I posted this topic on what seems to be the closest thing to an official Gigabyte forum,
http://forums.tweaktown.com/f69/. The resident electric guru posted the following reply:
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It sounds to me like the IBM keyboard is either drawing too much current for the adapter and/or the motherboards USB ports. Or through a combination of the above the adapter is sending corrupt Data via the USB Bus.
Without knowing what the max current draw is of the IBM keyboard in question and this article you linked too:- Model M keyboard - Wikipedia, the free encyclopedia states:-
"Sometimes it may be problematic to get them to communicate properly with a modern computer. A PS/2 to USB adapter would be necessary for computers without PS/2 ports, and the AT Model Ms additionally require an AT to PS/2 adapter. Some PS/2 to USB adapters are unreliable because the Model M tends to draw more power than some adapters expect. In response to these complaints, Unicomp added USB models to their lineup."
So as per my comments above, either the adapter is "falling over" due to the high current draw of this keyboard and or it could be exceeding (keyboard and adapter combination or even have a fault) the USB 2.0 current standard of a maximum of 500ma Universal Serial Bus - Wikipedia, the free encyclopedia
Either which way, i am slightly baffled why you wish to use a 20 year old keyboard that has been documentated as being troublesome when used with modern hardware and via an adapter.
Whilst it might be interesting to know what is specifically causing the "fault", the combination of a keyboard that wasn't designed for USB, using an adapter per-se, using an adapter with an adapter unfriendly keyboard makes it more likely that you've actually created a problem, rather than unexpectedly experiencing a fault.
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Try other adapters, or another version in the line of these keyboards?