You are right, the device will get 5 volts. However it is still part
of the spec that 2ms without a SOF will force the device into suspend
mode, so it will use less than 2ma. So depending on your definition of
function, there will be power, but no activity. If it uses more than
2ma, from the bus, problems will ensue.

Steve

On Mon, Jul 7, 2014 at 6:00 PM, Grant <emailgr...@gmail.com> wrote:
>> it isn't possible to disable Vbus on a USB host port and still use the
>> port.  To do what you want, you would have to physically cut the Vbus
>> wire in the USB cable and splice the device side of that wire to the
>> external power supply.
>
>
> I think I'll try that.  Can anyone confirm that it should work?  If I
> connect a 5V external power supply to the VBUS wire in a USB cable,
> the connected USB device should still function?
>
> - Grant
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