Electrical – How to test a laptop keyboard from its ribbon cable

What you describe is basically the way the 8 bit micros used to do it, before keyboards all got dedicated microcontrollers (which just did the same thing anyway).

Human user keyboard input isn't very fast. If you scan the key matrix once every 20 msec, and make it a rule that you only count the key as being pressed if you see it pressed on two consecutive scans, you will eliminate key-bounce.

As I understand, you connect your Arduino to two different target machines and on one it works and on the other it doesn't.

So it seems there is a difference between the initialization requirements of the two machines. On this page at the very bottom there is a listing of a possible initialization sequence. Start by comparing your initialization to that one.

It will be a lot easier by using a logic analyzer. I am using the Intronix Logicport, but there are both cheaper and better ones, though not at the same time.

Tapping into an open-collector bus is a bit cumbersome because you don't see which device is talking. However, if you put in a series resistor at the end where the pullup is not, you can tell by the voltage level which device is holding down the bus. Every open-collector bus (like PS/2) needs pullup resistors, usually they are built in in the PC. You can see the different voltage levels easily on a DSO. With only a LA you have to record twice with different threshold voltages.