Back in the early days of USB, Microsoft mouse devices often came with a USB plug at the end of the cable, but also came with a small green adapter to convert the USB type-A plug into a PS/2 plug. How did this adapter work?

USB and PS/2 are completely different protocols that are not compatible in any way. The adapter was purely mechanical (passive). It connected one set of pins to another, but it contained no circuitry. All of the smarts was in the mouse.

The mouse detected whether it received USB-like signals or PS/2-like signals on the pins and changed its behavior accordingly. The mouse did all the work.

It’s similar to the inexpensive electrical outlet adapters which convert between different national plug types. There are no smarts in the adapter. It just connects one set of plugs to another. The actual intelligence is in the appliance itself (or the power brick for the appliance).

So if you find one of these adapters in your junk drawer, be aware that it is not a universal mouse adapter. It is just a physical adapter; the actual conversion happens in the dual-bus mouse on the other end.

Bonus chatter: An Internet search shows that people still make these types of mouse adapters. I don’t know whether the ones you can buy today are smart adapters that translate between the PS/2 and USB signaling protocols, or whether they are purely mechanical adapters that rely on the mouse to do the work.