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Using USB As A KVM Switch?

Posted by Cliff on from the can-this-be-done? dept.

Don Faulkner asks: "One of the features of USB that people are really excited about is the 'multi-headed support.' In other words, I should be able to connect several sets of [monitor, keyboard, mouse (and other stuff like joystick, printer, etc.)] to one computer, maybe distribute these throughout my house, and move seamlessly from one to the next. What if we turn this around? What if I wanted to control several computers in a rack, all from one location? This is done today with KVM (Keybard/Video/Mouse) switches, some of which are very sophisticated. Could USB do the same thing? Most computers have two USB ports, so you could theoretically daisy-chain the computers together, but this would be bad for reliability (take down one computer, and everything past it on the chain disappears). So, we would probably want to use a hub, and attach the head to that. Has anyone tried this? Anyone seen it done?" Here's an interesting sidenote: the USB 2.0 specs were just released. You might check out what they have to say about KVM switches.

1 of 6 comments (clear)

  1. A bit about USB by Anonymous Coward · · Score: 3

    I work for a major corporation that is heavily involved with USB, and while I'm not directly involved with it, I am (organizationally) right next to the main USB group here.

    You can't 'daisy chain' USB. Every USB tree can have at most one PC on it. (This does not include special USB communications devices that allow communications between computers, similar in concept but not in execution to a null modem.) While you could connect multiple sets of keyboards and mice to your computer via USB (and a co-worker has told me about hooking 100 mice up to a computer at a time for testing), you have to remember that each link on the USB chain has to be five meters or less, thus limiting its usefulness for long-distance stuff. (See this link for a company that's trying to change that.)

    You can't have a USB monitor. While some monitors have USB hubs built in, and allow you to control things like brightness or picture shape with them, USB (even 2.0) is nowhere near fast enough to handle an uncompressed video signal.

    So, in response to your original question: Yes, you can use USB for a KM (but not V) switch, and it's easier and cheaper than the more advanced elecctronics that are necessary for a KM switch that can hide the connection and disconnection of the PS/2 keyboard / mouse from the computer. The best way to do it would be to plug a single keyboard and mouse into a hub, and then run a single USB cable from each computer to near that hub, and then either with a switch or manually change which computer is plugged into the hub.

    There are only four wires in a USB cable, and because it's hot swappable, any switch box that you create only has to be an actual switch, nothing more advanced is necessary. However, this does have the disadvantage that you can't switch too quickly. (Fast plugging-and-unplugging can screw up the Windows 98 USB stack.)