Two computers, one monitor, zero fiddling (2025)

I solved the multi-computer-to-multi-monitor problem with a Level1Techs KVM[1]. The price (~$500 for the variant I purchased) initially gave me pause, but the longer I've had it, the happier I am with my decision.

It handles all the switching at the hardware level and thus has no perceptible lag for video or anything else. I'm able to connect a single set of peripherals, in my case, two monitors, a keyboard, a trackball, and a USB audio interface, to both my Linux desktop and a CalDigit Thunderbolt dock connected to my laptop. The L1T KVM has hotkeys[2] that let me switch between systems, with only a 1–2 second delay.

The benefit, for me, of this extra box now mounted under my desk is that when I upgrade my monitor, I only care about how good a display it is, not whether there's some perfect confluence of KVM, refresh rate, aspect ratio, display technology, etc. I find the monitor I want and let a separate IO routing layer.

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[1]: https://www.store.level1techs.com/products/p/14-kvm-switch-d...

[2]: https://forum.level1techs.com/t/official-l1techs-kvm-faq-ult...

I am trying to do the same, and I absolutely don't understand what is happening in the 3-5 seconds while two gigahertz-level machines and a modern monitor negotiate something over tens-gigabit-per-second connection.

If Steve Jobs herded makers of all the parts into a single room, and told them they aren't leaving until this takes 10 ms, I would be immensely grateful and I bet it could be resolved in a week.

What a great idea. It should be obvious and easy but DDC commands are hard to find and should be documented better.

I have a Dell U4323QE in the office and look forward to trying this out. I wondered if it was the same DDC commands so I googled a little and found this gist (concerning DDM):

https://gist.github.com/nebriv/cb934a3b702346c5988f2aba5ee39...

Which has the very useful comment:

https://gist.github.com/nebriv/cb934a3b702346c5988f2aba5ee39...

Which states:

#define LUMINANCE 0x10 #define CONTRAST 0x12 #define VOLUME 0x62 #define MUTE 0x8D #define PBP 0xE9 #define SWAP_USB 0xE7 #define SWAP_INPUT 0xE5 #define INPUT 0x60 #define SUB_INPUT 0xE8 #define INPUT_ALT 0xF4 // alternate address, used for LG exclusively? #define STANDBY 0xD6

I much prefer simple DDC commands over using something like Synergy or Barrier. I think it is a much cleaner solution.

I did something similar last year with a monitor without built-in KVM but with good DCC support (Ultrasharp U3417W) and Synergy [0].

I use Synergy as part of my desk setup already, but needed a way to view the UI of a normally headless machine. The solution I built was a small shell script that terminated the active Synergy session and started a new one with a different config file (so keyboard/mouse input would map to the normally-headless machine), and fired off a DCC command to the monitor to change its input. The same script ran with a different argument would switch back to the normal display/control configuration. This solution worked pretty well until I was able to retire the headless machine early this year.

[0] https://symless.com/synergy

I love this approach, but a few years ago I tried it very unsuccessfully with my Xiaomi Mi 34" ultrawide.

By very unsuccessfully, I mean that the commands to change input didn't work. I can't remember if they did nothing or crashed the monitor, but subsequent investigation led me to realise I was lucky not to brick it, as some people found certain commands cause non-recoverable issues on that monitor!

So, I suggest caution in the form of maybe checking that others have successfully used DDC commands on your particular model.

https://github.com/rockowitz/ddcutil/issues/153

I have a Dell U3225QE with a built-in KVM and 2 macs.

One connects with Thunderbolt only. The other connects with Display Port for video and USB-C for the rest of the built-in dock.

It's OK most of the time with the nipple switch. My one piece of advice is *avoid HDMI*. I learned after getting this monitor that the HDMI protocol is a petulant unstable little shit that does not tolerate renegotiation well. Get yourself a USB-C to DisplayPort cable.

If you turn one computer display off as in "xrandr --output DVI-D-0 --off", the monitor automatically selects some other computer to display.

Thus I have 2 computers and 3 displays, and I can do sentences like "displays 13", which uses only displays 1 and 3 and sends ssh-command "displays 2" to the other computer.

Personally i prefer two computers, two monitors, one mouse/keyboard.

Deskhop has been a lifesaver https://github.com/hrvach/deskhop

For anyone that wants actually seamless switching of video and data, look to professional AV switchers and KVM. This Lindy 5x2 HDMI + USB KVM is the business [1]. I use one because I constantly switch between Mac and Linux while working on the same project and the couple of seconds of black really gets tedious pretty quickly.

https://www.lindy.eu/5-Port-Seamless-Multiview-KVM-Switch.ht...

Just dropping a note to say I’ve had the same monitor for a year and I absolutely love it. I don’t care about this seamless switching — I just use HDMI1 for Xbox, HDMI2 for my computer, and then swap hobby/work when needed. It’s also good motivation to turn off the work laptop when I’m done with the day.

The monitor is fantastic though. I’ve had no issues yet, knock on wood.

I'm going to have to look into this. I've got two machines using the same monitor (indeed work macbook and home linux). It's a curved 4K Samsung Oddyssey of some sort, and I can either switch through the monitor's menu, or set it to switch to the machine that gave the most recent input. The latter sounds perfect, but unfortunately both options suck. The monitor is incredibly slow to respond to anything: turning on, opening the menu, switching input, whatever. It always takes a couple of seconds, sometimes forgets to turn on at all, and often when I do something on one machine, the other suddenly wakes up, presumably by the monitor. The setup is unpredictable, and I blame the monitor.

With Windows and AutoHotkey and https://www.nirsoft.net/utils/control_my_monitor.html

You can add this .ahk script to run at startup:

    ^F4::
      Run C:\Users\YourNameHere\Documents\AutoHotkey\ControlMyMonitor /SwitchValue Primary 60 17 3
      Return

Where the file path is where you've put ControlMyMonitor.exe, "Primary" means the main Windows display, the "60" means input select, and the "17" and "3" are the values you observe in ControlMyMonitor when each display you want to switch between is enabled.

You can now press Ctrl+F4 to toggle inputs.

Isn’t the answer buy a kvm switch? If yes this could have been really short

I have a Iiyama with build in hub, at first it seemed ideal, just plug everything into the monitor. But my mouse glitches when used through the hub (MX Master 3S), the internet cable is only available to the active computer (so no tasks requiring internet can run on the other computer).

All in all, I just use the USB-C to HDMI switch functionality and couple the computers to keyboard/mouse via bluetooth, plus I bought a switch for the ethernet. It's a shame it's so suboptimal.

Another minor annoyance is that when I turn off one computer and want to switch to the other, the display turns off, taking about 5-10 sec to do so. Then I manually turn it back on and it's ready in another 3 sec. So I do this thing where I hover about the turn-off button, hit it and immediately switch so the monitor won't turn off.

It's hard to make convenient tech it seems.

I am just using RDP from one PC into another presently -- to solve this in a low complexity way. Tried a lot of approaches in the past -- none were reliable for me.

Yeah, this all sounds good in theory, but there's a lot of edge cases. For example, for me switching between Mac and Linux what often happens is that Linux just for some reason the it turns off the monitor port and it's black until I reboot and there was no easy way to get it back. This is while using fancy Dell monitors built-in KVM. Ultimately, I have settled on remote desktops as a more viable and quicker option.

Wait why not just buy a usb switch? I switch between 4 computers using the same monitor, keyboard and mouse.

Note that your monitor is a QD-OLED, not a QLED monitor.

Despite the similar names, these are two very different technologies. QD-OLED is a OLED panel that uses QD (quantum dots) to achieve a wide color gamut. QLED is a LED backlit LCD panel that uses QD in a similar way.

The difference in performance characteristics of these two panel types is dominated by the difference between OLED and LCD.

It never occurred to me that you can send commands across DDC to your monitor. Binding that to a key on the keyboard in different OSs to trigger the monitor's built in KVM is a nice touch. I only change between my computers a couple times a day else I'd be setting this up this evening

I gave up on a hardware solution to this and currently just use Screen Sharing to "remote" into my personal machine from my work machine, which I guess only works because they're both macs, although VNC probably solves this in a cross platform way. I have an Apple studio monitor so built-in KVM isn't possible, although maybe there's a jailbreak for it since it has its own processor and firmware? I still just vastly prefer the quality of Apple displays so I optimize for that first.

Also, OT but I have the same keyboard as OP and love it :) I want to hack a TouchID key from the Magic Keyboard I bought into the chassis. But it can't traverse the Screen Sharing hack, so I do still think about this from time to time.

For a run-down of various hardware KVM solutions that can handle high-resolution video at high refresh rates, see:

https://hardwarerecs.stackexchange.com/q/18561/7591

The UGREEN 8K Displayport KVM Switch has been running flawlessly for four months of continuous use.

https://www.amazon.ca/gp/product/B0CFFFHFJT

This is pretty cool, dunno if I'd want to commit to a specific monitor brand to get this functionality though. My primary concerns with monitor selection are ports and color quality.

I have an old TrendNet KVM that I've used for years but it doesn't support Thunderbolt so it's getting a bit long in the tooth.

Its amazing to me how annoying these problems can be and there's no real one size fits all solution. I have a desktop PC and macbook, and two monitors. I use a KVM and also switch inputs on the monitors themselves.

I tried very unsuccessfully to do something with DDC/CI when the KVM would switch between systems. The idea being when the OS detect the presence of the keyboard/mouse because the KVM had switched to them they'd send a change source command but DDC/CI is such a disaster in terms of support.

We need someone like Framework to make a "monitor for hackers" that actually has robust, well documented, DDC/CI support and I'd be all over it.

Thank you! I hadn't thought of using software to handle monitor switching.. I have a very similar setup but my monitor doesn't support usb upstream so I have a dumb usb hub with a button to switch sources then still have to manually use monitor input change.

You gave me the idea to setup scripts on the windows / macos system to 'react' to hub detach events (which I click the button) and execute cli commands to change the monitor source automatically to the other machine. Too good.

This is great. The Holy Grail of work/personal computing setups IMO. Too bad it’s so expensive.

I wish a KVM switch was a standard component of normal priced monitors these days. Especially one that also routed through all your peripherals, speakers, and everything.

I have my Windows gaming rig in a rack at home, and run Apollo [1] on it. Using that, I can game on any Apple TV (with an Xbox controller) or the MacBook (connected to a display/keyboard/mouse) anywhere in my home. With wired networking 60fps at 4K is no problem at all.

This would be easy to set up the other way around, too: having a gaming rig on your desk with Moonlight, and running Linux on another machine somewhere in the network with Apollo to host the development setup.

No KVM (or KVM-equipped monitor) or other special hardware needed.

1: https://github.com/ClassicOldSong/Apollo

Like many in this thread, I have a similar setup. I used to use I separate KVM, which did work but had a lot of cabling.

Eventually, I decided on just a simple USB SPDT switch that I designed [0], a USB hub, and manual input switching on the monitor. Even though it takes two actions instead of one, it's a simpler setup and has been quite reliable for years now.

[0] https://shielddigitaldesign.com/posts/2021/susb/

I have a couple of Eizo EV3285, which have enough separate physical inputs for the 3 machines I use to drive them. Only real PITA is having to press the input selector on both. Must admit I wasn't even aware of DDC!

As long as everybody is posting their favorite solutions. https://xpufx.com/posts/hundred-percent-software-kvm-switch/

I use 2 computers without a KVM. My keyboard, mouse, and soundbar all support Bluetooth. All wired to the PC by default. When I switch to the Mac, I just flip each device to Bluetooth mode.

I have a few boxes that I switch between, but for some software it's nicer that my "main machine" be on DVI, and everything else HDMI. I may have to look at some scripting option where if the keyboard / mouse disappear (KVM switched away) change the display to use the HDMI input.

I do worry that would just add more trouble / race conditions / issues around this stuff. I feel like nvidia + linux + monitors doing anything other than staying on + attached all the time causes some headaches.

In case anyone else likes to know the panel manufacturer before buying a display, it looks like this one is made by Samsung.

https://www.displayspecifications.com/en/model/f86e3770

https://www.displayspecifications.com/en/model/49bc3e67

Nice write-up! I wrote https://github.com/fiddyschmitt/SimpleKVM which makes the process easy on Windows.

As others have mentioned, you can use a cheap USB switch if you don't have an in-built KVM.

DDC/CI is quite interesting. Most OEMs seem to implement it just fine. But some (like LG) need special handling.

I'm using Apple Studio Displays and have been on the lookout for a solution like this for quite a while. My grail is a Thunderbolt KVM; there are a couple on the market, but they cost a fortune and have terrible reviews. I guess the use case is so narrow that nobody is investing in it.

An alternative could be a good 5K/6K monitor with a KVM, but this is a unicorn as well...

I have a Samsung Ultrawide monitor. It allows for two simultaneous video input displayed on the screen at once, in either 50/50 or 66/33 split. Combined with Barrier for mouse-keyboard switching and keyboard integration and I can use both devices simultaneously on the same monitor, almost as though they were a single device.

I have a similar setup for my MBP + Windows Desktop. I ended up using a simple USB switcher for all the devices and just running two inputs and manually switching between screen inputs. One button press for USB switch, one source switch on monitor. If I need something from the other computer during the use, I just remote in.

I've done the same with Lenovo and BENQ monitors with built in KVM, but DDC commands were new to me. I just switched source on the monitor buttons and it would also switch all USB peripherals at the same time.

FYI that if you have the right kind of Dell monitor, you can download their Display Manager software to do the same thing (and more): https://www.dell.com/en-us/lp/ddpm

Iogear (aten) kvms let you switch without moving your hands off the keyboard. There is a hot key you can activate the. Press the input number to connect and the KVM switches.

You can do this with even less fiddling just by getting a KVM that supports video. There are reasonably priced ones that can even do 4K 60Hz. This also means you don't have to deal with monitors that don't implement input switching via DDC/CI (thanks LG).

This is awesome. I chose my monitor among others because of the built in KVM switch. I always found it annoying that I had to use the navigation nipple to switch inputs. This is now in the past.

If someone wants to know I have an MSI MPG 491C QD-OLED.

Ah I went through the same thing myself, but with Windows

https://moshen.net/posts/virtual_kvm/

This is great, I have a monitor with built in KVM (CORSAIR XENEON 27QHD240 OLED)

m1ddc works fine on my Mac, but why isn't there a single multiplatform cli tool that can be ran on Mac/Linux/Windows?

I need a Windows one for this to be useful for me.

Tell the dude there're software KVMs - Synergy, Barrier, DeskFlow, InputLeap. I'm with Barrier for many years and it solves many use cases.

back in the '00s i used a hardware kvm that could be controlled by the keyboard with some weird key combo (~ ~ (1|2)? maybe?). these days i strongly prefer deskflow (oss version of synergy) for this sort of thing or just ordinary remote desktop for the secondary. (depends on the task, if you're just building for the secondary or reading email it doesn't really matter- but if you're developing interactive applications or you need to reboot a bunch or something, then having the physical hardware with a local head can help).

i have the same setup with my dell but i would love to use usbc for linux too . just one wire each to either device.

Honestly the best thing that worked for me, even if the hardware is not for everyone, is a Logitech MX Keys that supports 3 devices paired with another Logitech mouse that supports 3 devices, then flip the monitor through the inputs, super reliable. I could even have 2 devices side by side on the same screen (Dell utlrawide) and toggle between them.

I didn't know that DDC was a thing! Super cool.

Zero fiddling, but substantial financial outlay

I sometimes use a Raspberry Pi at work, and found the miniKVM to be a useful tool for controlling it:

https://www.crowdsupply.com/techxartisan/openterface-mini-kv...

now we need the same for two macs !