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Subject: Re: V1R running on USB power possible.
From: Manuel Dejonghe (limbus_at_gmail.com)
Date: 2004-09-02


afaik, you are only allowed to take more than n mA when you asked for
it in the usb-protocol, and got them assigned, where n is somewhere
about 50 mA.

So probably those Toshibas are refusing (shutting down the ports) if
you take more than you requested.
Your second dummy plug pretty certainly has NO usb-chip to ask for the
power he's gonna drain.
I would not have the heart to plug this device to my computer, risking
to damage my mainboard. I would get a selfpowered USB-Hub for that
task (self powered means with a proper wall wart)

Manuel

On Thu, 2 Sep 2004 12:38:14 +0100 , Stuart Tedford
<stuart.tedford_at_piresearch.co.uk> wrote:
> > You have to make sure that you draw a maximum of 500mA on the USB 5V,
> > if you draw more you could damage the USB port on your PC, or the USB
> > host PC (or hub) might decide to shut this port down to
> > prevent damage.
> >
> > So, it's not just a simple diode.
> >
> Yeah, in theory. But in practice, most USB ports ignore the standard and
> provide 1A or more.
>
> I have an external USB enclosure for a 2.5 inch HDD and out of maybe 10 PC's
> I've connected it to, only 2 refused to provide the 1A or so it needs to
> spin up. Both of those PCs didn't even provide the spec'd 500mA!!!
>
> The enclosure has a USB cable with a second dummy plug on it on the PC side
> so you can connect it to 2 USB ports simultaneously (only one of them
> provides the data link, but both provide power) That way it is within the
> USB 500mA spec. As a side note, those same PCs refused to power the disk as
> well, even with both ports connected! As a second side note, the 2 PC's
> that refused to work are both toshiba laptops.
>
> I also have a couple of Compaq laptops, and both of those can supply over
> 1.3A on a single USB port, I measured it when connecting a CD-ROM drive over
> USB.
>
> So I'm not really concerned about the current draw on the USB port, and it
> will only draw current when the batteries go below 4.7ish volts, and the
> disk will already be spinning which only consumes about 500mA anyway - (it's
> only the disk spinup that draws 1A).
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