I have configuration like this:
USB keyboard ---> USB-to-PS/2 adapter ---> PS/2 port.
Below is the image of the adapter.
My question is which scancode will be produced at the end of the chain? Will it be an USB scancode or PS/2 scancode (set 2 probably)?
After reading online, I'm not clear on this. Some say that the adapter has a chip which does USB to PS/2 translation, and others say that it is a straightforward pass-through connection.
If your "USB keyboard" does work in this combination (over PS/2 port), it means that this is a special "dual-mode" keyboard. It is designed to understand both USB 1.1 LS protocol and the alternative PS/2 protocol. Not all keyboards are made this way.
The adapter doesn't have any chip inside, it is just a passive connector. The keyboard switches automatically depending on what kind of initial signaling it sees from the port.
Not sure what do you mean by "scan code", but the port is PS/2, and operates as PS/2. It is the keyboard that enters the alternative PS/2 mode. and therefore work with PS/2 scancode SET. See also this SE EE post.
