"8042" PS/2 Controller: Difference between revisions

'''WARNING:''' PS/2 was never intentionally designed to support hot-plug. Usually it is fine as most PS/2 controllers have reasonably robust IO lines, however some PS/2 controllers (mostly those in old chipsets) may potentially be damaged.

Despite the warning, most OSs (Windows, Linux, etc) do support hot-plug PS/2. It is also relied on by old "mechanical switch" KVMs (which allow the same PS/2 devices to be shared by multiple computers by effectively disconnecting the device from one computer and connecting it to the next).

When a PS/2 device is removed the PS/2 controller won't know. To work around this, when no data has been received from the device for some length of time (e.g. 2 seconds), an OS can periodically test for the presence of the device by sending an "echo" command to the device and checking for a reply. If the device doesn't respond, then assume the device has been unplugged.

When a PS/2 device is first powered up (e.g. when it is plugged in to a PS/2 port), the device should perform its Basic Assurance Test and then attempt to send a "BAT completion code". This means that software (e.g. an OS) can automatically detect when a PS/2 device has been inserted. ''Note: If a device is removed and then another device (or the same device) is plugged in quickly enough, the software may not have had time to detect the removal.''

When software detects that a device was plugged in it can determine the type of device (see above). If the device was the same type as before software can re-configure it so that the device is in the same state as it was before removal. This means that (for example) someone using an old "mechanical switch" KVMs doesn't lose state (things like keyboard LEDs, typematic rate, etc) when switching between computers. If the device is not the same as before or there was no previously connected device, then software may need to start a new device driver (and terminate the old device driver, if any).