> I have been using a IOGEAR DVI 4-port 'KVMP switch' for a while now.
> It has an irritating problem, and I am wondering if it is defective,
> if anyone has similar trouble, or is it just the nature of the beast.
> Three desktops (All XP SP3), use the following DVI video cards:
> 1) RADEON X300 SE 128MB set at 1024x768 pixels.
> 2) NVIDIA GeForce 9300 GE 512MB set at 1360X765 pixels.
> 3) VISIONTEK RADEON 9250 AGP 128 MB 1024x768 pixels.
> All boot up correctly at the above resolutions.
> When I use the KVM to switch back and forth, the second and third
> machines quite often forget the display setting in favor a totally
> useless 640X480. On top of that, the boot setting is no longer
> selectable. Which means I guess that the system is no longer using my
> video card (yes I tried reloading its driver). Re-boot of that
> desktop is required to fix things which it always does.
This is related to auto-detection of connected monitors, on
the video cards.
The DVI connector has high speed serial signals for R, G, B.
They're differential pairs, with a termination resistor across
It's possible, at the video card, to detect the termination
resistor is missing. That is how the video card knows the
monitor is not connected. A KVM needs to prevent the computer
from seeing an "open circuit", when the KVM is switched
away from that computer.
If the KVM design is a good one, it should maintain termination
impedance on all the four computer DVI connectors, whether
the monitor is switched to that port or not. To do that,
the designers would use a digital multiplexing chip, rather
than some crude scheme involving relays for example. If relays
were used, the "glitch" on the diff pairs, would be enough
to trigger an attempt to auto detect the monitor.
The other part of the puzzle, is the DDC serial interface.
The monitor has clock and data signals. The computer queries
the monitor, via that interface. The interface, as far as I
know, was not designed for multi-master operation.
Computer #1 ---- KVM ----- Monitor DDC clock and data
Computer #2 ----
Computer #3 ----
Computer #4 ----
When you switch computers, the computer should read the
DDC, if it wants to know about the monitor. I doubt all
the computers are connected to those wires at the same
time. Each one of them is likely a separate bus master.
If Computer #2 attempts to read EDID via DDC in the picture,
it's currently got no connection. It can't get any data,
so it switches to a safe 640x480.
What the KVM should be doing, is "faking" the EDID, like
this box does. This box will "play back" the EDID, as it
is stored inside the box. The copy in the monitor now
doesn't need to be consulted. Of course, buying four
of those, at $79 each, completely defeats the low cost
of the KVM in the first place. I'm not suggesting
you buy these, merely illustrating there is a device
that can solve the problem of EDID reading. The chip
inside this thing, would cost less than a couple dollars,
and the KVM should have integrated that chip for you.
Then, the KVM could have "faked" the EDID readback, itself. http://www.gefen.com/kvm/dproduct.jsp?prod_id=4714
Computer #1 ---- Gefen --- KVM ----- Monitor DDC clock and data
Computer #2 ---- Gefen ---
Computer #3 ---- Gefen ---
Computer #4 ---- Gefen ---
Such a box, solves the "reading EDID" problem. Depending on
whether the little Gefen box also reclocks DVI data, it might even
stop impedance discontinuities or glitches, from triggering
It's possible to design the box, such that this doesn't
happen. Who knows why they don't bother... It could be
done with one custom chip, and done properly. And I would
expect, with a market demand for such a chip, there is
likely one already designed to do just that.
This isn't exactly the right chip, but what it does have,
is a block labeled "EDID memory". If any of the four
devices on the left, want to query the EDID via DDC, they
can get a copy. http://www.nxp.com/documents/leaflet/75016975.pdf
The only thing wrong with that chip, is the interface on the
right hand side isn't right. Since the chip is for usage inside
an LCD TV or the like, it's a front end chip, rather than
ready to be used in a KVM.
I suppose you could contact Iogear tech support and see if
your problem is "normal" or not. I'm sure they'll just
blame it on your setup, etc.