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Say I had usb port on my motherboard that only supports low speed devices, will full speed/ high speed devices not work on the low speed ports but at a lower speed?

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In general, no. There really is no such thing as a low-speed-only host port; it would violate the USB standard.

But even if such a thing did exist, the USB protocol does not support having a host negotiate a device down from full speed to low speed.

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But a high-speed device will enumerate at full-speed. It may not function correctly (e.g. a video camera that needs the full bandwidth) but it should still enumerate, and most devices like hard drives will continue to work at full speed instead of high speed. – Jim Paris Sep 14 '12 at 22:16
Yes, in fact, a high-speed device always identifies itself first as full speed, then negotiates the speed up to high speed if the host supports it. – Dave Tweed Sep 14 '12 at 22:41

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