I've been trying to get a couple of old PCs and servers running recently. I have (or at least plan to have) them connected up to a KVM switch so I don't have to faff around plugging and unplugging the keyboard / video / mouse when I want to switch to a different machine.
But the Pentium 4 era Windows XP machine doesn't seem to accept the PS/2 mouse when it is plugged in via the KVM. Plug the same mouse directly into the PS/2 port on the PC and it is fine, just not via the KVM. I initially thought it was a dodgy cable (I checked plugged one of the servers into the same ports on the KVM, and the mouse worked fine, so I knew it wasn't the KVM). So I purchased another couple of cables, which arrived today - but still the same issue.
I've now found the cause / fix for this, so I thought I'd post it here. In the BIOS Advanced > Peripheral Configuration > Mouse Contoller had to be set to Enabled.
The BIOS on the particular machine gives a helpful description of the meaning of this setting. You can set to Disabled to disable the PS/2 mouse port and free up an interupt input (IRQ). Mine was set to Auto, which is meant to free the interupt input if no mouse is plugged in, but use it for the mouse if one is detected. So it must be that due to the KVM, the mouse prescence wasn't being detected and so the PS/2 mouse port was being disabled. Changing the setting to Enabled (always enable to PS/2 port for mouse) fixed the issue.
