The National Marine Electronics Association

The National Marine Electronics Association is a non-profit association composed of manufacturers, distributors, dealers, educational institutions, and others interested in peripheral marine electronics occupations.(from "NMEA News")

 An NMEA standard defines an electrical interface and data protocol for communications
between marine instrumentation. These standards allow a single "talker", and several “listeners" on one circuit.  The recommended interconnect wiring is a shielded twisted pair, with the shield grounded only at the talker.  The standards do not specify the use of any particular connector.

The NMEA-0180 and 0182 standards say that the talker output may be RS-232, or from a TTL buffer, capable of delivering 10 mA at 4 V.  A sample circuit shows an open collector TTL buffer with a 680 ohm resistor to +12 V, and a diode to prevent the output voltage from rising above +5.7 V.

 NMEA-0183 accepts this, but recommends that the talker output comply with EIA-422.  This is a differential system, having two signal lines, A and B. The voltages on the "A" line correspond to those on the older TTL single wire, while the "B" voltages are reversed(while "A" is at +5, "B" is at ground, and vice versa)

In either case, the recommended receive circuit uses an opto-isolator with suitable protection circuitry.  The input should be isolated from the receiver's ground. In practice, the single wire, or the EIA-422 "A" wire may be directly connected to a computer's RS-232 input.

NMEA-0183  General Sentence Format

 Under the NMEA-0183 standard, all characters used are printable ASCII text (plus carriage return and line feed).  NMEA-0183 data is sent at 4800 baud. The data is transmitted in the form of "sentences".  Each sentence starts with a "$", a two letter "talker ID", a three letter "sentence ID", followed by a number of data fields separated by commas, and terminated by an optional checksum, and a carriage return/line feed.  A sentence may contain up to 82 characters including the "$" and CR/LF
The standard allows individual manufacturers to define proprietary sentence formats.  These sentences start with "$P", then a 3 letter manufacturer ID, followed by whatever data the manufacturer wishes, following the general format of the standard sentences.

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