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Originally Posted by David M  Nice writeup scotte! It clarifies some things I was unaware of....thanks. 
I just wanted to add that with a NMEA 0183 multiplexer you can take two or more sources that generate NMEA 0183 data and send that data to a single display unit. In that sense, it is possible to combine two NMEA data streams into one steam. This of course means one direction only.
I have heard that NMEA 0183 is an ASCII sentence but with a higher voltage...is this true scotte? |
You're mixing protocols and hardware. Above he says 183 is speced as RS-422. That's hardware which included voltage specs. What pattern the 1s and 0s is defined as is another matter. So if you send 100 0001, that means A in ascii. There is also number of start, stop and parity added to the protocol. EBIDIC is an example of another code. 183 uses ascii code.
ASCII - Wikipedia, the free encyclopedia
The next level is that only certain patterns of letters and numbers are recognized as valid sentences in 183. If you send a love letter from your computer to an NMEA listener it's going to be confused.
http://vancouver-webpages.com/peter/nmeatype.txt