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This article is from the Boats FAQ, by John F. Hughes with numerous contributions by others.

5.19 Interfacing NMEA0183 to your computer

Lots of people want to know how to interface NMEA 0183 instruments to
their laptops or other computers. One answer is to do it directly: NMEA
data out -> RS232 data in, and NMEA data return -> RS232 ground. The
signal is 4800 baud, no-parity, 1 stop bit.


But here's a better answer, courtesy of Bob Curtis (bc@gate.net): Here's
a simple circuit to keep your instruments safe:

     a ----/"/"/"/"----+         +---+------/"/"/"/"------ +12v
              5k         _         _    _          5k
                        ---      _/     +-------------------- to RS-232 rcv.
                        / "      _
                        ---      _"
                         _         _
     b ----------------+         +------------------------ to RS-232 common
                                   _
                                   _ <- might not need this connection
    gnd ------------------------+

You will have 100% isolation if you leave off the ground connection shown
(recommended). Some systems may work more reliably with a common
ground. The parts (2-5k resistors and a photo-optical isolator) will cost
about $4 at any Radio Shack.

 

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