[PLUG] 2-System (KVM) Switches

[Keith Lofstrom]

On Mon, Jan 16, 2012 at 03:34:14PM -0800, Chaz Sliger wrote:
> Has anyone had good experiences with any of the small
> 2-4 system kvm switches?
> 
> I've tried several and they all seem to suck pretty bad. 
> The latest was a Belkin SOHO.

Could you be a little more specific?  There are two major kinds,
DVI and VGA, what are you looking for?  And what do you find that
"sucks" about them, that is, what problems do you want to solve?
I assume you want to pass USB keyboard and mouse.

With a laptop interface, I'm guessing you want VGA, though decent
docking cradles often provide DVI.  DVI provides a crisper signal,
though I get by with VGA on a couple of setups.  I've seen VGA
dual port KVMs at the Free Geek Store, and deployed one of them.
I've seen some DVI switches at EcoBinary and Green Century.

The usual problem I have with these switches, DVI more often than
VGA, is providing EDID information from the monitor to the computer
during boot, so the computer knows what it is driving.  Some older
Linux video drivers expect the screen(s) to be connected at boot,
and won't access the EDID after a hot plug.  Other drivers, and
newer ones, work with hot plug just fine.  I got rid of one Intel
motherboard because it wouldn't talk to my favorite screen - another
Intel board with a different video chipset worked just fine.

If a screen doesn't work right directly connected to your computer,
a KVM won't improve things.  Some video chipsets get cranky when
the external screen has a different aspect ratio than the built-in.

Another issue is ghosting.  A high res screen and a cheap cable
will show ghosting.  A KVM implies two cables - if one or the
other is low quality, there will be ghosting on the screen, with
a ghostly replica of the image a few pixels to the right.  Again,
if two cables in series look awful, a KVM is unlikely to help. 
This is more of a problem with VGA, though when ghosting is
bad enough to interfere with a digital DVI signal, it can look
REALLY bad.

You should get high quality cables, "triple coax" for VGA
(fancy connectors and armored sheaths aren't needed).  Cables
should be no longer than necessary.  Longer cables echo more.
You can find a great selection of video cables at pchcables.com 
I use the super-dooper fat ones with the noise-suppressor 
cylinders around the cable.  $6 at PCH, $30 at Best Buy.

There may also be cheap KVMs which don't properly pass the EDID
information, and you get no display at all.  Or their bit rate
or bandwidth is too low for ultra-high-resolution screens. 
I've heard about those, but I've never encountered one.

On my wife's machine at work, we have a KVM selecting between
a Linux and a Windoze VGA source, driving two identical monitors
and two keyboards and mice after a USB hub and a VGA splitter.
The extra screen is turned on when she is working with a visitor
on the other side of her desk.  Pretty handy for showing stuff
or entering data together, without losing face contact.  That
is using a cheap old Belkin switch.  A little ghosting (three
cables in series!) but no problems otherwise.  The cables
from the computers to the switch, and from the switch to the
splitter, are all one foot long, with a three footer from
the splitter to one screen, and a six footer to the other
screen.  As short as possible!

I have an ioGear DVI switch in my office at home, selecting
between four computers, and another Belkin VGA switch fed
from two backroom servers at my wife's office.  The latter
may go away, it looks like we can combine servers.  We only
use the server screens for debug, anyway.

Finally, keep in mind that with Linux and a high bandwidth
ethernet connection, you can use an X client on one machine
with an X server (display/keyboard/mouse) on another.  So
KVM is often unneeded - linux/X11 systems aren't as tied
to a single video path as windoze and mac/aqua are.

Keith

-- 
Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs