NMEA 2000 101

[Gixer]

I’ve just installed a NMEA 2000 AIS unit and I’m going to buy a NMEA 2000 chart plotter next week.

The penny has just dropped that I can’t connect them directly together… Apparently I need a backbone!

First of all, is this correct?
If it is true, do I need 2x tee piece, power cable, 2x drop cables and a terminator?
This stuff is quite expensive, is there a recommended retailer to get this from?

Thanks in advance guys.

 

[wonkywinch]

This guide may be useful - https://actisense.com/wp-content/uploads/2021/08/Complete-Guide-to-NMEA-2000_Revision_4_2021.pdf

 

[Gixer]

Thank you Wonkywinch, that’s a really interesting read. Looks like this is going to get expensive.
I guess I’m future proofing myself.

 

[Martin_J]

Although once you start buying the bits, you'll find it never stops.

Two tee pieces, a power drop and the terminators will not keep you going for very long...

Plan ahead for when you need the backbone going forward for maybe a compass and depth transducer... then up towards the hatch area for wind displays... and then to the pedestal for yet more NMEA2000 devices.. not to mention the other kit that might appear at the chart table.

 

[KompetentKrew]

Topology of an NMEA2000 network illustrated:

The dotted red line indicates the backbone.​

Minimal NMEA network for 2 devices:

The backbone consists of the three blue T's, which has a blue terminator on each end. The purple cables are drop cables, power is supplied to the middle T.​

I like the Garmin NMEA 2000 Network Fundamentals PDF

www.HatLabs.fi used to be the cheapest quality supplier of "field installable" NMEA 2000 connectors - not sure if this remains true for UK since it left the EU. Farnell sell NMEA 2000 cable - I think it's listed as "canbus".

AliExpress also sell some NMEA 2000 stuff - they're cheap for T's and short drop cables. Although I don't think I've yet tested the ones I got there, I'm sure they're fine.

You cannot connect T's in arbitrary ways:

You should not do this, as your network will probably (certainly??) not work.​

The backbone should always be linear, all T-s arranged in the same way relative to the backbone and the terminators:

 

[wonkywinch]

The most useful NMEA device for me is the wind instrument. Saves me craning my neck and with the boat speed and heading, the plotter works out all kinds of things for me.

 

[Gixer]

Thanks for taking the time to give such a detailed response KompetentKrew.

I have a fully working Tacktick system at the moment and don’t plan to upgrade for a while. The NMEA0183 from the instruments passes through a converter to NMEA2000 mixed with GPS and AIS. I want to push all this information to a plotter which I can mount in the cockpit. I thought I could just connect the converter and the plotter together, how wrong was I?

 

[PaulRainbow]

I’ve just installed a NMEA 2000 AIS unit and I’m going to buy a NMEA 2000 chart plotter next week.

The penny has just dropped that I can’t connect them directly together… Apparently I need a backbone!

First of all, is this correct?
If it is true, do I need 2x tee piece, power cable, 2x drop cables and a terminator?
This stuff is quite expensive, is there a recommended retailer to get this from?

Thanks in advance guys.

Make that 3 tees and 2 terminators.

 

[PaulRainbow]

Thanks for taking the time to give such a detailed response KompetentKrew.

I have a fully working Tacktick system at the moment and don’t plan to upgrade for a while. The NMEA0183 from the instruments passes through a converter to NMEA2000 mixed with GPS and AIS. I want to push all this information to a plotter which I can mount in the cockpit. I thought I could just connect the converter and the plotter together, how wrong was I?

Now you need 4 tees.

One tee for each N2K device, plus one for power.

 

[Gixer]

Thanks PaulRainbow,

The NMEA converter, AIS and GPS is one unit and has one NMEA 2000 port for all the data. The plotter will have one port and the power connector another. What is the fourth connector for?

After adding this all up I’m tempted to ditch the NMEA 2000 idea and buy the Garmin plotter with NMEA0183 connectivity.

 

[PaulRainbow]

Thanks PaulRainbow,

The NMEA converter, AIS and GPS is one unit and has one NMEA 2000 port for all the data. The plotter will have one port and the power connector another. What is the fourth connector for?

In that case just 3.

After adding this all up I’m tempted to ditch the NMEA 2000 idea and buy the Garmin plotter with NMEA0183 connectivity.

Stick with N2K. What plotter are you considering ? Many don't have 0183, those that do usually only have one. 0183 is on it's way out for new equipment.

 

[Gixer]

Thanks Paul,

Two are on my list to go through at SBS this week are,
Garmin 723
Axiom+ 7
I used the Axiom with lighthouse charts on a boat recently and really liked the menu and layout.

Although the strong marketing that Orca have been doing recently has convinced me to take a look at them.

 

[PaulRainbow]

Thanks Paul,

Two are on my list to go through at SBS this week are,
Garmin 723
Axiom+ 7
I used the Axiom with lighthouse charts on a boat recently and really liked the menu and layout.

Although the strong marketing that Orca have been doing recently has convinced me to take a look at them.

The Axiom doesn't have 0183, but the Garmin does. If yo don't want radar the Echomap series are worth a look.

Orca looks horribly expensive.

 

[Gixer]

I feature I really want is to be able to mirror the plotter to a tablet so I can have a screen on deck and down below. I don't think the Echomap series can do this which is a shame as they are great value with the charts included.

 

[mullet]

This stuff is quite expensive, is there a recommended retailer to get this from?

Whilst I am all for buying local, I do have my limits. I get most of my NMEA cables, tees and so on direct from China via Aliexpress. Dramatically less costly than buying from a UK retailer, normally arrive within a week, and I have never had any problems with quality.

 

[PaulRainbow]

I feature I really want is to be able to mirror the plotter to a tablet so I can have a screen on deck and down below. I don't think the Echomap series can do this which is a shame as they are great value with the charts included.

Yes, that's correct. I'd go for the GPSMap if it was me.

 

[jakew009]

You cannot connect T's in arbitrary ways:

You should not do this, as your network will probably (certainly??) not work.​

The backbone should always be linear, all T-s arranged in the same way relative to the backbone and the terminators:

I think Garmin have lost the plot slightly with that diagram, there is absolutely nothing wrong with the first version. It is electrically identical.

You can even have tee’s branching off tee’s branching off more tee’s. Completely within the spec and works fine.

 

[PaulRainbow]

I think Garmin have lost the plot slightly with that diagram, there is absolutely nothing wrong with the first version. It is electrically identical.

You can even have tee’s branching off tee’s branching off more tee’s. Completely within the spec and works fine.

The diagram with the red X in it is incorrect and does not comply with N2K specs. I think Garmin might know a thing or two about N2K.

 

[Hoolie]

I think Garmin have lost the plot slightly with that diagram, there is absolutely nothing wrong with the first version. It is electrically identical.

You can even have tee’s branching off tee’s branching off more tee’s. Completely within the spec and works fine.

Electrically that layout would work in most leisure boat installations, but as PR has pointed out is non-compliant with the N2K specification that covers a wide range of applications.

 

[Martin_J]

Indeed... just like (from memory) the NMEA2000 spec. states that devices cannot be daisy-chained..

Raymarine are happy to provide the facility to daisy-chain though a number of their devices..

You end up with more than one device sharing a drop cable..



Edit: Although I now seem to recall that that requirement was something to do with a failure of electronics within one device should not be able to affect the connectivity of another device within the system.