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NMEA 0183 (or NMEA for short) is a combined electrical and data specification for communication between marine electronic devices such as echo sounder, sonars, anemometer (wind speed and direction), gyrocompass, autopilot, GPS receivers and many other types of instruments. It has been defined by, and is controlled by, the U.S.-based National Marine Electronics Association.
The NMEA 0183 standard uses a simple ASCII, serial communications protocol that defines how data is transmitted in a "sentence" from one "talker" to multiple "listeners" at a time. Through the use of intermediate expanders, a talker can have a unidirectional conversation with a nearly unlimited number of listeners, and using multiplexers, multiple sensors can talk to a single computer port.

At the application layer, the standard also defines the contents of each sentence (message) type so that all listeners can parse messages accurately.
Contents
1 Serial configuration (data link layer)
2 Application layer protocol rules
3 Vendor extensions
4 Software compatibility
5 See also
6 References
7 External links
Serial configuration (data link layer)
Typical Bit rate 4,800 Data bits 8 Parity None Stop bits 1 Handshake None
AIS units use a default baud rate of 38,400.
Application layer protocol rules
Each message's starting character is a dollar sign.The next five characters identify the talker (two characters) and the type of message (three characters).All data fields that follow are comma-delimited.Where data is unavailable, the corresponding field contains NUL bytes (e.g., in "123,,456", the second field's data is unavailable).The first character that immediately follows the last data field character is an asterisk.The asterisk is immediately followed by a two-digit checksum representing a hex number. The checksum is the exclusive OR of all characters between the $ and *. According to the official specification, the checksum is optional for most data sentences, but is compulsory for RMA, RMB, and RMC (among others).<CR><LF> ends the message.
As an example, a waypoint arrival alarm has the form:
$GPAAM,A,A,0.10,N,WPTNME*32
where:
GP Talker ID (GP for a GPS unit, GL for a GLONASS) AAM Arrival alarm A Arrival circle entered A Perpendicular passed 0.10 Circle radius N Nautical miles WPTNME Waypoint name *32 Checksum data
The new standard, NMEA 2000, accommodates several talkers at a higher baud rate, without using a central hub.

The NMEA standard is proprietary and sells for at least US$ 270 as of August 2007.[1] However, much of it has been reverse-engineered from public sources and is available in references like gpsd and Dale DePriest's.
Vendor extensions
Most GPS manufacturers include special messages in addition to the standard NMEA set in their products for maintenance and diagnostics purposes. These extended messages are not standardized at all and are normally different from vendor to vendor.

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