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The 3 easiest ways to diagnose NMEA network issues

Being able to diagnose issues on your boat’s electronics network can save you a huge amount of time, money and tears later. Our Solutions Engineer, Josh Keets, shares his top tips in tackling onboard diagnostics in this article. 

There is a wide range of equipment that can be used to test networks and devices, which range from basic tests through to functionality testing. I’m not going to cover all of these here, and I will instead just highlight some of the simple, easier to use pieces of equipment that allow for easy diagnostics.  

A digital multimeter (DMM) is extremely useful for testing and diagnostics, as it offers a number of different functions. This allows for battery testing, through to voltage testing an NMEA 2000 backbone to determine if there is enough power across the network. They can also be used for resistance testing on NMEA 2000 networks. Whilst there is also the option of testing NMEA 0183 devices using a multimeter, this isn’t always reliable or accurate, and there are better more straight forward methods for testing these.  

Along with a multimeter to take readings, a PC interface device or gateway is also great for making diagnostics easier. Two gateways are required, one for each data protocol, where an NMEA 0183 to PC Gateway (such as the Actisense USG-2) and an NMEA 2000 to PC Gateway (such as the Actisense NGT-1-USB) are needed respectively. These gateways allow the data to be sent from the device or network, via the gateway onto a PC where a terminal program or NMEA monitoring program can be used to view the data. 

This is a quick and easy way of identifying any missing data, or data which is corrupted or incomplete. 

NGT 1 Network diagram
NGT 1 Network diagram

Some basic testing 

Using a DMM, basic power and resistance tests can be done.  

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

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. 

Additional recommended products 

Alongside the earlier mentioned NMEA 0183 and NMEA 2000 devices and DMM, I would also recommend having a wireless gateway on the vessel as an additional tool for data monitoring and network analysis. The main benefit of this is that you have access to your NMEA 2000 network data from anywhere on the vessel where you have connection to the gateway. Some wireless gateways such as our W2K-1 come with additional benefits such as an internal SD Card, which continually logs all the NMEA 2000 data, essentially acting as a Voyage Data Logger. This offers a number of benefits, including post voyage data analysis and processing, but also acts as a ‘black box’ type product where there is a historic log of the journey. If there was an incident out on the water or a collision with another vessel, the data logged in the SD Card can prove useful into investigating how and why that has happened.  

Actisense-i Network Health Checker

Specific to the Actisense wireless gateway, there is a new feature called ‘Actisense-i’, which is a network diagnostic and monitoring tool, providing further insight into your NMEA 2000 network. This covers things from voltage monitoring on the network, through to displaying each device on the network. There is also the ability to monitor both the Transmit and Receive load percentage.

For a more comprehensive guide to NMEA 2000 and NMEA 0183, you can download our e-Books from here:  

NMEA 0183: https://actisense.com/everything-to-know-about-nmea-0183-guide/ 

NMEA 2000: https://actisense.com/complete-guide-to-building-an-nmea-2000-network/ 

We also have a knowledge base filled with various bits of technical info about our products, and a forum section where questions can be posted and discussed between users.