One Keyboard, Three Macs, No KVM

Three Macs share my desk: the Mac Studio (main), a Mac mini, and the Kaltura MacBook Pro. All three feed the same Dell U2720Q, and I type on one Kinesis Advantage360 Pro. I wanted a single keypress to move both the keyboard and the monitor to another machine. I had been evaluating hardware KVMs for exactly this (see ATEN CS1824 and GL.iNet Comet KVM), then realised the hardware I already own can do it in software: the keyboard's firmware plus DDC, the control channel built into every monitor. Total cost: zero.

What it solves

• One keypress (Mod 1/2/3 on the keyboard) moves keyboard and display to the chosen computer.
• No KVM box, no extra cables, no 4K60 compatibility roulette, no input lag.
• The monitor's three inputs are the "KVM ports": each Mac keeps its own direct video cable.

The mouse stays out of scope on purpose: the MX Ergo 2 has its own Easy-Switch buttons, and each machine that matters has a pointing device anyway.

How it works

Two facts make the design:

1. Monitors accept input-switch commands over DDC, the I2C channel that rides the video cable. BetterDisplay exposes this as a keyboard shortcut on macOS.
2. Only the computer currently shown on the monitor has a live DDC path. Once the monitor switches away, that machine cannot talk to it any more. So the switch command must come from the machine you are leaving, never the one you are joining.

The Advantage360 Pro runs ZMK firmware, which supports macros. Each Mod key becomes: tap a hotkey (received by the machine I am leaving, which still owns DDC), wait 300 ms for the keystroke to flush over Bluetooth, then hop the keyboard's Bluetooth profile to the target machine. The leaving machine's BetterDisplay hears the hotkey and flips the monitor's input. Keyboard and display arrive together.

Step by step

1. Cable each computer to its own monitor input. The Dell U2720Q has USB-C, HDMI, and DisplayPort, so three machines fit without any splitter. A USB-C-to-DP cable covers a Mac with only USB-C out.
2. Pair the keyboard once per machine. Select each Bluetooth profile on the keyboard, pair it from that Mac's Bluetooth settings.
3. Install BetterDisplay on every machine and create the same three shortcut slots on each: Ctrl+Opt+Cmd+1 switches the Dell to USB-C, +2 to HDMI, +3 to DisplayPort. Every machine needs all three, because the hotkey always lands on the machine being left. The free version is enough: DDC input switching and basic shortcuts are free (CLI switching is Pro-gated, but this design never uses the CLI). Set BetterDisplay to start at login.

4. Add the macros to the keyboard firmware. In the ZMK keymap, each Mod key gets a macro instead of the plain profile-select binding:

   sw_studio: sw_studio{
       compatible = "zmk,behavior-macro";
       #binding-cells = <0>;
       bindings = <&kp LC(LA(LG(N1)))>, <&macro_wait_time 300>, <&bt BT_SEL 0>;
   };
   

   Build via the repo's GitHub Actions, then flash the .uf2.
   5. Turn off Auto Select in the monitor's OSD. Otherwise the monitor falls back to a live input while the target machine renegotiates its link, and the switch bounces back.
   6. Test the loop in both directions from every machine.

Gotchas

• Flash both keyboard halves, always. I flashed only the left half and broke the half-to-half link: right half blinking red, no typing at all. Recovery: settings-reset.uf2 to both halves, re-flash both, re-pair everything.
• Copy .uf2 files with Finder or plain cp. A verifying copier reports a checksum error because the keyboard reboots mid-copy. That error is cosmetic, but it sent me chasing a "bricked" keyboard that was fine.
• USB-C is the one vendor-specific DDC input code. HDMI and DisplayPort use universal codes and worked from every machine immediately. The USB-C switch bounced until I found the entry in BetterDisplay's input-source menu that this Dell actually understands, and bound the slot to that. Click through the USB-C-flavoured entries manually before trusting the shortcut.
• Pick hotkeys nothing else owns. My first scheme used Ctrl+Opt+Cmd+A/S/D; two of those refused to record on one machine. Digits were free everywhere.

Further Reading

BetterDisplay

Display management for macOS, incl. DDC input switching with shortcuts

https://github.com/waydabber/BetterDisplay

Kinesis Advantage360 Pro

Split ergonomic keyboard with ZMK firmware and 5 Bluetooth profiles

https://kinesis-ergo.com/keyboards/advantage360/

ZMK Firmware

Open-source keyboard firmware: macros, Bluetooth profiles, split keyboards

https://zmk.dev/