A KVM seems like overkill for only 2 boxes but if you already have it, I guess it makes sense. KVM heads (the part that connects to your pc) are available in USB that is what we for our USB boxes.
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But then I would have to buy USB to PS2 converters for the primary machine (PS2 inlets) which I guess would not work.
Ok, maybe I’m not clear on your setup. My KVM has a bunch of ports that connect to cables which are mechanically Ethernet cables, those cables connect to adapters that connect to the test machine. We call them “heads” and we use different ones depending on whether we want to connect to the USB or PS2 ports of the test machine.