NMEA instrument interfaces

PC Active (NMEA/RS232) Interface Cable

If your system was supplied with a standard NMEA interface cable (9 pin D type connector and flying leads), then in all probability it will work well. However in noisy electrical environments or with long cable runs problems might be encountered. A solution is to use an active opto-isolated cable, this also reduces the risk of damage to your laptop during electrical storms (but can never fully protect)

Most personal computers communicate with the outside world via a protocol called RS232. NMEA and RS232 operate on slightly differing signal voltage levels and with some older equipment there is a danger of unreliable communication and possible damage to the computer serial port unless a suitable interface cable is used. Some GPS manufacturers supply suitable cables with their devices – if you have one of these, do check that it is fully opto-isolated and that it protects against reverse polarity before using it.

The interface cable Euronav supplies allows bi-directional transfer between NMEA and RS232 protocols, typically allowing data to come in to the PC from a GPS receiver or other instrument system, and simultaneously be transmitted to another device, such as an autopilot. As well as converting voltage levels between NMEA and RS232 levels, and protecting against reversed polarity, the listener (input) side is opto-isolated as per the NMEA specification. The electronics may be powered from the PC’s serial port, or from the boat’s 12V DC power supply (if your PC is unable to provide sufficient power). The electronics are housed in a 9-pin "D" connector, ready to connect to the serial port of most PCs; the 1m shielded cable has wires at the other end to connect to your NMEA device(s) and to a 12V power supply (if required).

Note: You will still need the appropriate cable to connect to the NMEA cable to your GPS,as this is specific to each GPS model. Top>>

NMEA Booster - for long distance cable runs and multiple instruments

Whilst the output from an NMEA device can be split to drive several others, eventually the signal becomes too weak and the data does not get through. This is increasingly common with the more complex systems found on larger yachts and in commercial installations. An NMEA Booster increases the signal strength, allowing a greater number of devices to be connected. The electronics are housed in a sealed box, with cable glands for NMEA input, 4 NMEA outputs, and a 12V DC power supply.Top>>

NMEA0183 – General Information

One of the main benefits of using computers and electronic instruments aboard boats is to take advantage of their ability to process, store and transfer information. Many marine electronic devices perform these functions and it has been typical that different instruments from the same manufacturer could communicate information between themselves, using a proprietary language, or "protocol".

This ability for them to communicate may make the instruments far more useful than if they were not connected, but serves to lock the purchaser into buying instruments from one particular manufacturer. Interfacing was also a headache for the marine electronics industry in general, which knew it was important that users could interface their equipment in a reliable and simple way, regardless of who made it.

A standard protocol developed which was adopted and defined by the (US) National Marine Electronics Association. This has been refined over the years and the current incarnation is version 0183. NMEA0183 is a combined electrical and data specification for communication between marine electronics and also, more generally, GPS receivers. (NMEA0183 is often pronounced "nema" for short since earlier versions are seldom found in use these days).

Data is transmitted from one compatible device (a "talker") to another (a "listener") as electronic text in the form of "sentences" which have a standard structure. By conforming to the standard, manufacturers can ensure that their instrument is able to read and understand information taken from other instruments and to send information back to them that they will understand.

In addition, there is scope for manufacturers to add-on their own proprietary sentences, thus, the now very commonly available marine instrument "network" systems tend to be based on the NMEA protocol, with enhancements to allow them to perform more sophisticated functions. This is very important for users of PC based electronic charting systems, since it means that the plotting system can understand information provided by a wide variety of instrument systems.

Although in theory, it is possible to "get away with" a simple two-wire connection, NMEA0183 is designed to protect devices and to minimize interference problems by having the data input opto-isolated (further interference protection is provided by specifying the use of shielded twisted pair cabling). For this reason, we strongly recommend the use of an NMEA/RS232 converter.

Finally, NMEA0183 compatibility does not guarantee that systems can talk to each other. There are many sentences available, so the same data can be transmitted in many different ways. Also, within a sentence, not all of the data fields need to be filled in, so you cannot always check comparability by looking at the lists of sentences transmitted/received.

In general though, the kind of information used by a chart plotting system (e.g. Lat, Lng, Course over the Ground, Speed over the Ground etc.) is reliably supplied in correct standard formats by the bulk of instruments available today.

When evaluating chart plotting systems, make sure that the one you choose has sophisticated control over the computer’s serial ports and how it handles different NMEA sentences. It should also have a "monitor" function to allow you to see what sentences are coming into your plotting system – in practice successful interfacing may require some trial and error, and less sophisticated systems may prevent you from achieving this.Top>>