Allow switching display inputs via DDC when a chosen USB device connects/disconnects

[waydabber]

Discussed in #3032

^{Originally posted by jgibson02 June 1, 2024}
Basically this is asking to replicate the functionality of haimgel/display-switch

Use case: I have multiple monitors that I use for both my work and personal setups and currently switch between them using a USB switch by pressing the switch's toggle and then using the OSD menus for each monitor to switch inputs. I heard about display-switch but then saw that it doesn't support M1 macs, but m1ddc does and that it's built by the creator of BetterDisplay which I already had installed! Currently I'm working on setting up display-switch to detect the USB connect/disconnect but have it call the BetterDisplay CLI to run the DDC commands.

[TheRevDrJ]

I use Hammerspoon to check for usb connections and switch video inputs using BetterDisplay.

Here's my work document, if it helps anyone.

Mac + PC sharing one monitor, one K/M — no KVM required

A working setup for sharing a single monitor, keyboard, and mouse (and any other USB peripheral) between a Mac and a Windows PC. Single physical button press flips both the keyboard/mouse routing and the monitor input. No expensive KVM. No software running on the PC side.

Built and tested on a 45" LG UltraGear OLED (45GX950A) — a monitor that breaks every standard DDC tool because LG uses an alternate DDC protocol. This guide handles that case specifically, but the pattern generalizes to any monitor BetterDisplay supports.

Architecture at a glance

[Keyboard + Mouse + any USB peripheral]
              |
              v
       [USB switcher with button] ←—— you press this
              |
       +------+------+
       |             |
       v             v
   [Mac (USB-C)]  [PC (USB-A)]
       |             |
       |             |  (both also connected to monitor inputs)
       v             v
     +-----[Monitor]-----+
                 ^
                 |
   Mac runs Hammerspoon watching USB events
       → calls BetterDisplay CLI
       → flips monitor input via DDC

When the user presses the USB switcher button:

1. K/M physically reroutes to the other PC (the switch does this in hardware)
2. On the Mac side, Hammerspoon detects the USB devices connecting/disconnecting
3. Hammerspoon calls a tiny shell script that fires BetterDisplay's CLI
4. Monitor flips inputs to match where the K/M now lives

Result: one button press, full environment follows. PC needs zero software.

Bill of Materials

Item	Cost	Notes
USB switcher with desk-button	~$80	UGREEN 2-in-7-out USB 3.2 (B0FDW6Z4J4) used here. Any cheap USB 3 switch works.
USB 2.0 male-to-female extension cable, 3-6 ft	~$8	Required if you use a Logitech wireless receiver. Forces USB 2.0 speeds at the receiver, distancing it from USB 3 RF interference.
BetterDisplay	Free	Free tier handles input switching. macOS App Store or Homebrew.
Hammerspoon	Free	Mac automation framework. Homebrew.

Why each piece

• USB switcher handles K/M routing in hardware. No driver, no software, no synthetic-keystroke shenanigans, works reliably across OS boundaries.
• USB 2.0 extension solves a well-documented but rarely-flagged issue: USB 3 emits significant RF noise in the 2.4 GHz band, which is exactly where Logitech Lightspeed/Unifying receivers operate. Plugging the receiver into a USB 3 hub directly makes wireless input drop/stutter, especially on Windows. The extension cable forces the receiver's link to USB 2.0 and distances it physically from the noisy USB 3 cabling.
• BetterDisplay is the only macOS DDC tool that handles the "LG alt" protocol used by newer LG monitors (45GX950A, GP850, GP950, and many other recent UltraGears). Standard DDC tools (display-switch, ControlMyMonitor, monitorcontrol) cannot switch input on these monitors at all. If your monitor responds to standard DDC, simpler tools work too — but BetterDisplay covers the LG-alt case.
• Hammerspoon watches USB connect/disconnect events on Mac and runs shell commands in response. Conceptually equivalent to display-switch, but routes through BetterDisplay's LG-alt-aware CLI instead of standard DDC.

Setup

1. Wire the hardware

• PC → USB-A out from the switcher to a USB 3 port on the PC.
• Mac → USB-C out from the switcher to a USB-C port on the Mac.
• Monitor → connected to PC via DisplayPort (or HDMI), and to Mac via USB-C.
• Keyboard, mouse, and any other peripherals plug into the switcher's downstream USB ports.
• If using a Logitech wireless receiver: plug a USB 2.0 male-to-female extension cable into one of the switcher's USB ports, then plug the receiver into the female end of the extension. Run the receiver out toward your keyboard for best signal.

2. Install BetterDisplay on the Mac

brew install --cask betterdisplay

Launch it once via GUI; grant Accessibility permission when prompted. Open Settings → Displays → your monitor → DDC Input Sources. If your monitor is a newer LG that needs LG-alt protocol, the input entries here will be labeled "(LG alt)". Note the numeric values next to your two relevant inputs — you'll need them in the next step.

3. Create the dp script

A tiny helper that switches the monitor input via BetterDisplay's CLI. Replace USB_C_VALUE and DP_VALUE with the numbers you noted in BetterDisplay. The example below uses LG-alt protocol; if your monitor uses standard DDC, remove --ddcAlt and change --vcp=inputSelectAlt to --vcp=inputSelect.

mkdir -p ~/.local/bin
cat > ~/.local/bin/dp << 'EOF'
#!/usr/bin/env bash
# dp — switch monitor input via BetterDisplay
set -euo pipefail
BD="/Applications/BetterDisplay.app/Contents/MacOS/BetterDisplay"
DISPLAY_NAME="lg"       # any substring of your monitor's name (case insensitive)
USB_C_VALUE=209         # value from BetterDisplay's input list for USB-C
DP_VALUE=208            # value from BetterDisplay's input list for DisplayPort
case "${1:-}" in

--mac) "$BD" set --namelike="$DISPLAY_NAME" --ddcAlt --vcp=inputSelectAlt --value="$USB_C_VALUE"; caffeinate -u -t 1 ;;

--pc)  "$BD" set --namelike="$DISPLAY_NAME" --ddcAlt --vcp=inputSelectAlt --value="$DP_VALUE" ;;

*)     echo "Usage: dp [--mac|--pc]" >&2; exit 1 ;;

esac

EOF

chmod +x ~/.local/bin/dp

Add ~/.local/bin to your PATH if it isn't already.

4. Identify your USB switcher's "tell" device

You need to pick a USB device on the switcher that's reliably present when K/M is routed to Mac and absent when routed to PC. A Logitech wireless receiver works well — its USB Vendor Name and Product Name are easy to match on. Find them with:

ioreg -p IOUSB -l | grep -i -B 2 -A 20 'logitech'

(Note: on macOS Tahoe, system_profiler SPUSBDataType was removed. Use ioreg as above.)

Capture idVendor (decimal) and kUSBProductString. For a Logitech Lightspeed receiver, these are typically 1133 and "USB Receiver".

5. Install Hammerspoon and write the watcher config

brew install --cask hammerspoon
mkdir -p ~/.hammerspoon
cat > ~/.hammerspoon/init.lua << 'EOF'
-- Flip monitor input when the USB switcher routes the keyboard/mouse
local watcher = hs.usb.watcher.new(function(event)
    if event.vendorID == 1133 and event.productName == "USB Receiver" then
        if event.eventType == "added" then
            hs.execute(os.getenv("HOME") .. "/.local/bin/dp --mac")
        elseif event.eventType == "removed" then
            hs.execute(os.getenv("HOME") .. "/.local/bin/dp --pc")
        end
    end
end)
watcher:start()
EOF

Launch Hammerspoon once via GUI; grant Accessibility permission. Quit (Cmd+Q) and relaunch — macOS sometimes caches the permission state and needs a restart to pick it up.

6. Press the button

Press the USB switcher button. K/M routes to the other machine; a moment later the monitor follows. Done.

Sleep settings for the Mac side

To keep this working without manual intervention, the Mac side benefits from:

sudo pmset -a disablesleep 1     # system never sleeps (so Hammerspoon always runs)

For OLED protection, don't use displaysleep — when macOS sleeps the display, USB power state changes and some peripherals (Stream Decks, etc.) lose connection. Use a black/dark screen saver instead:

• System Settings → Lock Screen → "Start Screen Saver when inactive" → 15 minutes
• System Settings → Screen Saver → choose a black or solid-dark screensaver (Apple's drone-footage built-ins are bright and bad for OLED)

Troubleshooting

Logitech receiver isn't detected on Windows after switching: USB 3 RF interference. Confirm you have a USB 2.0 extension cable between the switcher and the receiver. Run the receiver out closer to the keyboard.

Monitor doesn't switch when you press the button: Open Hammerspoon's Console from its menu bar icon. If you see no event firing when you press the button, your USB switcher might enumerate devices differently than expected. Re-run the ioreg command above with the K/M routed to Mac vs PC and adjust the vendorID/productName match in init.lua.

BetterDisplay's CLI returns no error but the monitor doesn't switch: Your monitor likely uses LG-alt protocol but the script is using standard DDC (or vice versa). Try toggling --ddcAlt and inputSelectAlt vs --ddc and inputSelect.

Hammerspoon stops working after a macOS update: Re-grant Accessibility permission. macOS sometimes invalidates the entry silently after system updates.

What didn't work (so you don't have to try)

• Synergy 3 hotkeys driven from Stream Deck: synthetic keystrokes from a client machine don't fire the hotkey listener on the server, and the keystroke-injection workarounds (osascript, AHK over SSH, PowerShell SendKeys) all hit Windows session-boundary issues.
• display-switch: elegant pattern, but no LG-alt support. Works fine if your monitor responds to standard DDC.
• Stream Deck + monitor input switching as a one-button workflow on this monitor: the LG-alt DDC quirk + Stream Deck's session model made this fragile. A hardware USB switcher with Hammerspoon-on-Mac as the bridge ended up cleaner.
• macOS Picture-by-Picture toggle via DDC: unstandardized across monitors; no published code for the 45GX950A. PBP from the monitor's OSD works fine, but software-triggering it is brittle.

Credits and sources

• BetterDisplay for handling the LG-alt protocol
• Hammerspoon for the USB watcher
• haimgel/display-switch for the architectural pattern (this is essentially display-switch adapted to BetterDisplay's LG-alt-aware backend)
• BetterDisplay Discussion #3032 — the open feature request to bake USB-event triggering into BetterDisplay itself; when this lands, the Hammerspoon piece becomes unnecessary

# Mac + PC sharing one monitor, one K/M — no KVM required

A working setup for sharing a single monitor, keyboard, and mouse (and any other USB peripheral) between a Mac and a Windows PC. Single physical button press flips both the keyboard/mouse routing and the monitor input. No expensive KVM. No software running on the PC side.

Built and tested on a 45" LG UltraGear OLED (45GX950A) — a monitor that breaks every standard DDC tool because LG uses an alternate DDC protocol. This guide handles that case specifically, but the pattern generalizes to any monitor BetterDisplay supports.

Architecture at a glance

[Keyboard + Mouse + any USB peripheral]
              |
              v
       [USB switcher with button] ←—— you press this
              |
       +------+------+
       |             |
       v             v
   [Mac (USB-C)]  [PC (USB-A)]
       |             |
       |             |  (both also connected to monitor inputs)
       v             v
     +-----[Monitor]-----+
                 ^
                 |
   Mac runs Hammerspoon watching USB events
       → calls BetterDisplay CLI
       → flips monitor input via DDC

When the user presses the USB switcher button:

1. K/M physically reroutes to the other PC (the switch does this in hardware)
2. On the Mac side, Hammerspoon detects the USB devices connecting/disconnecting
3. Hammerspoon calls a tiny shell script that fires BetterDisplay's CLI
4. Monitor flips inputs to match where the K/M now lives

Result: one button press, full environment follows. PC needs zero software.

Bill of Materials

Item	Cost	Notes
USB switcher with desk-button	~$80	UGREEN 2-in-7-out USB 3.2 (B0FDW6Z4J4) used here. Any cheap USB 3 switch works.
USB 2.0 male-to-female extension cable, 3-6 ft	~$8	Required if you use a Logitech wireless receiver. Forces USB 2.0 speeds at the receiver, distancing it from USB 3 RF interference.
BetterDisplay	Free	Free tier handles input switching. macOS App Store or Homebrew.
Hammerspoon	Free	Mac automation framework. Homebrew.

Why each piece

• USB switcher handles K/M routing in hardware. No driver, no software, no synthetic-keystroke shenanigans, works reliably across OS boundaries.
• USB 2.0 extension solves a well-documented but rarely-flagged issue: USB 3 emits significant RF noise in the 2.4 GHz band, which is exactly where Logitech Lightspeed/Unifying receivers operate. Plugging the receiver into a USB 3 hub directly makes wireless input drop/stutter, especially on Windows. The extension cable forces the receiver's link to USB 2.0 and distances it physically from the noisy USB 3 cabling.
• BetterDisplay is the only macOS DDC tool that handles the "LG alt" protocol used by newer LG monitors (45GX950A, GP850, GP950, and many other recent UltraGears). Standard DDC tools (display-switch, ControlMyMonitor, monitorcontrol) cannot switch input on these monitors at all. If your monitor responds to standard DDC, simpler tools work too — but BetterDisplay covers the LG-alt case.
• Hammerspoon watches USB connect/disconnect events on Mac and runs shell commands in response. Conceptually equivalent to display-switch, but routes through BetterDisplay's LG-alt-aware CLI instead of standard DDC.

Setup

1. Wire the hardware

• PC → USB-A out from the switcher to a USB 3 port on the PC.
• Mac → USB-C out from the switcher to a USB-C port on the Mac.
• Monitor → connected to PC via DisplayPort (or HDMI), and to Mac via USB-C.
• Keyboard, mouse, and any other peripherals plug into the switcher's downstream USB ports.
• If using a Logitech wireless receiver: plug a USB 2.0 male-to-female extension cable into one of the switcher's USB ports, then plug the receiver into the female end of the extension. Run the receiver out toward your keyboard for best signal.

2. Install BetterDisplay on the Mac

brew install --cask betterdisplay

Launch it once via GUI; grant Accessibility permission when prompted. Open Settings → Displays → your monitor → DDC Input Sources. If your monitor is a newer LG that needs LG-alt protocol, the input entries here will be labeled "(LG alt)". Note the numeric values next to your two relevant inputs — you'll need them in the next step.

3. Create the dp script

A tiny helper that switches the monitor input via BetterDisplay's CLI. Replace USB_C_VALUE and DP_VALUE with the numbers you noted in BetterDisplay. The example below uses LG-alt protocol; if your monitor uses standard DDC, remove --ddcAlt and change --vcp=inputSelectAlt to --vcp=inputSelect.

mkdir -p ~/.local/bin
cat > ~/.local/bin/dp << 'EOF'
#!/usr/bin/env bash
# dp — switch monitor input via BetterDisplay
set -euo pipefail
BD="/Applications/BetterDisplay.app/Contents/MacOS/BetterDisplay"
DISPLAY_NAME="lg"       # any substring of your monitor's name (case insensitive)
USB_C_VALUE=209         # value from BetterDisplay's input list for USB-C
DP_VALUE=208            # value from BetterDisplay's input list for DisplayPort

case "${1:-}" in
    --mac) "$BD" set --namelike="$DISPLAY_NAME" --ddcAlt --vcp=inputSelectAlt --value="$USB_C_VALUE"; caffeinate -u -t 1 ;;
    --pc)  "$BD" set --namelike="$DISPLAY_NAME" --ddcAlt --vcp=inputSelectAlt --value="$DP_VALUE" ;;
    *)     echo "Usage: dp [--mac|--pc]" >&2; exit 1 ;;
esac
EOF
chmod +x ~/.local/bin/dp

Add ~/.local/bin to your PATH if it isn't already.

4. Identify your USB switcher's "tell" device

You need to pick a USB device on the switcher that's reliably present when K/M is routed to Mac and absent when routed to PC. A Logitech wireless receiver works well — its USB Vendor Name and Product Name are easy to match on. Find them with:

ioreg -p IOUSB -l | grep -i -B 2 -A 20 'logitech'

(Note: on macOS Tahoe, system_profiler SPUSBDataType was removed. Use ioreg as above.)

Capture idVendor (decimal) and kUSBProductString. For a Logitech Lightspeed receiver, these are typically 1133 and "USB Receiver".

5. Install Hammerspoon and write the watcher config

brew install --cask hammerspoon
mkdir -p ~/.hammerspoon
cat > ~/.hammerspoon/init.lua << 'EOF'
-- Flip monitor input when the USB switcher routes the keyboard/mouse
local watcher = hs.usb.watcher.new(function(event)
    if event.vendorID == 1133 and event.productName == "USB Receiver" then
        if event.eventType == "added" then
            hs.execute(os.getenv("HOME") .. "/.local/bin/dp --mac")
        elseif event.eventType == "removed" then
            hs.execute(os.getenv("HOME") .. "/.local/bin/dp --pc")
        end
    end
end)
watcher:start()
EOF

Launch Hammerspoon once via GUI; grant Accessibility permission. Quit (Cmd+Q) and relaunch — macOS sometimes caches the permission state and needs a restart to pick it up.

6. Press the button

Press the USB switcher button. K/M routes to the other machine; a moment later the monitor follows. Done.

Sleep settings for the Mac side

To keep this working without manual intervention, the Mac side benefits from:

sudo pmset -a disablesleep 1     # system never sleeps (so Hammerspoon always runs)

For OLED protection, don't use displaysleep — when macOS sleeps the display, USB power state changes and some peripherals (Stream Decks, etc.) lose connection. Use a black/dark screen saver instead:

• System Settings → Lock Screen → "Start Screen Saver when inactive" → 15 minutes
• System Settings → Screen Saver → choose a black or solid-dark screensaver (Apple's drone-footage built-ins are bright and bad for OLED)

Troubleshooting

Logitech receiver isn't detected on Windows after switching: USB 3 RF interference. Confirm you have a USB 2.0 extension cable between the switcher and the receiver. Run the receiver out closer to the keyboard.

Monitor doesn't switch when you press the button: Open Hammerspoon's Console from its menu bar icon. If you see no event firing when you press the button, your USB switcher might enumerate devices differently than expected. Re-run the ioreg command above with the K/M routed to Mac vs PC and adjust the vendorID/productName match in init.lua.

BetterDisplay's CLI returns no error but the monitor doesn't switch: Your monitor likely uses LG-alt protocol but the script is using standard DDC (or vice versa). Try toggling --ddcAlt and inputSelectAlt vs --ddc and inputSelect.

Hammerspoon stops working after a macOS update: Re-grant Accessibility permission. macOS sometimes invalidates the entry silently after system updates.

What didn't work (so you don't have to try)

• Synergy 3 hotkeys driven from Stream Deck: synthetic keystrokes from a client machine don't fire the hotkey listener on the server, and the keystroke-injection workarounds (osascript, AHK over SSH, PowerShell SendKeys) all hit Windows session-boundary issues.
• display-switch: elegant pattern, but no LG-alt support. Works fine if your monitor responds to standard DDC.
• Stream Deck + monitor input switching as a one-button workflow on this monitor: the LG-alt DDC quirk + Stream Deck's session model made this fragile. A hardware USB switcher with Hammerspoon-on-Mac as the bridge ended up cleaner.
• macOS Picture-by-Picture toggle via DDC: unstandardized across monitors; no published code for the 45GX950A. PBP from the monitor's OSD works fine, but software-triggering it is brittle.

Credits and sources

• [BetterDisplay](https://github.com/waydabber/BetterDisplay) for handling the LG-alt protocol
• [Hammerspoon](https://www.hammerspoon.org/) for the USB watcher
• [haimgel/display-switch](https://github.com/haimgel/display-switch) for the architectural pattern (this is essentially display-switch adapted to BetterDisplay's LG-alt-aware backend)
• [BetterDisplay Discussion #3032](Allow switching display inputs via DDC when a chosen USB device connects/disconnects #3032) — the open feature request to bake USB-event triggering into BetterDisplay itself; when this lands, the Hammerspoon piece becomes unnecessary