KVM/Scroll Lock behavior

milk niteshadez at gmail.com
Wed Jan 18 20:21:38 PST 2012

As far as I know, the KVM switching should be hardware triggered. It should
not be related to X at all because it does not require any software at all.
If it did, then you wouldn't be able to switch inputs when no computers are


On Wed, Jan 18, 2012 at 20:07, Todd Pytel <tppytel at sophrosune.org> wrote:

> Hi all,
> I'm digging around at a quirk involving Scroll Lock and KVM switching
> that I think X is responsible for and hoping someone here can explain
> it. At this point I can work around it myself. But I'd really like to
> understand it, or perhaps provoke a change to make it work better for
> others less inclined to dig at this than I am.
> Like many other KVM's out there, mine triggers switches through double
> Scroll Lock taps. This has often required some fiddling to get working
> properly in the past, usually by adding Scroll_Lock to mod3 using
> xmodmap. But sometime recently, (I think) something changed in X11's key
> handling. A while back, my GNOME desktop stopped switching on Scroll
> Lock (even with the xmodmap) but still switched on Num Lock. Then it
> stopped working entirely in GNOME, XFCE, or Openbox.
> What I found after poking around some more tonight is that I can get the
> KVM trigger working by twiddling the LED's with xset...
> xset led 3 && sleep 0.2 && xset -led 3
> That toggles the LED's and triggers the KVM.
> In some sense, this solves my problem. I can put that in a script and
> bind it to a key. But I'd like to actually understand what's going on.
> Also, since Scroll Lock is the most common KVM trigger, it would be nice
> to fix this so that it "just works" for people with less enthusiasm for
> this sort of detective work. It seems like this shouldn't be an
> earth-shaking architectural change, but I admit I don't know jack about
> the internals of X.
> Specifically, here's what I see on Debian Sid...
> 1) In the console, Scroll Lock and KVM switching work properly with no
> tweaks.
> 2) In an untweaked X session, the Num Lock LED works while the Scroll
> Lock one does not. Both keys register in xev output.
> 3) After "xmodmap -e 'add mod3 = Scroll_Lock'", the Scroll Lock LED
> works. But it still doesn't trigger the KVM.
> 4) Even without the xmodmap line, twiddling the LED's via xset both
> toggles the LED's and triggers the KVM.
> What I'd really like to understand is what's different internally
> between toggling the LED via xset and doing it through an xmodmap'd
> Scroll Lock. Clearly the KVM is seeing those events differently and I
> don't know enough about X to understand why.
> If it's not too much of an undertaking, I'd also suggest that Scroll
> Lock should work as expected, or at least have an easy xorg.conf option
> to make it work as expected. A little Googling will show a decade's
> worth of posts across the web from KVM users trying to get their boxes
> to switch properly. And since the old xmodmap solution doesn't work
> anymore, it would be helpful to have a solution that doesn't involve
> hacky scripts and keybinding.
> Cheers!
> Todd
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Hope is a dimension of the spirit. It is not outside us, but within us.
When you lose it, you must seek it again within yourself and in people
around you -- not in objects or even events.    -
-Vaclav Havel
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