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NMEA 0183

NMEA 0183 is a combined electrical and data specification for communication between marine electronics such as echo sounder, sonars, anemometer, gyrocompass, autopilot, GPS receivers and many other types of instruments. It has been defined by, and is controlled by, the National Marine Electronics Association. It replaces the earlier NMEA 0180 and NMEA 0182 standards. In marine applications, it is slowly being phased out in favor of the newer NMEA 2000 standard.

The electrical standard that is used is EIA-422, although most hardware with NMEA-0183 outputs are also able to drive a single EIA-232 port. Although the standard calls for isolated inputs and outputs, there are various series of hardware that do not adhere to this requirement.

The NMEA 0183 standard uses a simple ASCII, serial communications protocol that defines how data are 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.

There is a variation of the standard called NMEA-0183HS that specifies a baud rate of 38,400. This is in general use by AIS devices.

As an example, a waypoint arrival alarm has the form:

Another example for AIS messages is:

The new standard, NMEA 2000, accommodates several talkers at a higher baud rate, without using a central hub, or round-robin packet buffering.

Typical Baud rate 4800
Data bits 8
Parity None
Stop bits 1
Handshake None
ASCII Hex Dec Use
<CR> 0x0d 13 Carriage return
<LF> 0x0a 10 Line feed, end delimiter
 ! 0x21 33 Start of encapsulation sentence delimiter
$ 0x24 36 Start delimiter
* 0x2a 42 Checksum delimiter
, 0x2c 44 Field delimiter
\ 0x5c 92 TAG block delimiter
^ 0x5e 94 Code delimiter for HEX representation of ISO/IEC 8859-1 (ASCII) characters
~ 0x7e 126 Reserved

  • All transmitted data are printable ASCII characters between 0x20 (space) to 0x7e (~)
  • Data characters are all the above characters less the reserved characters (See next line)
  • Reserved characters are used by NMEA0183 for the following uses:
  • Messages have a maximum length of 82 characters, including the $ or ! starting character and the ending <LF>
  • The start character for each message can be either a $ (For conventional field delimited messages) or ! (for messages that have special encapsulation in them)
  • 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 remains blank (it contains no character before the next delimiter – see Sample file section below).
  • The first character that immediately follows the last data field character is an asterisk, but it is only included if a checksum is supplied.
  • The asterisk is immediately followed by a checksum represented as a two-digit hexadecimal number. The checksum is the bitwise exclusive OR of ASCII codes 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.
  • GSV records, which describe satellites 'visible', lack the SNR (signal–to–noise ratio) field for satellite 16 and all data for satellite 36.
  • GSA record, which lists satellites used for determining a fix (position) and gives a DOP of the fix, contains 12 fields for satellites' numbers, but only 8 satellites were taken into account—so 4 fields remain blank.


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