I am quite aware you can get ready made off the shelf adaptors to connect old ps/2 keyboards to USB. But I am curious as how would one do it themselves? I have tried just rewiring an old keyboard to a USB cable as they both have 5V and ground and 2 data connections. Though USB has a differential DATA +- and ps/2 has a data and clock wire.

So after trying a basic rewiring it didn't work so there has to be some conversion going on but what? The puzzling thing is going from USB to PS/2 works but going the other way doesn't work. Why?

Edit: Ok so in some more research, I think I understand the why now. As USB devices need to be registered and be able to give data to the OS about what it is when requested by the OS. ie. report back a VID and HID code. Although all the 'scan codes' that the keyboard sends to the system should still work with modern systems. As this was based on the original IBM PS/2 (AT) keyboards and this protocol has been carried though to today.

As to the how... still more research needed.