NMEA 0183 is RS422 which is different to RS232, actually if everything is working within their respective specs then the RS422 will not provide the voltage levels for the RS232. Luckily modern RS232 chips work beyond the spec and can cope with the RS422 signals. I have never yet found an RS232 connection not to work with an RS422 input but it is not guaranteed.
You can get isolating RS232 to RS422 cables with a DB9 connector already fitted. This will also give you protection against any potential ground loop problems.
It should be noted that you shouldn't use the B/- wire if your device has one. This is not compatible with the RS232 connection, just use the ground (pin 5) and the A/+/Data (pin 2) wires.
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