Monday, June 25, 2012

NMEA Reboradcasting, again

This post is more like a question...
This is always the same problem, most of the NMEA devices use a Serial Port, which is always accessed exclusively (i.e. by no more than one program at a given time).
The way around this exclusive access is to have the exclusive reader to rebroadcast the NMEA data using another channel, accessible from several other programs. This other channel can be HTTP, TCP, UDP... Those channels are machine, system and language agnostic. From Java to Java, RMI is also an interesting alternative.
SailMail reads the NMEA Stream to get the position of the boat, and thus is able to find the best SailMail station, provides the possibilty to rebroadcast the data it reads. Available channels are TCP, UDP, and even COM (rebroadcast on another serial port). It rebroadcasts the NMEA Sentence as it has been read, assuming - appropriately in my humble opinion - that if it is NMEA you are interested in, you know how to parse it.
There is also something else, which OpenCPN is aware of, this is GPSd.
It runs as a daemon - on the systems where daemons exist - and is only good for GPSs, not all the NMEA messages, because it's rebroadcasting the data after reworking them in another format, some proprietary classes over json (JavaScript Object Notation).
The GPSd protocol is not exactly documented, and the project leader does not return my emails...
In the NMEA Console, you can read any of those channels (Serial, TCP, UDP, RMI) and rebroadcast on HTTP, UDP, TCP and RMI (simultaneously if needed). I'm also working on a GPSd rebroadcast, running anywhere Java runs (no need for a daemon).
Anyway, both approaches have pros and cons (even if I'd rather rebroadcast the data as they come).
If anyone has any comment or idea, please speak up!
Thanks in advance.

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