In the recently discussions about my problem with the interface of my
autopilot with my GpsMap 276C, hapened discussions about NMEA, DSC and
AIS and to me this was very good to learn a litle bit about these
things, but now, I must confess that I am totally lost on this topic of
NMEA.

For example, I only knowed that NMEA 183 has versions 1.5, 2.0, 3.1
and was used to interface my simple gps with my old autopilot, the
speed was 4,800 bps and the messages were ASC II characters.

Now I am learning that this animal has a protocol type the old
start/stop, using CR/LF and "silence" bytes, each byte has 10 bits
instead of 8, it does do not alows more than 1 talker, can send
messages to no more than 3 listeners, the actual gps/ploters wich have
"Two NMEA Serial Ports" can not use more than one set as NMEA, some
says that NMEA 183 is only 4,800 bps, others says that DSC/DSCE are
NMEA 183, but uses 9,600 bps, but others says they DSC interface is
working well at 4,800, one says that AIS uses NMEA 2000, other says
that AIS is NMEA 183, but uses 38,400 bps, no one can show me what are
the format of the "secret" DSC, DSCE, AIVDM messages, no explain me how
NMEA can use only one port and can work at 4800, 9600 and 38400 bps?

Could the NMEA experts please, teach us here a litlle more, just about
NMEA 183 (please not NMEA 2000 yet) so that a simple sailor with an
autopilot, a VHF radio and dreaming of purchase an AIS receiver to plot
the ships in his Garmin 276C gps/ploter could understand?

Thanks


Pascal