On Mon, 2002-12-02 at 18:08, Poul Petersen wrote: > question is is the root hub a requirement of the USB > controller design?
Yes. > Also, the 127 device limit is per controller right? Yes; however, the figure "127" is mostly symbolic, since it is not practical to have more than 10-20 *typical* devices connected. Your desk will look like a snake kingdom. You also are likely to run out of bandwidth, and maybe out of power as well. I can think of some very special USB hardware that fans out fast and wide, but that's not your normal keyboard, mouse or a printer... > I add another controller, then I can have another 127 devices > on that bus? Yes. And the full bandwidth too. I tried to use two PCI add-on cards, each with 4 independent OHCI controllers. Works great, gave me 9 busses with 12 Mbps each. > And finally, the USB serial device limit is 256 > which I could theoretically attain with at least 3 > independent USB controllers? If you really want 256 serial devices connected through USB then probably you need to look at digiboard stuff. It will be cheaper and faster. Generally, what you are trying to do makes as much sense as connecting a CD-RW through a parallel port - can be done, but a pain. You should keep in mind that USB is generally a CPU-intensive bus, compared to PCI bus master solutions (which need almost zero CPU use). Dmitri
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