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Toggling two monitors with a single port KVM Switch

The problem

I have two PCs and two monitors and a single port HDMI KVM switch that toggles one monitor, keyboard and mouse between the PCs. This leaves the second monitor that I need to press buttons on (go into the menu, select an input) whenever I toggle the KVM.

The solution

Then I discovered ddcutil which is command line tool that can interact with monitors and change their settings (e.g. color profiles, change inputs).

I took a look at dmesg to see if either of my PCs logs anything when the KVM toggles. I found that the PC notices that the USB keyboard dongle is removed when the KVM toggles to the other PC and is added the the KVM is toggled back.

The means that we can create a udev rule that monitors for specific hardware changes. When it sees the dongle go away, switch the screen to the other input. When it sees the dongle appear, toggle the screen back to the local input.


On Ubuntu, install the ddcutil package (sudo apt install ddcutil). Then have it probe your monitors:

ddcutil detect
Display 1
I2C bus: /dev/i2c-1
EDID synopsis:
Mfg id: ACR
Model: Acer KA241
Serial number: T6RSA0014200
Manufacture year: 2018
EDID version: 1.3
VCP version: 2.1

Display 2
I2C bus: /dev/i2c-2
EDID synopsis:
Mfg id: ACR
Model: Acer KA241
Serial number: T6RSA0014200
Manufacture year: 2018
EDID version: 1.3
VCP version: 2.1

Invalid display
I2C bus: /dev/i2c-4
EDID synopsis:
Mfg id: LGD
Serial number:
Manufacture year: 2014
EDID version: 1.4
DDC communication failed
This is an eDP laptop display. Laptop displays do not support DDC/CI.

My two external monitors (the final entry is the built-in display on my laptop) show up on /dev/i2c-1 and /dev/i2c-2.

Running the same command on my other PC shows the second monitor as not supported. I assume that this is because it is connected via DVI instead of HDMI. This means that only one PC can control that second monitor.

sudo ddcutil capabilities --bus 1
MCCS version: 2.1
Command: 01 (VCP Request)
Command: 02 (VCP Response)
Command: 03 (VCP Set)
Command: 07 (Timing Request)
Command: 0c (Save Settings)
Command: 4e (unrecognized command)
Command: f3 (Capabilities Request)
Command: e3 (Capabilities Reply)
VCP Features:
Feature: 02 (New control value)
Feature: 04 (Restore factory defaults)
Feature: 05 (Restore factory brightness/contrast defaults)
Feature: 08 (Restore color defaults)
Feature: 0B (Color temperature increment)
Feature: 0C (Color temperature request)
Feature: 10 (Brightness)
Feature: 12 (Contrast)
Feature: 14 (Select color preset)
05: 6500 K
08: 9300 K
0b: User 1
Feature: 16 (Video gain: Red)
Feature: 18 (Video gain: Green)
Feature: 1A (Video gain: Blue)
Feature: 52 (Active control)
Feature: 6C (Video black level: Red)
Feature: 6E (Video black level: Green)
Feature: 70 (Video black level: Blue)
Feature: AC (Horizontal frequency)
Feature: AE (Vertical frequency)
Feature: B6 (Display technology type)
Feature: C0 (Display usage time)
Feature: C6 (Application enable key)
Feature: C8 (Display controller type)
Feature: C9 (Display firmware level)
Feature: CC (OSD Language)
00: Reserved value, must be ignored
01: Chinese (traditional, Hantai)
02: English
03: French
04: German
05: Italian
06: Japanese
08: Portuguese (Portugal)
09: Russian
0a: Spanish
0c: Turkish
0d: Chinese (simplified / Kantai)
0e: Portuguese (Brazil)
14: Dutch
16: Finish
1e: Polish
Feature: D6 (Power mode)
01: DPM: On, DPMS: Off
04: DPM: Off, DPMS: Off
05: Write only value to turn off display
Feature: DF (VCP Version)
Feature: 60 (Input Source)
01: VGA-1
03: DVI-1
11: HDMI-1
Feature: FF (manufacturer specific feature)

I want to target feature 60. I want to be able to change between 03 and 11. Just to frustrate people, that 11 is a hex value. The setvcp command we will use in a moment expects a decimal input. 11 becomes 17.

Switch the first monitor to DVI input:

sudo ddcutil setvcp 60 03 --bus 1

Switch the first monitor back to HDMI input:

sudo ddcutil setvcp 60 17 --bus 1

If that worked for you, your display hardware is compatible.

USB device


Start the udev monitoring tool:

sudo udevadm monitor --kernel --property --subsystem-match=usb

Toggle your KVM back and forth. If the above command returned a bunch of events then there are hardware events that we can attach to.

Look for something like this:

KERNEL[1741870.274946] remove   /devices/pci0000:00/0000:00:14.0/usb1/1-4/1-4:1.0 (usb)

The PRODUCT seems to be unique to my USB dongle and correlates with the output of lsusb:

Bus 001 Device 049: ID 046d:c52b Logitech, Inc. Unifying Receiver

udev Rules

ACTION=="add", SUBSYSTEM=="usb", ENV{PRODUCT}=="46d/c52b/1211",
RUN+="/bin/sh /usr/local/bin/ddcswitch"

Handler script

The script that is executed on matching events can be found here: [ddcswitch-unlink-2_monitor_kvm/ddcswitch). Put the file in /usr/local/bin

  • Change the "PRODUCT" string on line 5
  • Change the Feature Code and values on lines 8 and 10

Make the script executable:

chmod +x /usr/local/bin/ddcswitch

In it's default form, the script logs a lot of information to /tmp/ddcswitch. If it doesn't work right away, look to this file for clues.