On 1 May 2011 18:51, Mark Knecht <markknecht@email-addr-hidden> wrote:
> As far as I can tell, no, I don't. I own a couple of USB hubs and 4
> external hard drives. None of them functioned at all when attached
> without their power supplies.
According to the proper USB1/2 specs, you're only supposed to feed
0.5A at 5V (a measly 2.5W!) to an individual port. Now, most USB
chipsets don't have the necessary hardware to achieve this and will
provide up to 2A for a bus of four, letting any device draw as much
current as they require (10W!). Most hard disks, on the other hand,
will require more than 0.5A at 5V. Thus if you have a compliant
machine, they won't work. If you have a dodgy machine with no
isolation between the USB devices, it's more likely to work! Hubs
without their own power supply draw the power from the mother board,
making the power issue even more of an issue (4 devices served by a
0.5A current source).
All of this means that since none of your hard drives works, your
machine has a good, proper implementation of the spec with isolation
between individual ports plugged into the same internal bus. Also it
means any device that's completely broken in power requirement will
struggle.
Even different machines from the same manufacturer has issues. My Dell
laptop can't drive one of my disks but a Dell PC can.
USB3 has increased the individual current amount to 0.9A at 5V (4.5W).
Most likely the implementation will be broken and a whole pile of USB
powered devices will arrive soon, drawing massive amount of power
(18-20W?).
-- Hakan (m1fcj) - http://www.hititgunesi.org _______________________________________________ Linux-audio-user mailing list Linux-audio-user@email-addr-hidden http://lists.linuxaudio.org/listinfo/linux-audio-userReceived on Mon May 2 16:15:02 2011
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