[Wylug-discuss] Powered USB [WAS: Recommendation: Linux-compatible Netbook Under GBP 250?]

[Dave Fisher]

On 25 November 2011 16:16, Paul Brook <paul at codesourcery.com> wrote:
> Note that "powerd USB" may mean different things to different people.
> Standard USB allows 2.5W per port.  This should be fine.
>
> I suspect you're thinking ot some devices that used an unpowered hub to
> provide their USB ports.  These can only supply 0.5W/port,
> .  Enough for mice, but not more power hungry devices like 3G dongles or
> harddrives.  Manufacturers caught on fairly quickly that this was a terrible
> idea and I haven't seen it recently.
>
> Some devices (typically large spinning disks) draw more USB power then they're
> allowed to, especially at startup.  Typically these come with evil "Y"
> splitter cables to spread the load.  If you're running multiple of these you
> may exceed the total USB power available on the machine.  I guess slightly
> more likely to occur with a netbook given they have less ports total, but
> relatively easy to workaround (powered hub) and really not a fault in the
> netbook - the USB spec says that hosts can/should deliberately cut off devices
> that draw too much power.
>
> The other feature is a "charging" port that stays powered when the machine is
> turned off.  Support entirely depends on the device, my guess is fairly rare
> on netbooks.


Thanks Paul,

That was a very useful clarification/reminder of stuff I sort of knew about.

My vagueness has previously caught me out, when I've expected to use
one of those 'evil' Y cables or to charge some device or other on the
move.

Thanks again.

Dave