Show HN: Multi-monitor KVM using just a USB switch (opens in new tab)

Good KVMs, specifically the ones from Level1Tech[1], are smart enough to keep all monitors connected virtually so all your windows stay in place when you switch back and forth between machines. There is no delay of plugging devices in because from the perspective of the client PCs the USB/display port devices were never gone. Of course you can connect a bunch of peripherals to a dock and switch the upstream thunderbolt connection, but that doesn't work correctly.

Crappy KVM solutions are easy. Good ones are hard.

[1] https://www.store.level1techs.com/products/hardware (no affiliation)

This is nice. I know some digital nomads are using KVM to evade detection by their employers.

Haha, just writing a "me too here", I just came to the same conclusion, but I think my setup is better and cheaper, if a tiny bit more complex. For example, the above would need to install to all computers which is a no go for me. Also this is dependent on the USB switching, cannot be triggered fully remotely.

- DDC commands for monitor input switching

- USB KVM that has only one button to switch - mine was 25$, there are plenty of models out there

- Stream deck or prebuilt macropads are super expensive (80$+) for what they are - a few buttons. Get a cheap AliExpress macropad, they go for 5-10-15$ and have all kinds of layouts. They are also much smaller, you can get just one, or 2-3-4 buttons. Don't use the crappy software they ship with, there are github projects to program them. https://github.com/rOzzy1987/MacroPad

- FingerBot 16$ (if you already have HomeAssistant, Zigbee+BT it's faster) or Switchbot 28$ (bluetooth, has a bit more delay) to press the USB KVM button.

- Then an AHK switch to watch for keyboard shortcuts, and trigger both the DDC commands and the USB switch button.

It has a few seconds delay, but it's controlled from one source only, and can be also fully triggered remotely.

After researching KVMs I also came to the same conclusion that ddc is the solution.

My use case is a bit different. I have two Windows machines and a dual monitor setup. I use NirSoft ControlMyMonitor which has a CLI and a Elgato Stream Deck.

In a normal KVM approach both monitors are showing only one computer. My use case is a bit odd- I want one computer on monitor A and the other on monitor B. I may also want to flip that. I use a portrait and landscape setup. Then I want to be able to switch mouse/keyboard independently of monitor configuration.

Each computer runs the Stream Deck software and the Stream Deck is connected to the USB switch.

I found even these simple USB Switchers to be quite expensive, and sometimes they require drivers that don't work on Linux. I quickly sketched a simple circuit board with USB hubs and a switch and ordered them assembled (!) from one of those DIY-fabs (JLCPCB or PCBWay). They cost 25€ for 5 pcs, each with one 2-to-1 switch. I don't have any experience in circuit design and didn't have to solder anything, and they work perfectly.

These would go pretty well with this software, maybe I'll give it a try.

Before settling back to one computer, I've done the cable replug thing manually, then used the urgreen USB switch as in the README then settled to using a monitor with built-in KVM switch, and it was the best. The problem is when you want one monitor for work and gaming - 4K (and/or high-refresh) KVM switches are incredibly expensive and tend to be unreliable, so what you usually do is use USB switches and switch input source on the monitor manually, the software here saves you clicking the "change input source" button, but seems to be Windows only, so unless both machines run Windows it'll be a one way switch and you'll have to resort to manual switch back.

The monitor that is suitable for gaming and has high DPI high refresh and built-in KVM switch is Gigabyte M28U, clicking one button switches input source and USB peripherials, works quite nicely, there's also M32U but with less DPI.

There's a blog post about a similar thing but without any extra hardware. It uses barrier and a few simple scripts. (monitor does have to support switching via dcc).

It's not perfect especially since the input switching for the display is quite slow but there's leg in hardware kvms too.

https://xpufx.com/posts/hundred-percent-software-kvm-switch/

To clarify (the first thing I wondered) each computer needs its own HDMI cable plugged into its own input on the monitor. So not true KVM as many people would think of it, but this saves you from having to use the monitor's input selector.

At least one of the computers must be on and running Windows, but maybe only one?

Why is it callled a "KVM"? What does it stand for?

Just in time project! Do you know if it is possible to just switch a Thunderbolt dock? I mean: you have, fir example, four computers all supporting Thunderbolt and you just switch a USB-C between those comouters?

I have not tried because I am concern with affecting the hardware. Based on your inspiration I just found Thunderbolt KVMs such as [1] and [2].

[1] https://sabrent.com/products/sb-tb4k

[2] https://www.reddit.com/r/Thunderbolt/comments/11gos6n/kvm_sw...

Nice project.

I wrote sth similar for myself, without requirement of USB switch or other hardware. it sends DDC command to HDMI/USB-C monitor and HID++ byte code to Logi MX keyboard and mouse.

Code is too messy to share but here's snippet if anyone want to write their own:

    hid_devices = [
        {
            'name': 'Logi Bolt Receiver',
            'receivers': hid.enumerate(0x046D, 0xC548),
            'command': [  # MX Keys Mini as first device on the receiver
                bytes([0x11, 0x01, 0x09, 0x1A, 0x00] + [0] * 15),  # change host to 1
                bytes([0x11, 0x01, 0x09, 0x1F, 0x01] + [0] * 15),  # change host to 2
                bytes([0x11, 0x01, 0x09, 0x1D, 0x02] + [0] * 15)]  # change host to 3
        }, {
            'name': 'Logitech Unified Receiver',
            'receivers': hid.enumerate(0x046D, 0xC52B),
            'command': [  # MX Anywhere 3 as first device on the receiver
                bytes([0x11, 0x01, 0x0A, 0x1A, 0x00] + [0] * 15),  # change host to 1
                bytes([0x11, 0x01, 0x0A, 0x1E, 0x01] + [0] * 15),  # change host to 2
                bytes([0x11, 0x01, 0x0A, 0x19, 0x02] + [0] * 15)]  # change host to 3
        }, {
            'name': 'MX Keys Mini Bluetooth',
            'receivers': hid.enumerate(0x046D, 0xB36E),
            'command': [
                bytes([0x11, 0xFF, 0x09, 0x1E, 0x00] + [0] * 15),  # change host to 1
                bytes([0x11, 0x01, 0x09, 0x1F, 0x01] + [0] * 15),  # change host to 2
                bytes([0x11, 0xFF, 0x09, 0x1D, 0x02] + [0] * 15)]  # change host to 3
        }, {
            'name': 'MX Anywhere 3 Bluetooth',
            'receivers': hid.enumerate(0x046D, 0xB025),
            'command': [
                bytes([0x11, 0xFF, 0x0A, 0x1A, 0x00] + [0] * 15),  # change host to 1
                bytes([0x11, 0x01, 0x0A, 0x1E, 0x01] + [0] * 15),  # change host to 2
                bytes([0x11, 0xFF, 0x0A, 0x19, 0x02] + [0] * 15)]  # change host to 3
        }
    ]

I have been doing this for years without any additional software: just make sure your monitor goes to blank when locking the screen:

1. lock screen

2. Switch USB switch to the other source

3. Press space to activate the other PC

4. Monitor should automatically sitch to the "new" active source.

I have always found that KVM switches have side effects on USB devices.

So I use mechanical USB switches.

for keyboard and mouse, I use two of these:

https://www.amazon.com/dp/B01I0Y3GEE/

They have no extra keyboard/mouse latency, and no weird side effects like boot keys that don't work or modifier keys that are in the wrong state.

Issues I've had before with mouse/keyboard:

- hotkey conflicts

- mac boot keys break (hold down <key> for <some boot function>)

- modifiers stick in wrong state

- weird delays

- combinations with mouse+keyboard break

Any advantage over Synergy?

My mid-range LG monitor has a built in KVM as well as PBP (picture by picture??) --- Whatever it stands for, I am able to split my 34 inch wide screen between my work Macbook and my personal computer (Linux) - I use an app that I've had forever called Synergy to share my mouse and keyboard between them seamlessly. You can even copy/paste across machines. I personally am hooked on my PBP widescreen+Synergy combo.

For software KVM you can use https://github.com/debauchee/barrier

I use it between a Windows PC & a Macbookpro (Linux version available but I don't have Linux)

With my COVID obsession of live streaming while writing code, I ended up building something a little more elaborate. https://github.com/timgws/kvm-switch

It doesn't use DDC commands direct to the monitor(s), instead swapping it for a HDMI matrix combined with a traditional KVM for swapping around inputs smoothly when you roll your mouse over the edge of different operating systems.

The switching client runs on Windows, MacOS and Linux (if you use X).

I’ve been using display-switch[0] for this for a while now. No problems whatsoever. Works on windows/mac/linux.

[0] https://github.com/haimgel/display-switch

Looks pretty useful if you don't want to spend more on your KVM switch than you spent on your monitors combined!

Well not for me because my boxes run multiple operating systems, but for someone with just multiple windows computers it would be great.

For this to work, does my primary computer need to be on all the time? The way I understand this, the 'host' computer will be running this piece of software and fires the event for the monitor to switch inputs.

Would a USB bluetooth adapter to connect to wireless keyboards and mouses work with this??

If you only have two PCs, you only need to run the program on one of them.

When it detects a USB device being plugged in, it changes the monitor input to this computer. When a USB device is removed, it changes the monitor input to the other computer.

How would this work if your monitor uses HDMI?

I use udev for this purpose (send a DDC command when the USB switch appears):

  ACTION=="add", SUBSYSTEM=="usb", ENV{ID_VENDOR_ID}=="1a40", ENV{ID_MODEL_ID}=="0101", RUN+="/usr/bin/ddcutil --sn ABCDEFGHI setvcp 60 0x0f"

One thing to note here with these kind of tools, on displayport they typically only allow commands on the active input. Once you switch to another you can't switch back (unless you do it from the other computer). Which is kind of a hassle.

HDMI on the other hand typically allows the ddc-commands even on ports that are not the active input.

Not sure if the above must be the case but it has been the case for all displays I've used.

After trying many hardware and software solutions for easily switching between my rigs, I settled on something much simpler: a monitor with a KVM built-in. It does not have to cost you an absolute fortune. I personally chose the Gigabyte 34" M34WQ; it's probably one of the very best purchases I made in the past two years, and it simplified moving between my work and personal rigs an absolute breeze.

I haven't reviewed the code for this -- but I like this idea a lot!

I would love to see a future piece of software, one that could use any number of USB KVM's like this software does -- but could also serve as the main GUI for any number of local VM's and/or remote VM's -- connected over USB or Ethernet or software-controllable "smart" hardware KVM switch or some other intermediary layer.

In other words, abstract:

a) VM GUI;

b) Connection medium to that VM (USB, Ethernet, Smart Hardware KVM Switch, Firewire, ?, ???)

c) How/where the a given VM GUI is displayed on the local monitoring computer, i.e., be able to shrink screen, display on a multi-row, multi-column display (like a Zoom meeting!), group various related VM GUI's, have a local directory and search for VM (for example, if I've got a software development environment of various x86 Windows VM's and various ARM Android VM's -- then I might want to group by CPU or Operating System or software stack version, etc., etc.)

Now, probably all of this software infrastructure exists in one form or another.

For example, in QEMU, local or remote VM's can be created and then they can be connected to via TCP/IP...

But, to centrally manage all of those QEMU VM's and non-QEMU ones as well, as well as regular PC's running any OS on "bare metal" over any connection medium (a heterogenous network of computing machines) -- that would be the goal of this software!

VNC, or something like it, could be used on the client-side of bare-metal PC's, but wouldn't be necessary for VM's...

Anyway, KVM over USB (the original article), is a great idea!

Only after I've got my ham radio license that I learned how these USB switches are annoying sources of RFI, even the more expensive ones. KVM switches are fine though.

What I ended up doing is making sure all my computers support Thunderbolt 3/4 (or USB 4 at minimum), getting a single good Thunderbolt dock (CalDigit TS4+) and just swap out which computer’s TB cable is plugged into the dock. To ease this I have a little magnetic pad stuck to my desk next to the dock which holds each computer’s cable when not in use.

It works great for my needs. Perfectly for switching between Macs or PCs with Intel iGPUs. It almost works perfectly with my gaming PC (which sometimes does double duty as a Windows/Linux workstation) but its Nvidia GPU can get cranky when used with Thunderbolt GPU passthrough for some reason under both operating systems. I think the issue is actually in drivers, but good luck getting Nvidia to care about this use case. I’m considering putting an old AMD GPU in the second slot on the PC and using that for passthrough duty.

There are pretty reasonable 4-port KVM switches on Aliexpress that support 60Hz 4K and USB-3 for $55 or even less.

I got this https://www.aliexpress.com/item/1005002941646654.html a year ago. It has some minor compatibility problems with my Macbook Pro (or Apples' multiport adapter) which require me to remove and reinsert the USB-C cable from the Macbook once in a while, but it otherwise works very well.

A year later, there are multiple such devices starting from $30, or even less.

I am using this https://a.co/d/4Bl8opp to set up my 2 MacBook Pros with 8k Monitor. The good thing about this is, it also has power delivery so that I don't have to use another cable to manage power and replaced my 2 docking stations with this. However, this still sometimes loses signal and I had to switch to other monitor and back with remote to get the signal back which is not ideal but have to live with because of limited options out there. Another limitation is, it supports 8k with 30Hz refresh rate only. I am open to suggestions if anyone has solved this problem.

I've been pretty happy with a monitor acting as a kvm, where the usb output (usb-c or usb-3) can be mapped to a display output.

It's nice for a wfh setup where I have a personal desktop and a work laptop, and I can quickly switch between them just by changing the display output.

I realize it’s not the use case in OP, but if each of your machines has a display, Barrier is like a super power.

Also allows copy-paste across OSes.

https://github.com/debauchee/barrier

It’s not really a switch but I just simplified my connection to just a single thunderbolt cable on the computer. With that, they all get the same two monitors, keyboard, mouse, and anything plugged into the dock.

This is great, but currently very limited; partial support for Windows. Seems inclined more towards Apple.

https://github.com/feschber/lan-mouse

I have a Mac and Windows hooked up the same monitor and keyboard/mouse. I used a USB-switch and connected the monitors separately with USB-C and VGA. Then I don’t have to route the video though the USB-switch.

wow, great! (in theory, sadly my monitors suck. Dell one refuses to see dp and the cheap one does not react to hdmi when set through the tool... I got one specific setup with normal kvm that mostly works, all others don't...)

I use dell display manager with dell monitors and one of the switches in the article.

I use DDC and synergy. So no hardware. It is mostly working OK.

Does this work? This would be amazing.

This is genius

I have a posix script does the same thing but after a few months the screen was dead, somehow I feel manual switch screen input is safer.

Cool project!