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From: Daniel James on 19 Dec 2008 11:54
In article news:<gidlu7$11u$2(a)localhost.localdomain>, Martin Gregorie
wrote:
> > The secondary power USB cable of the LG "USB Powered" DVD-writer
> > we were discussing doesn't connect to the drive with a USB
> > connector, but with a simple centre-positive push-in barrel
> > connector (marked "DC IN 5V").
> >
> I have a Formac 2.5" HDD with two USB plugs, which is the
> arrangement I thought we were discussing.

Maybe I wasn't clear. The 'power' lead with the LG drive connects to a
second USB port on the PC, but doesn't use a USB connection at the
drive end of the cable.

So, both leads connect to USB ports at the PC end, but at the drive end
one is standard mini-USB and the other is a typical concentric power
connector.

> My hypothetical microbrick would go between the second cable and
> the PC (if it was a stand-alone) or replace the second plug and
> would be just bright enough to do the negotiation and then supply
> power to the device on the first plug.

Yes, I got that. The point I'm making is that if the device was
designed to draw power from a USB port and did not contain the "smarts"
to negotiate for 500mA it could not could not draw more than 100mA
without exceeding the USB spec. Putting a microbrick such as you
describe between the device and the PC might achieve the negotiation
that would lead to the PC supplying 500mA, but the device would not
know that the negotiation had taken place and so could still not
legally draw more than 100mA.

In fact, I doubt very much that any PC actually supplies more power to
its USB ports because negotiation has taken place. I suspect that PCs
either have 500mA per port or they don't, and the purpose of
negotiation is to allow the device to determine whether it's safe to
draw more than 100mA.

Cheers,
Daniel.




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