Re: High Power USB hub port...

BillW50 wrote:
In news:46d66e4e$1_1@xxxxxxxxxxxxxxxxxxxxxx,
M.I.5¾ typed:
That is not actually quite correct. The USB spec requires a port to
supply a minimum current of 100 mA. Anything larger is entirely by
negotiation with the host and will only be available if the power
supply can support it once the other loads are accounted for. In
general terms, this usually means that USB ports on desktop machines
can deliver 500 mA. Also ports on self powered hubs can generally
deliver 500 mA. However, ports on non powered hubs are usually
limited to 100 mA. Some laptops also limit the current on their
ports to 100 mA.
Also note that according to the USB spec., 500 mA is specified as a
maximum not a minimum. Although most ports will, in practice,
deliver a bit more than 500 mA, they shouldn't deliver much more
before the overcurrent protection kicks in. Any port designed to
deliver more than 500 mA (and I know that do exist) are strictly
non-compliant.

I've seen some USB powered 2½ inch external hard drives that actually come with two USB plugs. The extra one taps into extra 5VDC so it doesn't overload using a single USB port.

Yes, this is quite common. They tell you to use two if necessary.

However if you look at how the USB power circuit is designed on notebooks, you'll see why this is rarely necessary. Four USB ports will share a single power controller. The power controller will limit total current through the four USB ports to 2.0A (500mA per port). However each individual port won't cut off with over-current protection at 501mA, in reality they'll cut off somewhere closer to 1.0A. Of course you can't use all four of them at 1.0A because the total current is limited to 2.0A. Since most USB devices are low-current devices or are devices that are not powered by the bus, you're usually just fine with a single power plug on the higher current drives.
.

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