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NMEA 2000 decoder in Erlang

An NMEA 2000 decoder in Erlang. Decodes the following formats:

  • Yacht Device's RAW format
  • Yacht Device's DAT format
  • Yacht Device's CAN format
  • CANBOAT's PLAIN format (CSV)

The program bin/n2k can be used from the command line to convert or pretty print files with NMEA 2000 frames or messages in one of the supported formats.

The functions n2k_raw:decode_raw/1 and n2k_csv:decode_csv/1 can be used to decode captured packets into NMEA 2000 frames. The frames can be decoded into messages by calling n2k:decode_nmea/2.

Build

Handling of PGNs

At build time, src/canboat.xml and any user-defined PGNs XML files are first compiled into src/pgns.term and then src/pgns.term is compiled to src/n2k_pgn.erl, which contains code for decoding NMEA 2000 binary messages into Erlang terms.

Here's an example of the generated code:

decode(130306,
       <<Sid:8/little-unsigned,
         WindSpeed:16/little-unsigned,
         WindAngle:16/little-unsigned,
         _6:5, % reserved
         Reference:3/little-unsigned,
         _/bitstring>>) ->

    {windData,
      [{sid,chk_exception2(255,Sid)},
       {windSpeed,chk_exception2(65535,WindSpeed)},
       {windAngle,chk_exception2(65535,WindAngle)},
       {reference,chk_exception1(7,Reference)}]};

%% {int, Resolution, Decimals, Unit}
type_info(windData,windAngle) ->
    {int, 0.0001, 4, rad};
type_info(windData,windSpeed) ->
    {int, 0.01, 2, 'm/s'};

User-defined PGNs

Custom PGNs (or proprietary PGNs that are not part of canboat's XML) are defined in the same XML format as canboat uses.

For some PGNs, we can't decode with only the canboat.xml definitions, but we need additional code to perform the decoding. If this is the case, write an erlang module that implements the behavior n2k_pgn_callback (see that module for details).

In order to compile the custom PGN definition files, add a file system-config.mk to the top directory, and define the following variable:

CUSTOM_DIR = path/to/dir

Place custom PGN definition files and erlang files in this directory. The custom PGN definition files must be on the form *-pgns.xml.

If some PGN needs an additional erlang module, set CUSTOM_PGN_ERL:

CUSTOM_PGN_ERL = <pgnid>:<erlangmodule>(,<pgnid>:<erlangmodule)*

For example:

CUSTOM_PGN_ERL = garminCustomChannel:x_n2k_garmin

Control the size of the generated code

By default, all manufacturer proprietary PGNs are compiled into n2k_pgn.erl. By setting the variable MANUFACTURER_CODES in system-config.mk, this behaviour can be tweaked. The variable must be on the form all | none | <integer>,<integer>,.... For example, to generate code for Airmar and Garmin only:

MANUFACTURER_CODES = 135,229