Subject: Re: soft control of +5V at a USB hub port?
To: J Chapman Flack <>
From: Steven M. Bellovin <>
List: tech-kern
Date: 05/04/2005 10:51:28
In message <>, J Chapman Flack w
>Here's something I've been wondering for a while:
>I have a USB scanner.  It gets its power from an independent wall wart,
>and it has no power switch.  It does go into a powered-down mode when
>the computer is off, though, and I can make it do that by unplugging the
>USB cable.  Apparently the scanner, even though it is independently powered,
>watches the +5V on the USB cable and powers down when that drops.
>I would like to be able to keep the scanner powered down the vast majority
>of the time when I am not using it at all.  I have heard just enough about
>the inner workings of USB to wonder if it's possible on some or all USB hubs
>to command down the +5V on a particular port through software.  Whether
>there's driver support is another question - right now I'm just curious if
>anyone who knows USB better than I do thinks the idea is even possible.

I don't know about +5V; I have noticed that under Windows, the LED in 
my USB flash disk goes off when I "stop" the device.  The same does not 
happen when I umount it on NetBSD, so clearly there's some power hook 
we're not using.

		--Prof. Steven M. Bellovin,