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IS there a necessity for the mouse to know a different protocol or a different pinout to be able to be connected to a PS/2 port ?

Many vendors provide USB - PS/2 adapter, but some don't.

From what I know PS2 and USB interfaces even have a different number of pins. How would such an adapter work ?

Will the device have the same performance as before ?

Is it necessary for the USB device to 'know' it is connected to a PS/2 port ? I suppose it is necessary since USB to PS/2 adapters are just that : adapters. not converters. They do not do anything to the signal so probably the keyboard or mouse should know and adapt their signal accordingly.

And another question: Is there any latency added when connecting a USB keyboard or mouse to a PS/2 port using an adapter ? I suppose the adapter itself does not add latency since it only connects some pins to some others, but inside the mouse or keyboard something has to change. The mouse has to switch to PS/2 data serialization instead of USB. In theory PS/2 should be faster but I am thinking that if that particular device is made to work naitvely on USB it could have sort of internal signal converter which may generate some latency.

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These adapters are purely mechanical. There is no converter or whatever inside. So, yes, the mouse/keyboard needs to do PS/2 by itself.

Whether the controller IC inside the input device will perform worse with PS/2 will depend on how the protocol switch is made.

It is unlikely that PS/2 will perform noticably different than USB. From just looking at the connection specs, USB is vastly superior, both in speed and latency. Only USB host controller inefficiencies could lead to a worse overall experience.

PS/2 and USB share some properties that make an adapter feasible:

  • 5 V
  • Low current
  • 2 “data” wires (though one is clock on PS/2).
  • (two pins on PS/2 are unused)
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    actually PS/2 ports have a lower latency than USB due to the fact that they send the signals directly to the processor via interrupts signals which is WAY WAY more efficient than the way USB connects to the processor. A lot of people say that there is no difference which is bs if you are into gaming because 10 ms difference is huge there. If you check your emails it does not matter sure. The reason I asked this question is because I want to connect my peripherals to the PS/2 and I was curios about the internal circuits that make it possible to communicate to both USB and PS/2 – yoyo_fun Nov 27 '16 at 11:59
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    @Daniel,u write "It is unlikely that PS/2 will perform noticably different than USB. From just looking at the connection specs, USB is vastly superior, both in speed and latency" <--- Looking back to when we had computers with both USB and Ps2, it was clear Ps2 was vastly superior, much faster, much more responsive.I remember slower responsiveness hitting num lock on and off.. it was available on ps2 right from the start after num lock scroll lock and pause/break had flashed which was straight away. With USB it took a while to get itself together, and even then possibly not as responsive. – barlop Nov 27 '16 at 12:02
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    Sorry, come back with the "USB and PS/2 are equivalent" argument when USB keyboard interfaces start supporting n-key rollover. – Cody Gray Nov 27 '16 at 17:36
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    @barlop I have at least three keyboards that didn't work with an adapter: MS Natural Keyboard 4000, Cooler Master Quickfire TK Stealth and some noname keyboard. And even if all keyboards were to support it, my statement would still be 100% true. – Daniel B Nov 27 '16 at 21:45
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    The USB HID standard supports NKRO. But it requires keyboards to boot with 6KRO for the sake of lazy BIOS programmers, and doesn't require them to implement NKRO. So lazy keyboard programmers take the path of least resistance. I think. en.wikipedia.org/wiki/… usb.org/developers/hidpage/HID1_11.pdf – Matt Nordhoff Nov 27 '16 at 22:08
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I can definitively answer the first part of the question,

"IS there a necessity for the mouse to know a different protocol or a different pinout to be able to be connected to a PS/2 port ?"

The answer is yes, a USB HID device (mice and keyboards) must understand the alternative PS/2 protocol in order to function in PS/2 port using USB-PS/2 adapter.

Not every USB mice or keyboard support the PS/2 mode. Presence of the green adapter in the OEM package warrants this compatibility, but the opposite is not true - some HID devices do support PS/2 mode even if the adapter is not included with the product, some don't, all depending on IC in use. For example, the Microsoft Notebook Optical Mouse P/N: X08-86056 does not work in PS/2 port.

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