RS423 is somewhere in between RS232 and RS422 regarding transfer speed and connection distance. You of course need a line driver to convert TTL level signal from the controller to an RS423 compatible signal. For small distance connection (10 meters), I suggest to use standard RS232 for you have already build in that interface in your PC. Take a line driver like MAX232 or compatible chip for the controller.

The protocol is the 'language' that your PC and your controller are using to exchange data. For sending only keystrokes, make your own protocol as simple as possible. Use parity control to check for transmission errors.