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IOGEAR DVI KVM Problem?
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. Anyone? Frederick |
#2
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IOGEAR DVI KVM Problem?
Frederick wrote:
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. Anyone? Frederick 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 |
#3
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IOGEAR DVI KVM Problem?
On Wed, 07 Dec 2011 18:58:16 -0500, Paul 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 |
#4
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IOGEAR DVI KVM Problem?
Frederick wrote:
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 It might just be the way it is designed. To prevent the computer from seeing a "monitor change", you must 1) Maintain termination resistors on the end of the monitor cable. 2) Support DDC EDID reading at any time. If you can't maintain termination resistance, at least eh EDID should always be readable. The computer may make a video output switch, but eventually return to the normal resolution. If the design doesn't support these features properly, then switching away from the monitor, may cause the computer to revert to 640x480. That's my guess as to what is happening. You'd need to find a product review for the KVM, to see if other users also see the problem. Check Newegg and see if they carry your product, and see if the reviews there detected a problem. Paul |
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