Subject: Re: soft control of +5V at a USB hub port?
To: J Chapman Flack <>
From: Lennart Augustsson <>
List: tech-kern
Date: 05/04/2005 19:55:27
It depends on the hub if you can control power to the port
individually.  But my guess is that it doesn't monitor the
+5V, but instead checks if the port is enabled.  You can
always enable&disable ports individually.  But there is no
way to do this from outside the kernel at the moment.

	-- Lennart

J Chapman Flack wrote:
> 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.
> -Chap