NMEA isolation

When designing DC-powered products for marine networks, isolation must be considered. It is not recommended to connect NMEA devices to a PC directly. Without some form of isolation, this practice opens up the risks of ground loops which can damage both the NMEA device and PC (we will unpack this later in the article). Whilst there are many electronic devices without isolation due to cost reductions or the belief that an old version of the NMEA standard allowed for that, there is general agreement that isolation in DC-powered systems is wise and arguably critical to reliability.

What is isolation and why is it important?

Isolation prevents the two biggest issues plaguing DC power systems and the devices on them; ground loops and ground potential differences. A ground loop exists when two or more paths to the ground connection of the ship’s DC battery supply are created. When these paths to the ground are at different potentials, which is easy to do with DC-powered systems, issues can range from intermittent communication problems to complete device damage.

Marine galvanic isolation prevents current flowing between two electrical circuits, whilst still allowing data to be shared through optical, mechanical or electromagnetic methods. Boat galvanic isolation gives the user peace of mind that ground potential differences and ground loops cannot occur so the resultant damage cannot occur either.

A small ground potential difference can be glimpsed as random data corruption, whilst a larger ground potential difference can prevent data communication entirely. Over time, depending on the size of the potential voltage difference, any device without marine DC isolation will become damaged. If you’re 80 miles from shore with no GPS because it suffered a complete failure, your appreciation of isolation will increase greatly!

Marine opto isolators (“Optos”) safely transfer data between two individually isolated circuits using light. In most cases, they are made up of an LED that converts electrical energy into light, and a photosensor such as a phototransistor that converts light into electrical energy.

So what does this mean?

Most boaters will want to connect their NMEA devices to their PCs at some point, but doing so safely is critical in order to protect your onboard equipment. We always stress the importance of creating a reliable marine data network, where isolation must be considered. Depending on your network requirements, we have different options to safely connect your NMEA devices to your PC.

NMEA 0183 to PC

We’ve noticed a few new PC connection cables on the market that offer no isolation. As this article has highlighted, it is certainly not recommended to connect NMEA devices to a PC directly without isolation. Therefore, to ensure full protection of both the NMEA device and your PC, we recommend using something like the USG-2, which has isolation on both the input and the output, as well as a USB connection to easily connect with the onboard PC.

Another option for isolation for PC interfacing is the Actisense OPTO-4, which provides isolation to the PC’s input but not the output. If your PC does not have a serial port, you will require a USB to serial adapter.

NMEA 2000 to PC

The Actisense NGT-1 is an NMEA 2000 to PC Gateway that allows you to easily transfer NMEA 2000 data from the NMEA 2000 network to a PC. The NGT-1-USB variant is selected in most scenarios (where the PC has a USB port) as it allows for ‘plug and play,’ as well as being fully isolated to prevent dangerous ground loops.

For more on connecting your NMEA devices to your PC, read our article here.

Our belief in the importance of isolation stems back to the founding years of our company. To discover more about our own ISO-Drive TM  technology, take a look at this article.