msg wrote in news:13qo9l9paiau127
@corp.supernews.com:

A quick look at NMEA-0183 sentences shows blank data between
commas at certain field positions when no fix is available,
but I didn't see a description of completely empty data and
shouldn't each sentence begin with a dollar sign?


Anytime there is no data in a data field, you'll see ,, so the reading
instrument can count the commas from the header. NMEA is really a
primitive comma-delineated data field from way back in the DOS days. ,, is
the only way it can show a blank field in a system that counts commas from
the header without screwing up the count and putting the wrong data in the
wrong field at the terminal end.

,, just means "I have no data to report in this field.

HEADER,,,,,,,,,,,
would mean I have no data at all for this function.

Most of the equipment is too stupid to raise a flag and warn you there's no
data. It just simply sits and reads nothing with no explanation at all to
confuse the helmsman. To put in warnings would require us to hire real
programmers during development.