Context: I have four little PCs doing always-on things, mostly routing. I don't want four monitors, keyboards, mice -- too much clutter and space. So I connect each computer to a single set of peripherals through a KVM switch. The problem: one day, none of the computers could use the display. Clue: journalctl showed that the computers complained: "EDID has corrupt header" Solution: remove power from the KVM briefly. This probably reset it and caused it to re-fetch its copy of EDID information from the monitor. Things worked after this. What I think is going on: - EDID (Extended Display Identification Data) is provided by each monitor to the computer so that the computer knows what the monitor can do. For example, resolutions and refresh rates - a good KVM fetches the EDID information once from the monitor and makes it available to each attached computer, even the ones not currently selected. If it doesn't do this, unselected computers would incorrectly "understand" the monitor. - there was a power flicker in my house. Short enough that not all devices were reset. Perhaps the KVM was partially reset (i.e. got into a bad state)
On Mon, Jun 22, 2026 at 8:58 AM D. Hugh Redelmeier via Talk < talk@lists.gtalug.org> wrote:
- there was a power flicker in my house. Short enough that not all devices were reset. Perhaps the KVM was partially reset (i.e. got into a bad state)
Given that it would draw so little power, doesn't it make sense to have the KVM switch plugged into the same UPS as your PCs? (And if at least some of your PCs aren't on UPSs, confused EDID is probably the least of your risks.) - Evan
From: Evan Leibovitch via Talk <talk@lists.gtalug.org>
Given that it would draw so little power, doesn't it make sense to have the KVM switch plugged into the same UPS as your PCs?
(And if at least some of your PCs aren't on UPSs, confused EDID is probably the least of your risks.)
Yes it does, but only because of this bug. I don't have monitors on a UPS so why put the KVM switch on the UPS? If I want to access the servers, I can SSH in from a laptop (with a built-in UPS). BTW, many KVMs are only powered by USB from the host computers. That's a little inconvenient if all the hosts are off: the selection is lost. I run into this when only one host is on, and I reboot it. I have to quickly reselect that host if I want the intereact with the boot sequence. I do that so rarely that I forget and have to reboot a second time.
ignoramus here, who owns multiple KVMs. If kvm comes back before monitors (likely as less electronics to reset), won't it have bad info about the monitors to give out when asked? Carey
On 06/24/2026 8:19 AM CDT D. Hugh Redelmeier via Talk <talk@lists.gtalug.org> wrote:
From: Evan Leibovitch via Talk <talk@lists.gtalug.org>
Given that it would draw so little power, doesn't it make sense to have the KVM switch plugged into the same UPS as your PCs?
(And if at least some of your PCs aren't on UPSs, confused EDID is probably the least of your risks.)
Yes it does, but only because of this bug. I don't have monitors on a UPS so why put the KVM switch on the UPS?
If I want to access the servers, I can SSH in from a laptop (with a built-in UPS).
BTW, many KVMs are only powered by USB from the host computers. That's a little inconvenient if all the hosts are off: the selection is lost. I run into this when only one host is on, and I reboot it. I have to quickly reselect that host if I want the intereact with the boot sequence. I do that so rarely that I forget and have to reboot a second time. ------------------------------------ Description: GTALUG Talk Unsubscribe via Talk-unsubscribe@lists.gtalug.org Start a new thread: talk@lists.gtalug.org This message archived at https://lists.gtalug.org/archives/list/talk@lists.gtalug.org/message/PSZVZON...
From: CAREY SCHUG via Talk <talk@lists.gtalug.org>
ignoramus here, who owns multiple KVMs.
I do. I have three going now. Various models. Each for a different collection of computers. Each supports four computers: switches supporting more are much more expensive. I have a couple that are no longer used: - PS/2 KM and DVI V - PS/2 KM and VGA V I have three broken ones: USB part no longer working. Roughly this model, but red: <https://www.amazon.ca/TESmart-Switching-Switcher-Computers-Keyboard/dp/B0CC8DR8SF/ref=ast_sto_dp_puis> Bought long ago at a significantly lower price. Why not PiKVM? - much more expensive per host -- starts at about $100. (recently I paid $40 for a 4 port KVM from AliExpress) - lower resolution
If kvm comes back before monitors (likely as less electronics to reset), won't it have bad info about the monitors to give out when asked?
Apparently not. My other KVMs work. The KVM could know when a monitor is connected and powered on.
sorry, my "multiple kvms" meant I have several. 24 port daisy chainable ones, though never got a daisy chain cable, and never reconnected enough old computers to actually need them. At this point only using one modern 4 port one. aside (i think) some computers won't boot up through kvm, the bios can't handle the usb hub before the keyboard/mouse. My suggestion was that one kvm re-initializes faster than the others, and gets bad info from the monitors who are only partially reinitialized and so give out info saying "not ready" or something like that to the kvm. or when it gets the "not ready" from the monitor, it gives up, whereas the other kvms try again. Have you experimented with unplugging parts and plugging back in in different orders (simulated power outage)? Carey
On 06/24/2026 10:43 AM CDT D. Hugh Redelmeier via Talk <talk@lists.gtalug.org> wrote:
From: CAREY SCHUG via Talk <talk@lists.gtalug.org>
ignoramus here, who owns multiple KVMs.
I do.
I have three going now. Various models. Each for a different collection of computers. Each supports four computers: switches supporting more are much more expensive.
I have a couple that are no longer used: - PS/2 KM and DVI V - PS/2 KM and VGA V
I have three broken ones: USB part no longer working. Roughly this model, but red: <https://www.amazon.ca/TESmart-Switching-Switcher-Computers-Keyboard/dp/B0CC8DR8SF/ref=ast_sto_dp_puis> Bought long ago at a significantly lower price.
Why not PiKVM?
- much more expensive per host -- starts at about $100. (recently I paid $40 for a 4 port KVM from AliExpress)
- lower resolution
If kvm comes back before monitors (likely as less electronics to reset), won't it have bad info about the monitors to give out when asked?
Apparently not. My other KVMs work.
The KVM could know when a monitor is connected and powered on. ------------------------------------ Description: GTALUG Talk Unsubscribe via Talk-unsubscribe@lists.gtalug.org Start a new thread: talk@lists.gtalug.org This message archived at https://lists.gtalug.org/archives/list/talk@lists.gtalug.org/message/2FLA6U2...
Speaking of.. are they any cheap kvm over ip that you know of?? On Wed, Jun 24, 2026 at 11:58 CAREY SCHUG via Talk <talk@lists.gtalug.org> wrote:
sorry, my "multiple kvms" meant I have several. 24 port daisy chainable ones, though never got a daisy chain cable, and never reconnected enough old computers to actually need them. At this point only using one modern 4 port one.
aside (i think) some computers won't boot up through kvm, the bios can't handle the usb hub before the keyboard/mouse.
My suggestion was that one kvm re-initializes faster than the others, and gets bad info from the monitors who are only partially reinitialized and so give out info saying "not ready" or something like that to the kvm.
or when it gets the "not ready" from the monitor, it gives up, whereas the other kvms try again.
Have you experimented with unplugging parts and plugging back in in different orders (simulated power outage)?
Carey
On 06/24/2026 10:43 AM CDT D. Hugh Redelmeier via Talk < talk@lists.gtalug.org> wrote:
From: CAREY SCHUG via Talk <talk@lists.gtalug.org>
ignoramus here, who owns multiple KVMs.
I do.
I have three going now. Various models. Each for a different
collection > of computers. Each supports four computers: switches supporting more are > much more expensive. > > I have a couple that are no longer used: > - PS/2 KM and DVI V > - PS/2 KM and VGA V > > I have three broken ones: USB part no longer working. Roughly this model, > but red: > < https://www.amazon.ca/TESmart-Switching-Switcher-Computers-Keyboard/dp/B0CC8DR8SF/ref=ast_sto_dp_puis>
Bought long ago at a significantly lower price.
Why not PiKVM?
- much more expensive per host -- starts at about $100. (recently I paid $40 for a 4 port KVM from AliExpress)
- lower resolution
If kvm comes back before monitors (likely as less electronics to reset), won't it have bad info about the monitors to give out when asked?
Apparently not. My other KVMs work.
The KVM could know when a monitor is connected and powered on. ------------------------------------ Description: GTALUG Talk Unsubscribe via Talk-unsubscribe@lists.gtalug.org Start a new thread: talk@lists.gtalug.org This message archived at https://lists.gtalug.org/archives/list/talk@lists.gtalug.org/message/2FLA6U2...
Description: GTALUG Talk Unsubscribe via Talk-unsubscribe@lists.gtalug.org Start a new thread: talk@lists.gtalug.org This message archived at https://lists.gtalug.org/archives/list/talk@lists.gtalug.org/message/3YFF2IQ...
Gad, I used to have 2-port and 4-port KVM (PS/2 keyboard mouse, VGA video) with mechanical rotary dial. I threw them away, but I think I still have the cables. On 2026-06-24 11:43, D. Hugh Redelmeier via Talk wrote:
I have a couple that are no longer used: - PS/2 KM and DVI V - PS/2 KM and VGA V
On Wed, Jun 24, 2026 at 08:48:54AM -0500, CAREY SCHUG via Talk wrote:
ignoramus here, who owns multiple KVMs.
If kvm comes back before monitors (likely as less electronics to reset), won't it have bad info about the monitors to give out when asked?
EDID is stored in an i2c eeprom and is powered by the video cable, so it is readable even if the display is off as as far as I understand it. Certainly VGA pin 9, pin 18 on hdmi and DVI pin 14 provide +5V power to eeprom. Displayport on the other hand does not provide eeprom power over the cable so those can only be read while on. -- Len Sorensen
participants (6)
-
CAREY SCHUG -
D. Hugh Redelmeier -
Evan Leibovitch -
Lennart Sorensen -
Nick Accad -
William Park