Assumptions are being made. Is your adaptor
rs232 or rs422? Wire accordingly, but it should be 422 as that is
nmea 0183 signaling - I believe without double checking.
422 is differential, connect both - & + wires if that is the adaptor type. If it is an
rs232 adaptor, you might just might, get away with connecting the 2 data - to ground. In all cases, connect ground from the device to the adaptor as it is likely passed through to the pc's USB.
The pc and rs232 are unrelated as you are using USB input. There hasn't been an rs232 port on a computer for at least 10, maybe 15 years.
Sorry if this is too much info.
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