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I haven't had a PS/2 mouse in a long time, but I thought I remember that if you unplugged a PS/2 mouse and then replugged it back into the PS/2 port while the computer was on, that the mouse would no longer respond.

I'm curious if this is true or not because someone I know is telling me they're having issues with their PS/2 mouse and I think they may be plugging it in/out while the computer is running. They're running Windows XP.

Also, if the mouse no longer responds, why is this? Why does't a USB mouse NOT have this problem?

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USB is designed for hot-plugging. Quite often, PS/2 mice talk directly to the keyboard controller; not only does it often not initialize the mouse when plugged in after system boot, it doesn't and can't notify the system that the mouse has been connected. Here's some discussion from the Linux Kernel mailing list about hotplugging PS/2 devices which I recall leading to the kernel explicitly ignoring any attempts to hotplug PS/2 devices for a while. I don't know the situation with respect to Windows in detail, but as many of the problems are rooted in BIOS-level initialization and the keyboard controller chip on the motherboard, it's not appreciably different.

On cheap machines, it may also not have a hardware buffer, which in extreme cases could lead to the motherboard or mouse electronics being damaged when the mouse is plugged/unplugged.

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this applies to keyboards too! I have a desktop computer and a laptop on my desk and it is quite frustrating if I switch on the PC and then realize the keyboard is connected via USB/PS2 converter into laptop. Then I have to shut down the pc again using just mouse, connect the PS2 keyboard to it and switch it on again to have keyboard working :-\ Putting it into standby and connecting the keyboard does not make it work either. – Vitas May 10 '15 at 18:33

PS/2 mouse unplugging depends a lot on both the machine and the operating system.

PS/2 was always designed to be a permanently connected type device while USB was designed from the get-go as a removable device connection and all the devices made respect that. As such a PS/2 device when unplugged and replugged can be in an indeterminate state and may require some reinitialisation to make it work properly and this would typically be done by the BIOS as the computer is starting up, with USB this initialisation is done every time the device is connected by design.

I believe that Vista and onwards are much better with hotplugging PS/2 keyboards and mice but it is not guaranteed to work on any given machine.

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It's all explained here:

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Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. – BloodPhilia Apr 5 '11 at 22:00

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