Findings faults


The real beauty of NMEA-based systems is that they may be checked for performance, in particular when they fail to work as expected. Little hardware is required to check what they are doing, what data is transmitted and the quality of data. And in case of NMEA 0183 checking may be achieved by simple computer means.

Due to the technical similarity of NMEA0183 and the rs232 standard on computers, many PC with a 9-PIN sub-serial interface might be able to display NMEA data.

If no 9-PIN connector is available anymore, a serial-to-USB adapter might work as well instead. For displaying of data a cheap serial-to-USB adapter might suffice, but in some cases they have shown to fail so a specific high quality model should be employed.

On the PC either a simple terminal program is required (like Microsoft’s Hyper Terminal on Windows OS upto XP or). Alternatively ACTISENSE provides software for testing NMEA 0183 and N2K or you may look for an alternative elsewhere.

The computer port then has to be set in the software to 4,800 Baud, 8 bit, No parity, 1 stop bit (4800, 8, N, 1) or 38,400 Baud, 8 bit, No parity, 1 stop bit (38400, 8, N, 1) for AIS data. Then NMEA data should be visible on screen. Most terminal-type programs allow to save the data in a text file which can be sent in to suppliers to advise on problems.

Alternatively if NMEA data is already transmitted via a WIFI INmeaLogger will make display of data possible.

A little bit on data looks:
NMEA data is always coded by the unit identifying itself followed by an alpha-numeric data string. This string can be understood by looking up the individual identifier. There are numerous sites on the internet that explain NMEA coding in detail. A typical phrase from a GPS is shown here

$GPRMC,170546,A,5723.25,N,01201.22,E,000.0,357.1,290707,000.4,E*7E

where GP identifies the phrase as from a GPS, RMC stands for Required minimum information, positioning data is in degrees, minutes, seconds North and East and finished by a so called „checksum“.

Note, this checksum may be missing in data-sets sent by old NMEA 0183 equipment, ie sending data according to Version 1.X. Today’s software will most likely require the checksum. In that case one has no other way then to install a device that will add the checksum like the ACTISENSE NDC-4 Multiplexer or for transmission over WIFI the router made by vyachts installation should do this task (information by the manufacturer). This way we have made even 20 year old NMEA units become visible in the most state-of-the art software packages eg on an IPAD. In the contrary, most modern software packages allow to delete the checksum should this be required.