h2obo wrote:the adaptor does not work. The reason, I believe, is that NMEA data works with 5v logic and the USB-to-TTL console adapter uses 3.3v logic.
Partially, but perhaps not sufficiently, correct.
I'll try to restate it more clearly. We are talking about representing data by a series of symbols that can have one of two states - Mark or Space; True or False, etc. Binary.
TTL represents these two states with 2 voltages: 0v for one of them, and the 'logic level' - in your example 3.3v - for the other.
RS232 also represents these two states with 2 voltages: in this case (using 5v logic as the example) +5v (a positive signal wrt ground) as one state, and -5v (a negative signal wrt ground) as the other. RS232 does not recognise 0v (ground) as a valid signal state.
Depending on implementation, sometimes you can get away with connecting TTL and RS232 signals together. But more often than not it will not work, or will not work reliably. And there is a risk that a high voltage rs232 signal could permanently damage a ttl receiver circuit.