NMEA information

NMEA stands for "National Marine Electronics Association", the organization that has defined standards for exchange of data between marine instruments. “NMEA 0183 ” refers to the standardized data-format consisting of so-called NMEA sentences which contain data-strings in ASCII-code. Values are separated by commas and sentences are terminated by CR-LF, which makes it easy to display or print. NMEA 0183 also defines the electrical interface and the serial transmission method.

The text of all NMEA standards is copyrighted. Copies of the standards can be purchased directly from the NMEA. The address is:

NMEA National Office
P.O. Box 3435
New Bern,
NC 28546-3435
USA


Brief description.

NMEA 0183 defines the standard for  one-way serial data transmission from a single TALKER (transmitter) to one or more LISTENERS (receivers). Listeners and receivers may be marine instruments, computers, chart plotters, etc. Many devices are transmitters and receivers with one or more input and output ports.

Multiple listeners may be connected to a transmitter (talker) port, but transmitters cannot share the same connection, as this would result in data collisions and garbling of the data.

The data is in printable ASCII form and is transmitted in "sentences" with a length of up to 80 characters.

Sentences may be terminated by a checksum, which should be re-calculated by the "listener" and if not equal to the transmitted checksum, the data should be ignored. The type of error checking is LRC.

Sentences of most talkers are sent with a frequency of once per second (1Hz), but some talkers such as electronic compasses may send at the higher frequency of 5 or 10 Hz.  Sentences are retransmitted even if the data does not change, so occasional transmission errors do not affect the overall performance of the communication.

The contents of sentences depend on the type of talker.  For example GPS sentences will contain data such as latitude and longitude, depth sounders transmit water depth in feet or meters, etc.

Each sentence starts with a $-sign, followed by a 2-character mnemonic that defines the type of talker and a 3 character mnemonic that defines the type and format of the sentence. AIS sentences start with !AI. 

The transmission line between transmitter and receiver(s) is 2-wire and should meet the RS422 standard. However, for practical reasons many manufacturers do not strictly adhere to this standard. In most cases electrical compatibility is not a big problem.   

Detailed NMEA 0183 information

Peter Bennets webpages have been a popular source for general NMEA 0183 information for many years:

vancouver-webpages.com/peter/nmeafaq.txt

More useful NMEA information including  sentence formats can be found on the following website:

http://www.gpsinformation.org/dale/nmea.htm