Networking Marine Instruments: – 02 | NMEA 2000

NMEA 2000 networking fundamentals

Repeating my brief introduction to NMEA2000, (N2k) from my last article:

N2k is a bus network based on the CAN Bus (Controller Area Network) specification, which was originally designed for vehicular use.  This is a single cable system that consists of a backbone (or trunk) cable. Instruments and devices connect to the backbone at T-connectors via drop cables. There is an almost standard system of T-connectors and device connectors. The cable, for backbone and drops, contains five conductors. A shield (or drain), a power pair, and a data pair. Power is supplied to the network via a standard T-connector in most cases, although only the two conductors of the power pair are used.

These features make the system almost plug-and-play. If all of your devices and instruments are from the same manufacturer, you connect them all together, and it should just work. When it doesn’t all just work, a little bit of knowledge can help you to figure out the issues…. Read on!

Power supply specifications:

As usual with all electronics, the quality of the power supplied to them is a major factor in their reliability. N2k requires a power supply within the range of 9 to 16Vdc. Note: that this is the N2k specification, and your instruments may require a supply with smaller variation than this.

I’m going to say here that the voltage supply to the N2k network must be a nominal 12Vdc supply. Whilst it may be possible, in certain circumstances, to use a 24Vdc supply, many devices will not tolerate it… You do not want to let the magic smoke out of your new instruments, do you!

Whilst detailed discussion of the subject is outside the scope of this article, I strongly recommend feeding power to your instruments via a power conditioning device such as those from the Victron Orion range of DC/DC converters. These will take a quite dirty and variable supply and give you a stable, clean output voltage. A good power supply will help with the longevity and reliability of your system.

The components that make a NMEA2000 network

Cables

Cable is available in three flavours:

• Mini: Approximately 12mm (0.5”) diameter, current capacity 8A
• Mid: Approximately 8mm (0.33”) diameter, current capacity 4A
• Micro: Approximately 6mm (0.25”) diameter, current capacity 3A

For small networks, usually, only the micro cable is used for both backbone and drops.

Bend radius – You can’t, well, shouldn’t, put sharp bends in any multicore cable. Manufacturers will provide two bend radii in the specification. The installation radius is the tightest radius that you may bend the cable whilst handling it during the installation. The installed, in-service, or fixed radius is larger and what you need to allow for in your cable route planning.

Observing the bend radius is particularly important at connectors to ensure that excessive strain is not applied to the connectors. A typical final fix bend radius for micro cable is 7 times the diameter; therefore, it is approximately 40mm (remember this is ‘radius’, not diameter). All of this is common sense if you have to force a bend into a cable to make it mate with a Tee, you are risking early failure. Ignore this at your peril!

Drop cables

Drop cables are typically available in lengths ranging from 0.5m up to 6m; the in-between values vary depending on the manufacturer, but 0.5, 1, 2, 3, and 5m are common.

• max length of 6m per drop
• Total of all drop cables on the network: 78m

Backbone cables

For a small network using the micro connector standard, there is no difference between drop and backbone cables other than the available lengths.

• maximum cumulative length, 100m end to end (terminator to terminator)
• Larger conductor sizes can extend this range and amperage, but if you need a longer backbone than 100m, you have other things to worry about; so let’s not!

The connectors

The most common connector that you will come across in small networks and used on both drop and thin backbone cables is the micro connector. This is an M12 5-pin connector (IEC 61076-2-101 is the specification) which is rated to carry up to 3A of current.

For larger networks using the thick cable, the connector is larger and rated up to 8A.

Cables generally come from the factory pre-assembled with a male connector on one end and a female on the other. You can buy bulk cable and use field-fit connectors if you feel handy enough.

Note: When referring to the gender of the connectors, we are referring to the way that the pins mate together; males go into females; it has to be this way, or the magic smoke will come out.

T-connectors

The Tee connector has three terminals: two female and one male. The female connector at the base of the Tee, where the drops connect, is female, and the through connectors for the backbone are one male and one female.

• You require one for each instrument, plus one for the power connector.

Terminators

• You need two, not more, not less
• These are resistors with a value of 120 Ohm (Ω) (the two in parallel have a value of 60Ω)
• Some devices that are designed to be installed at the end of the network may have a built-in termination resistor; sometimes this is user selectable, sometimes not; check your manual.

Optional – If you want to read up on resistors in electrical circuits, here is some info (resistance in series and parallel)

Power connection

The power supply connector should be located in the middle of the circuit (roughly)

• You can buy purpose-made power connectors or make your own using a cut end of cable (red to +12V and black to ground).
• It should be fused with a 3A fuse
• Voltage drop across the network must not exceed 1.5Vdc
• Ideally, you want to connect power in the middle of your network, but for a small network, you can inject the power wherever convenient.

Load Equivalency Number and maximum load

All devices designed to connect to a N2k network will be marked with their Load Equivalency Number (LEN) value. This is a measure of how much load the device pulls from the circuit.

• The total current draw of the network should be less than 3 amps (60 LEN)
• Each LEN is equivalent to 50mA

Many modern instruments, with their bright screens, pull much more than 1LEN. For example, the popular IS42 display from SIMRAD has a LEN value of 3. Don’t worry though, it still takes a lot of instruments and devices to exceed 60LEN.

Testing & Troubleshooting

You will need a basic multimeter for this – I’m assuming that if you are tackling a job like this that you know how to use a multimeter. If you think that an article on multimeter use would be useful, drop a note in the comments.

You have connected up your shiny new network and are happily playing with all the new integrated features…. pat yourself on the back and have an(other) beer. Or…. it doesn’t work; you’ll have to put down that beer and get into some troubleshooting. 

You’ll need a spare cable with one end cut off, or better, cut a cable in half so that you have two cables with a male and female connector. Ideally, grab some banana plugs and fit them to the red, black, white, and blue wires. Put the bananas on the half cable with the male micro connector.

1. Plug the black lead into the common socket of the meter and the red lead to the positive voltage socket, and set your meter to dc Voltage (Vdc or =V). You should see approximately 12V, i.e., your battery voltage. Note: if your battery is charging, this could be as high as 14.7V
2. Disconnect power from the network!
3. Connect the blue and white wires to your multimeter and set it to read resistance (Ohms). You should see a reading of 60Ω.

If the voltage is outside the limits mentioned above, you’ll have to fix it. If it varies from your 12V system’s operating voltage by much (say 0.5V), you should check for faults such as high resistance in connectors and cables supplying your network. Check close to your power injection Tee and at each end of the network. The end voltage may be slightly lower than at the test points near the power Tee, which is OK provided that it doesn’t fall outside the specifications.

If the resistance check gives a reading more than a few Ohms away from 60Ω, you most likely have too many or too few terminators on your network. Double-check that you have the power disconnected, as any voltage on the network will result in unpredictable resistance readings.

• A higher resistance (probably 120Ω) suggests that you only have one terminator installed.
• A lower resistance indicates that you have one or more extra terminators installed.

If your physical troubleshooting turns up no faults and you still can’t see data from all of your devices, you probably don’t have the data source selected. I won’t go into the subject in this article, but if you search your user manual for “source selection”, you should find some useful information.

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