Turing wrote:

I am looking at KVMs on EBAY - will choose DVI this time. I notice
the pics show only one USB to support mouse and keyboard. How do they
get away with that?

Multiple devices can connect to the same USB port. That is how, for
example, passive USB hubs work: many ports to one port. The USB port
can only support a fixed amount of power, so how many USB devices you
can connect to the port depends on whether they are low- or high-power
USB devices. Keyboards and mice are low-power USB devices. A single
USB controller can manage up to 127 devices (7 bit address = 2^7
addresses = 128 addresses, but the host controller gets one). To get
that many chained together means having to use powered USB hubs (they
have their own source of power, like an A/C adapter). A hub itself
counts as a device. While I've seen 10-, 20-, and 30-port powered USB
hubs that you could chain together (four 30-port hub + one address for
each hub = 124 addresses) at newegg.com which get pretty pricey, I
didn't bother hunting around for one with 126 ports (126 ports + 1 for
itself for the 127-port maximum). I did happen to find the following:

97-port USB hub
https://www.youtube.com/watch?v=4OS6L80Fe3Q
16 7-port powered USB hubs = 112 ports minus 16 ports to chain them
together = 96 ports, so I don't know how they get a count of 97 unless
they're counting the downstream port+cable to the computer. Whether
such a frankenjob works on your computer depends on its USB host
controller's capabilities. As the video show, plug-n-play detects takes
a long time to discover all those ports.

Remember those old movies showing a telephone switchboard manned by many
telephone operators? Large-numbered USB ports chained together would
make the same mess. You'd probably want Bluetooth or wi-fi devices
instead of the cost of such a hugh-ported USB hub and all those USB
cables and the space for it all.

You didn't give a URL to the KVM with just one USB port for both
keyboard and mouse. Perhaps in its description is listed a Y-adapter
cable: 2 USB ports to 1 USB port. Those have been around a long time.
I remember using one for an external USB HDD that needed to use 2 ports
because the drive needed more power than one port could deliver. In
that case, I was using 2 USB ports for 1 device. In your case, the
Y-adapter would go the other way: 2 devices to 1 port. Maybe the KVM
comes with a passive or powered hub that lets you connect more than 2
input USB devices to the KVM. We don't know because you didn't show us
(by giving a URL). Tough to explain something that is vague.