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I have a few computers connected through a KVM and one of my macOS systems has two monitors attached: the KVM attached display and a second display. When I switch off of the Mac, the still connected display becomes the primary display and things shift around, however I'd like to find a way to keep the non-active display that's attached through the KVM still "alive" to macOS such that both displays remain active as far as the operating system is concerned.

I'm not seeing any software mechanisms that facilitate this - is there a setting I'm missing that can allow this?

Or would some kind of a "dummy" dongle for the system be helpful to attach the KVM through to the display to make macOS think the display is still active?

Any thoughts or options to make this work would be appreciated!

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  • I was thinking more expensive KVM might keep the display enabled on MAC. Are you using powerless or powered KVM ? First search should be powered. Commented Oct 8, 2023 at 11:39
  • A dummy dongle ended up working well enough.
    – ylluminate
    Commented Oct 9, 2023 at 1:23

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A “dummy dongle” that passes through the video will likely solve your issue. There are pass-through dongles and ones that don’t for reasons of keeping a display and/or GPU alive for a “headless” server, it’s a hardware workaround to a software limitation.

What you are likely looking for is a “fast switching” KVM. Most KVM switches disconnect the display completely and so when the connection is restored the computer goes through the process of establishing a new connection. In the early days of VGA the “headless dongles”, or whatever one might call them, would keep the connection alive by shorting pins together to communicate that a display was attached and which resolutions are supported. Later on displays used a slow serial connection on one or two pins, and this carried over into DVI, HDMI, and perhaps other systems. The delay and/or the reset of the display settings is from this serial connection being lost and then reestablished. The higher end KVM switches listen in on this serial connection, relay that to the computer, and then leave the connection in place even while switched to another computer.

The pass-through “dummy display dongles” and the “fast switching” KVM switches are really doing the same thing, it’s just the fast switching KVM switch has the pass-through dongle integrated internally for all computers attached to it.

I do recall seeing software fixes for this problem and some are more elegant than others. One fix is a non-standard manipulation of the display output settings on the computer, to get this desired behavior people will modify bits of code that aren’t meant to be user accessible. They can’t always put things back the way they should. I recall people finding drivers from the GPU manufacturer that can sometimes manipulate the video behavior and have much lower risk of doing damage.

A KVM that has that integrated “dummy dongle” is the lowest risk option but also highest dollar cost. I recall seeing free utilities for changing video settings but I don’t recall their names. Knowing they exist is half the battle. What these utilities can do will likely depend greatly on the model of computer, especially if there’s an external GPU or only the one integrated into CPU.

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    Hey @MacGuffin do you have a link to an example product? Just as I'm searching for fast switching and can't find anything sadly matching this kiind of thing, but its exactly what I'm after!!! Commented Nov 16, 2022 at 10:40
  • Examples of "fast switching" KVM switches: startech.com/en-us/server-management/sv231usbddm startech.com/en-us/server-management/sv431dviuddm
    – MacGuffin
    Commented Nov 16, 2022 at 20:18
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    Thank you pal, I'll need to hunt down a HDMI one of these then :) Commented Nov 17, 2022 at 12:31
  • @JamieHutber Remember that HDMI has a lot of overlap with DVI, Displayport, and USB-C so it may be possible to get a passive adapter or cable for one of these to work with HDMI. They all have a "config" or "aux" channel for handshaking between the display and the video controller, if they speak the same language then you can have HDMI fast switching and some "future proofing" from a KVM switch using another video technology.
    – MacGuffin
    Commented Nov 17, 2022 at 18:01
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There's no software setting for this, unfortunately.

Instead I think you will want a headless HDMI dummy dongle with pass through. These exist in various forms and price ranges, such as for example the Fueran HDMI Pass-Through EDID Emulator found on Amazon for $21. At the other end of the price range is the Atlona Etude Sync 4K HDR HDMI EDID Emulator, which I found at B&H for $364.

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