The Arduino uses a shift register to interact with the keyboard’s 8×10 matrix, and the sketch translates any serial data it receives into the keypresses the clone is expecting.

[Damian Peckett] has implemented an Apple ] [, its 6502 processor, and a cassette port, all on an Arduino Uno. If that wasn’t enough, he also uses a PS/2 keyboard for input and outputs analog VGA.

Arduino Uno How a cheap microcontroller is making it possible for anyone to design and build hardware.