NMEA 2000 backbone

[KompetentKrew]

I'm hopefully in the final stages of planning an upgrade to my boat's electronics, which will see an upgrade largely to NMEA 2000.

I see that Force 4 sell NMEA 2000 backbone kits comprising a long cable, power injector and various connectors, but this cable would need to be run down some very narrow conduit and I don't think the terminators will fit.

Am I correct in thinking that I can just run 4-core shielded cable and terminate it myself?

I found this pin-out diagram and was thinking of using this cable or, more likely, this thicker cable and terminating with plugs and sockets from Hat Labs (seems like a weird place to source them, but they're a fraction of the price I can find them anywhere else).

I figure to supply 12v at both ends, effectively running 12v in parallel to make sure there's enough current and I'll make sure the 12v all comes from a single switch by the nav station so that there's no potential between them.

Is there anything else I need to consider or look out for please?

 

[AngusMcDoon]

While your cable choices would probably do, DeviceNet cable is more appropriate for CAN Bus as it's designed specifically for the job - like these...

DeviceNet Thin Cable
DeviceNet Thick Cable

These have the correct colour cores, screening and thicker power than data cores. DeviceNet thick specification is hefty, more expensive, and may not fit your terminals. DeviceNet thin is fine for most small boat installations - backbone and spurs.

DeviceNet thick was originally intended for the backbone and thin as the spurs. However, it comes from a factory instrumentation background where the backbone may be 1000m in length. NMEA2000 specifies 2 cable thicknesses and the thinner one which is like DeviceNet thin is suitable for your size boat with a typical installation.

Chapter and verse is on pages 7-8 here...

https://www.nmea.org/Assets/20090423 rtcm white paper nmea 2000.pdf

 

[Sandy]

Chop off the terminator and then refit once the cable pops out the far end and refit.

 

[KompetentKrew]

While your cable choices would probably do, DeviceNet cable is more appropriate for CAN Bus as it's designed specifically for the job - like these...

DeviceNet Thin Cable
DeviceNet Thick Cable

These have the correct colour cores, screening and thicker power than data cores. DeviceNet thick specification is hefty, more expensive, and may not fit your terminals. DeviceNet thin is fine for most small boat installations - backbone and spurs.

Thanks.

I'm not yet certain of where everything goes - if I were using the cheaper cable I would just buy a whole reel for £11 or £45 and start laying the backbone now in preparation.

I would prefer to use the proper cable, but it's a bit more expensive - can I run a length through the first conduit, terminate it with male and female terminators, and then later plug in another length as an "extension cable" (to extend my backbone) when I know where other things go?

This Garmin NMEA 2000 Network Fundamentals PDF document seems clear (on page 4) about the right way to connect devices using T-pieces. I guess it doesn't matter which end of the backbone is male and which end female, as both ends will be terminated (one male and one female); when adding devices a T-piece is always used, so it's just turned whichever way around needed to fit? In other words there is no "front" and "back" (or "top" and "bottom") to the NMEA backbone.

 

[Keith-i]

I would try and get the proper cable. Also, whilst you can buy field installable connectors they are not easy to fit. I don’t think it’s good practice to have 2 power feeds to the same backbone but you can get a specific T connector that breaks the power wires and allows you to feed each side independently with power.

 

[PaulRainbow]

This all sounds misunderstood. The terminators do not go on the ends of the backbone cable, or you don't have anywhere to connect your equipment. You also do not connect the power cable to the backbone cable, it goes to a tee. You should only have a single power connection.

Some useful examples here: Maretron | Examples | Sailboats

 

[KompetentKrew]

This all sounds misunderstood. The terminators do not go on the ends of the backbone cable, or you don't have anywhere to connect your equipment. You also do not connect the power cable to the backbone cable, it goes to a tee. You should only have a single power connection.

The Garmin document I linked below seems quite clear on the matter of termination, Paul - I'll post some pics below. Is it me that's wrong or Garmin?

Where do the terminators go, if not at the ends of the backbone?

I guess the terminology that Garmin are using here is that the T-pieces form part of the backbone - could that be the source of our misunderstanding?

I'm aware that supplying it at both ends is not in the spec - you have me second-guessing myself, but I thought a good supply would be better for redundancy and to ensure no voltage drop.

 

[ithet]

I used a very similar socket (with screw connections) as the OP posted to make up a cable to connect my AIS to my SeatalkNG network. Found it works fine and was easy to fit.

 

[Keith-i]

One other thing regarding cables. As N2K is canbus I think you will find the data cables are twisted pair to limit interference. Standard cable won’t really be compliant.

 

[PaulRainbow]

The Garmin document I linked below seems quite clear on the matter of termination, Paul - I'll post some pics below. Is it me that's wrong or Garmin?

Where do the terminators go, if not at the ends of the backbone?

I guess the terminology that Garmin are using here is that the T-pieces form part of the backbone - could that be the source of our misunderstanding?

I'm aware that supplying it at both ends is not in the spec - you have me second-guessing myself, but I thought a good supply would be better for redundancy and to ensure no voltage drop.

​

​

Looking at the above, you can see that the terminators are at the ends of the backbone, but as you said, the tees are part of the backbone, so more often than not the terminators will be fitted to the tees at either end of the backbone. The image above is a decent example.

A common arrangement would be a bunch of connectors (or a 4 way one) at the helm, with a terminator, then another bunch down below, connected to transducers etc and any instruments at the chart table, with the second terminator down there too. The two sets of tees would be connected with a backbone cable.

If the backbone cable connectors won't go through where they need to go, you can cut and splice the cable. Field installable connectors are available but are fiddly. I wouldn't use non-N2K compliant cable.

 

[Red Panda]

I updated all the instruments to N2K a few years ago. My B&G instrument set came with a basic backbone kit.
I managed to acquire some single-ended cables from a local installer, where he'd had to cut off one connector. I used the moulded end for external console fitment, passed the cut end through small deck glands, and used field-install connectors down below.

 

[salar]

The Garmin document I linked below seems quite clear on the matter of termination, Paul - I'll post some pics below. Is it me that's wrong or Garmin?

Where do the terminators go, if not at the ends of the backbone?

I guess the terminology that Garmin are using here is that the T-pieces form part of the backbone - could that be the source of our misunderstanding?

I'm aware that supplying it at both ends is not in the spec - you have me second-guessing myself, but I thought a good supply would be better for redundancy and to ensure no voltage dro

​

​

That is exactly as I have installed mine. In fact the backbone only runs across the back of the dash and is a load of T pieces locked together. Everything else is connected with a drop cable, and as most of them (plotter, vhf, ais etc) are local I only needed a longer drop cable to the GPS antennae. You only need a long backbone if you have a bunch of units in a separate location.

 

[KompetentKrew]

So am I right in thinking that the NMEA network must always be "complete"?

You can't simply unplug a device from the network, instead you must remove its T-piece at the backbone?

What happens if you buy a multi-port T-connector? Presumably all ports must be in use at all times?

 

[Spirit (of Glenans)]

I'm hopefully in the final stages of planning an upgrade to my boat's electronics, which will see an upgrade largely to NMEA 2000.

I see that Force 4 sell NMEA 2000 backbone kits comprising a long cable, power injector and various connectors, but this cable would need to be run down some very narrow conduit and I don't think the terminators will fit.

Am I correct in thinking that I can just run 4-core shielded cable and terminate it myself?

I found this pin-out diagram and was thinking of using this cable or, more likely, this thicker cable and terminating with plugs and sockets from Hat Labs (seems like a weird place to source them, but they're a fraction of the price I can find them anywhere else).

I figure to supply 12v at both ends, effectively running 12v in parallel to make sure there's enough current and I'll make sure the 12v all comes from a single switch by the nav station so that there's no potential between them.

Is there anything else I need to consider or look out for please?

The system should have only one power supply and it should be to the centre (more or less) of the backbone.

 

[KompetentKrew]

The system should have only one power supply …

Why tho?

 

[Keith-i]

You do not have to have all ports or T's connected to something. They are fine left without any equipment plugged in. The only critical aspect is having a terminating resistor at each end.

 

[KompetentKrew]

You do not have to have all ports or T's connected to something. They are fine left without any equipment plugged in. The only critical aspect is having a terminating resistor at each end.

Thanks very much. That was really bugging me,

 

[Spirit (of Glenans)]

Why tho?

Because the NMEA2000 Protocol says so. I'm sure someone will be along shortly with chapter and verse.