For the IBM compatible: Use the Roland serial port interface cable (RSC15AT). Put the selector switch on the KC20 to "I". Make sure to install the Roland serial port driver on the computer.

For the Mac: Use a standard serial cable and put the KC20 selector switch to "M".

It is important that you use the Roland style serial interface cable. I have found several other similar cables that do not work and the only tried and true one was the Roland style.