Richardco,

I've been down this path before. Its pretty tough to multiplex NMEA data with a pic and not loose data from at least one source.

You would be better off learning the basics before tackling this project!

Trust me, I've waisted hours trying to do this


The only thing I can suggest to you is rather than use the pic to serial receive / send the data, use the pic to control a digital switch (isolated).

The switch would then cycle and pass through each talker's NMEA data. You could also use the PIC to detect the CR/LF at end of each string to trigger the digital switch to cycle to the next talker.

Cheers
J