The basics of building and testing your NMEA 2000 Network

NMEA 2000 is an open standard designed by the NMEA to help them achieve their mission by standardising the messages and connector system used for devices in the marine industry. There is a common misconception that the term ‘open standard’ means freely available when in fact it means that the standard is available to buy at a reasonable cost from their website.

It is important to remember that the NMEA is not for profit (NFP) and the standards create an important source of income so that they can continue to exist.

When building your NMEA 2000 network, there are some minimum requirements;

1.      A fused power insertion point (an Actisense A2K-MPT-2 connected to a fused battery supply for example)

2.      2 NMEA 2000 Certified Devices plus cables and connectors to build the network

3.      2 Termination resistors

In order to achieve a compliant NMEA 2000 network, the following table indicates the technical specifications required for your NMEA cables:

Inserting power into the NMEA 2000 network

Power Insertion using A2K-MPT-2:

The Actisense Micro power T-piece has a twin pair of power wires, one for each side of the T-piece. Both pairs must be connected to a 12/24 Volt power supply (via fuse panel) so that the electrical load can be distributed evenly.

When using 24V systems, a warning must be installed on the network to ensure anyone installing new devices is aware. This is because the NMEA 2000 specification doesn’t officially support 24V but this level of power supply can be used if installed devices are capable of supporting it. All Actisense devices support 24V power supplies.

Power using A2K-SBN-1 / SBN-2:

The power cord for the Small Boat Network is over moulded to the case of a 4-way / 8-way T-piece to prevent water ingress. Power is available to all 4 / 8 ports once the pair of bare wires are connected to a 12V or 24V power supply.

Power Insertion using Quick Network Block (QNB-1):

The Actisense Quick Network Block (QNB-1) provides a versatile and easy to install solution for any NMEA 2000 network, utilising high quality NMEA 2000 barrier strip connections. With glands fitted to the QNB-1, it allows for custom lengths of cable and cable mixing, meaning that the QNB can be implemented with Raymarine STNG cables for example.

The QNB-1 splits the power feed with separate fusing on either side, enabling longer networks with isolation due to the ATO fuses. The other version (QNB-1-PMW), comes with 6 x M12 Standard Female (Micro) Connectors, allowing for ‘plug and play’ with drop cables.

You will hear some common phrases used when discussing NMEA 2000 networks. Here’s what they mean:

Backbone: The main trunk of cable that runs through the boat to form the network. T-pieces are connected along the network for devices to plug in to.

Drop Cable: The length of cable that connects an NMEA 20000 device to the backbone. It is important to highlight that the maximum length for a drop cable as per the NMEA 2000 standard is 6m.

PGN: Stands for ‘Parameter Group Number’. The messages used by NMEA 2000 devices for sharing information.

Testing and diagnosing the network

Testing Power Supply and Termination Resistors

Ideally, the power supplied to the network should be tested at 2 points; the insertion point, and the extremities of the network.

Power insertion points should be tested to determine how much power is actually being supplied to the network. Any device which does not have an external power supply is powered from the backbone, thus there needs to be sufficient power to run all of the connected instruments.

The power should never exceed 14V, and be below 9V. With network devices using up some of the supplied power and impedance from cabling also dropping the voltage, it is important to know that the voltage at the end of each backbone leg is adequate.

Temporarily remove a termination resistor from the end of the network and use a voltmeter to measure across the NET-S and NET-C pins. The measured value should be at least 9V.

When testing the resistance on the network, the value expected is 60 ohms. This test can be performed by powering down the network and placing a DMM across the NET-H and NET-L pins. If 120 Ohms is measured, only one termination resistor is installed. A second resistor will need to be installed at the opposite end of the network to the one that is already present.

If 40 Ohms is observed, there are 3 termination resistors installed on the network so 1 will need to be removed. If it’s not obvious where the third resistor is, check for any devices on the network with installed termination resistors and ensure they are switched off. Network devices with termination resistors installed do not achieve NMEA 2000 certification as this practice can cause confusion on the network.

Further Testing

If the issue is still present after verifying that both the power supply and termination resistance are good, then further testing is required: A common technique used for investigating a network is to split the network into sections by moving the terminator: Disconnect one half of the backbone and move the termination resistor to the new, temporary end of the network.

If the issue remains you have narrowed it down to the half of the network that is still powered and active. If the issue does not remain it is in the half of the network that has been disconnected.

Keep performing this half split method until you are able to narrow it down to a single device, if possible.

Don’t forget to move the termination resistor each time to ensure the network is correctly terminated. Once the issue has been narrowed down, try swapping out t-pieces or cabling with ones that are known to be good.

If Field Fit Connectors have been used, check the wiring connections inside to ensure that they are correct and firmly secured in place.

If possible, check the wiring inside the device is properly and securely terminated, ensuring to observe anti-static precautions if exposing internal electronics. It is always best to check that this is ok with the device manufacturer before performing this step to ensure you do not void any warranty.

If an issue still remains you now know there is a problem with the device and it’s time to contact the manufacturer of that device.

For a complete guide to installing your NMEA 2000 network and using Actisense products, please download our NMEA 2000 e-book.

View the full range of Actisense NMEA 2000 products