One thing that people do to break the first CIA chip is to connect a gamepad from a Sega Master System or for the Mega Drive/Genesis. They work fine on other sy

One thing that people do to break the first CIA chip is to connect a gamepad from a Sega Master System or for the Mega Drive/Genesis. They work fine on other systems that accept "Atari-compatible" joysticks, such as the Atari VCS, Atari 8-bit, Amiga, etc. so you'd think they'd work fine here too, but no. Sega had put the pull-up resistors in the controller instead of on the motherboard, thus providing positive current on each input pins when a button/direction is not pressed. But for the C64, the inputs are supposed to be disconnected when not active, and when CIA #1 is reading the keyboard, the extra current could overload the chip. You could make an adaptor though with a diode on each input line. (The adaptor should also route Vcc to pin 5 which is Sega's Vcc pin)