat (NMEA 2000) (06/2018)

My first boat had some 80's vintage speed and depth instruments. Purely analog, each transducer connected to a display and that was that.

My second boat didn't have much in the way of instruments but it had a GPS hooked up to the VHF via NMEA 0183. That was enough to show me I didn't want to deal with NMEA 0183 ever again. Far too limiting to be worth the effort.

I'd been casually reading about NMEA 2000 but had never had a chance to set it up until now so had to do some research first. Here are some notes which might help if you are looking setting up a new network and have not done it before.

Backbone and drops

If you did any ethernet wiring in the 80's or early 90's, you'll feel at home.

Here is one good overview document from Maretron:

And one from Garmin:

Hopefully the links still work when you read this. I won't repeat the details they describe more thoroughly already.

But basically, there is one backbone cable which runs between the two most distant parts on the boat where devices need to be plugged in. At each extreme of this cable there is a terminator (so, exactly two terminators in the system). Throughout the length of this backbone cable, there can be any number of T-connectors which is where the actual devices plug in via drop cables. Somewhere on the cable (can be anywhere) there is also one (and only one) special T connector for powering the network.

On my Catalina 270 the backbone cable runs from the bow (below the V berth, where the transducers are) to the stern and up the pedestal guard to the navpod (where the instruments are). I'd guess that's pretty typical.

Read the details on the documents linked above on backbone cable and drop cable maximum lengths to make sure your network is in spec. The limits are quite large so on a small boat like mine not really an issue. I suppose on a large boat you'd have to think a bit more carefully about the layout so you don't exceed limits.

Also check the documents on how to compute the current load limits. This might influence where on the backbone the power T connector needs to be.

It all sounded very straightforward after I read the above documents so at this stage of my research I felt it'll be very easy to set up.

But then as I started reading about specific devices and reading the manufacturer sites and product documentation, the picture started to get foggier. I saw photos online of different cables that clearly didn't work together and lots of questions with half-answers and conflicting answers on various forums. So what actually works with what and is the NMEA 2000 network a standard or not?

Cables and connectors

The good news is that there is a standard NMEA 2000 connector and cable.

The cable has five inner wires: red (+12V), black (ground), blue and white (data), silver/bare (drain). The connectors correspondingly have the same 5 connections (M or F). This is true for both backbone and drop cable (to the devices) cables and connectors.

Here is a standard T connector from Maretron:

Notice the pin pattern with one pin in the center and four around it.

Here are two views of a Garmin T connector. As you can see it has the same pattern:


And here is the connector from some Garmin cable, again with the same pattern:

So far so good. The answer is that you can mix and match cables and connectors from any brand which complies to the standard.

I used backbone cable from Maretron and most of the T connectors I used are also from Maretron. I used some T connectors and drop cables from Garmin as well as some drop cables from Maretron. It all works together in any combination, no issues.

Unfortunately as with most standards, someone always spoils the party.

Dealing with Raymarine

So here's the confusing part. Raymarine uses two completely different connector types (I guess introducing one non-standard connector wasn't enough, they had to introduce two.. aaargh! All to make you buy more cables.)

Here is a Raymarine T connector. It has blue (backbone) and white (drop cable) connections just like a standard T.

But look at the pin layout. Here is the Raymarine backbone cable. It has five wires as expected but in a non-standard incompatible pattern:

And here is a look at a Raymarine drop cable. This has six wires, in a yet again incompatible pattern:

Oh oh.. trouble. So now what?

I'd say the best recommendation is to only buy standard-compliant devices and make your life easier (and also to vote with your wallet to promote vendors who stick to the standard instead of making customers life difficult). If you can only buy NMEA 2000 compatible devices you'll have an easy time and not have to deal with this nonsense.

So that was my plan. Unfortunately... nobody really makes a small boat wheel autopilot other than Raymarine. So I was forced to deal with this.

The good news

Fortunately there's also good news here. Even though Raymarine has two different incompatible cable connectors, electrically they are NMEA2000-compatible and the devices process standard NMEA2000 data packets. So it's just a matter of getting the plugs to physicall work on an NMEA 2000 network.

To plug Raymarine devices into a NMEA2000 network, simply take the drop cable that comes with the Raymarine device and cut off one of the connectors. Plug the proprietary non-standard connector into the Raymarine device and splice in a standard NMEA2000 connector into the other end. Done!

When you cut off the Raymarine drop cable you'll find that it has six wires. It has the five NMEA2000 wires using the standard color code, just match them up color-to-color with a standard cable or standard connector. The sixth wire is yellow and it's not used for anything. Cut it short and ignore it, it's not needed.

A Raymarine backbone cable has just the five wires in the expected colors.

I also used an Raymarin iTC-5 (translates old analog depth & speed sensors to NMEA 2000) which is wacky in its own way because it needs to be wired onto the backbone instead of using a drop cable like a normal device. The same applies though, I just wired a Raymarine backbone connector to the standard backbone cable by matching the wire colors.

You can use any standard connector (like Garmin or Maretron) cut off from another cable. A somewhat easier way is to get a Maretron field-attachable connector ( which has screw terminals for each wire. I used several of these and they make the job easy.

Finally, another way is to get DeviceNet Adaptor cables from Raymarine. It gets more expensive though as you'd need to buy an adapter cable for every connection. And philosophically it's annoying to have to buy an extra cable from Raymarine just to plug in the device I just bought from them because they couldn't be bothered to use the standard connector in the first place! I didn't need to use any of these so can't comment on them, but presumably should also work fine, extra expense aside.

I hope Raymarine comes to their senses in their next generation of products and use proper standard cables and connectors. But until then, at least it's easy (although somewhat tedious) to work around this problem.

Back to my Catalina 270 page.