>> The real keyboards are PS/2 and I can't change that because it runs >> on a wire physically passing a /real/ firewall, [...] > >(a) Is it possible to run USB over the same conductors used by the PS/2 >cable? (This is a real question; I don't know enough about layer 1 of >either to answer it.)
Great question. The answer is "yes, if you're desperate". Keyboards are low-speed or full-speed USB devices, and the signaling is relatively non-critical -- especially if you find a low-speed keyboard. However: >(b) There exist devices that adapt PS/2 to USB in the >PS/2-keyboard-to-USB-host direction. Since the problem is that the real keyboards are PS/2, the adapters sound perfect. https://www.newegg.com/Mouse-Keyboard-PS2-Adapters/SubCategory/ID-3024 US$ 6.... pretty reasonable. Put it in the machine room, use the existing PS/2 keyboards, and... isn't the problem solved? If you need USB as well from another source, use a hub (in the machine room). But I confess I'm not clear about that part of what you're trying to do. --Terry