On Mon, Dec 02, 2002, Poul Petersen <[EMAIL PROTECTED]> wrote: > I just wanted to clarify a few points about limitations > of the USB bus. > > The stated maximum number of USB devices per controller > is 127, where hubs also count as one device. The Dell PC I > have has one controller with a "root hub", so it would seem > that that the two USB ports on the back of the machine are > actually on the same controller and consequently I will have > a maximum of 126 devices. If this is correct, then my > question is is the root hub a requirement of the USB > controller design? Will there always be a "root hub" that > takes up one device?
Kinda, yes. The root hub is not a device per se and doesn't need it's own device number. In fact, older USB code doesn't assign the root hub a device number. However, newer code does for managability reasons. We want to expose the root hub as just another device and to do that we need to give it a device number. It might be possible to hack the code to assign it a number other than 1, maybe -1 or 128. I think the major problem with that will be usbdevfs. > Also, the 127 device limit is per controller right? If > I add another controller, then I can have another 127 devices > on that bus? And finally, the USB serial device limit is 256 > which I could theoretically attain with at least 3 > independent USB controllers? Possibly. There are some limitations with bandwidth too. The HC may not be able to poll all of the serial devices in one frame. The USB serial device limit is strictly a limit with the number of minor numbers in one major. One could presumably hack the driver to support more major numbers to support more devices. JE ------------------------------------------------------- This SF.net email is sponsored by: Get the new Palm Tungsten T handheld. Power & Color in a compact size! http://ads.sourceforge.net/cgi-bin/redirect.pl?palm0002en _______________________________________________ [EMAIL PROTECTED] To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-users
