On Wed, 07 Dec 2011 18:58:16 -0500, Paul <nospam@needed.com> wrote:


>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
>them.
>
>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
>auto detection.
>
>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.
>
> Paul


Thanks again Paul.

I guess I will just have to live with this problem. I didn't tell
you, but this KVM replaced its twin, that died on me about a year ago.
I don't recall ever suffer my current problem with that one. So, I am
going to assume I have a faulty KVM now. Sobeit.

Frederick