Tech Notes And Miscellaneous Thoughts

problems with Linux and Aten CS1784A KVM switch

Dear lazyweb,

I bought an Aten CS1784A DVI+USB KVM switch a couple of months ago. I’m pretty happy with it except for one annoying problem.

It has a Dell multimedia USB keyboard plugged into it (picked it up from a swap meet for $15 – nice. it's a pretty good keyboard in its own right but i mostly bought it to match what I use at work), and a Logitech MX518 mouse connected. Also two computers – one runs Linux only. The other is switched off most of the time, but is occasionally powered up to boot into either Linux to experiment with stuff i don't want to do on my main system (like gnome 3) or sometimes Win7 to play games.

OK, that’s the background data. Here’s the problem

Whenever I reboot either of the Linux installations, the KVM stops recognising the keyboard. I have to unplug the keyboard and plug it back in before any input is recognised. Keyboard input doesn't get through to the currently active system, and I can't even use the KVM's magic key <ScrlLock><ScrlLock>n<Enter> to switch between systems – it doesn't even beep after the 2nd ScrlLock as it should, so it seems as if all keyboard input is ignored by the KVM, or the keyboard is totally confused and not sending anything. The mouse continues to work during this.

Rebooting Win7 doesn't do this. The keyboard just works as it should.

The problem occurs sometime between the time Linux is selected from the Grub2 menu and the gdm login screen is displayed (same result with both gdm on my main system and gdm3 on my secondary system).

The keyboard works when plugged directly into either system, without the KVM. I’d been using it for several months before I got the KVM, and I’ve tested that it still works correctly across a reboot if i plug it in directly.


Googling for "+linux +CS1784A" just gives me hundreds of pages from web sites selling the KVM, but no useful info.

Anyone seen anything like this before? Can anyone think of anything that linux might be doing in it's console or X initialisation that would confuse the KVM?

it's not a huge problem because I really don't reboot all that often – more of an annoyance that I'd like to go away.


  1. Paul Tötterman

    I just thought I'd point out that the +-operator doesn't work like that anymore on google. "linux" "CS1784A" is the new form. And there's also "verbatim" in the search tools menu on the left.

    1. cas

      thanks, useful to know.  

      I added the + in front of "linux" to try to refine the search to pages mentioning linux.  didn't help.  Google doesn't find anything useful on the topic either way.

      btw, my original post was ambiguous – i searched for +linux +CS1784A (without any quotes), not +”linux CS1784A” – that would have made google search for the two words immediately adjacent to each other as a phrase

  2. Hi Craig,
    I also have the same problem with Wheezy, w/o any KVM switch, using any keyboard, be it on PS/2 or USB. It stays there, with the power LED lit, but useless as a piece of junk. Re-plugging it into its port, and works like a charm.
    It also triggers somewhere between grub menu and X startup. More than that, I could only reproduce this error in cold start conditions, I mean powering on the PC from the unplugged PSU condition. Turning off and on works, and so do reboots.
    Haven't got the time searching bug reports on that though, but now you're just making me curious. Stay tuned, maybe I'll find something soon.
    Oh, and leave a comment or something if you manage to fix it. It used to drive me crazy back in the days my PC was lying under my desk.

  3. Timo Juhani Lindfors

    Interesting. I would probably approach this problem by capturing the USB traffic in both broken and working scenario and comparing the traffic. Unfortunately if you need to do this during bootup this can be hard to do without expensive hardware or virtualization.

  4. Avi

    I had the same problem. Downgrading to a “stupider” keyboard fixed it for me. There’s something about the Dell multimedia keyboards that KVMs appear to hate, I find. Might be the onboard USB hub. I’ve never looked into it much. Went a very simple old Microsoft Comfort keyboard and that works a treat.

    1. cas

      Thanks, didn’t think of that. I’ll try it with another keyboard tonight.

      The only other decent keyboard I’ve got also has a USB hub (and a USB audio device which i’ve never used). Be interesting to see if it has the same problem.

      The cheap plain keyboards I have aren’t worth using long-term. I’ll try one as a test, but the cure would be far worse than the disease.

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