With reference to the OSI communications model: RS-232 and RS-422 are layer 1 (physical layer) electrical specs. The signal patterns on those layer 1 interfaces (e.g. data rate, parity, stop bits, etc.) are layer 2. The application data elements (meaningful sentences of data displayed by human interface devices) are all the way up at layer 7. You can easily convert layer 1 with electrical devices. The higher you go up the OSI stack, the more complex it is to convert.

http://en.wikipedia.org/wiki/NMEA_0183