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Isn’t the latency lower on ps/2 than usb?
Not necessarily, modern having keyboards can operate at upwards of 1khz polling speed. The main difference is, that usb needs polling in the first place, whereas ps/2 is interrupt based.
That essentially means that ps/2 tells the computer "hey a key was just pressed" instantly and usb waits to be asked whether a key was pressed or not. If the latter is done often enough the difference becomes negligible.
Higher than that, I have a keyboard with 8000hz polling and a mouse with 4000hz polling. Anything over maybe 2k at most is hard to perceive, at least for me
I think a high polling rate matters more with N-key rollover (lots of successive keypress messages in one bunch), which in turn matters for some gaming scenarios.
Afaik PS2 juat injects inpits straight to the CPU
but the protocol is slow. with high speed usb and fast polling rates, even though ps/2 starts sending instantly, usb will often have had time to poll and send its whole packet before ps/2 has finished sending.
Not necessarily. Even though PS/2 operates with a superior protocol, latency-wise, the clock speed is atrocious, resulting in an effective polling rate of about 1500hz, give or take. We could account that it doesn't need to wait for request to send keystrokes like USB keyboard do, effectively doubling it even more, but then we'd have to account for whatever delay Super I/O chips introduce and I'm not qualified to talk about that. But, if your keyboard is not from a dollar store shelf then it probably runs on at least 1000hz, at which point we are talking about sub-millisecond differences which would be quite hard to notice. 4000hz keyboard definitely beats PS/2 though.