Connect both Picos to their outputs, then plug in just the keyboard to board A. LEDs should flash quickly back and forth (once).

This means board A talks to your USB device correctly and can send messages to the other board. Repeat with USB port on board B.

If both LEDs flash in both cases, all is good!

If only one LED blinks, the board talks to your USB device correctly but cannot send messages to the other board. Check #1 or #2 in Wiring and Assembly section depending on the board.

If no LEDs flash after plugging in, this means board A can't talk to your USB device. Check #4 in Wiring and Assembly section.

• Check ADuM1201 pin 3 connected to pico B pin 21 (GPIO 16)
• Check ADuM1201 pin 6 connected to pico A pin 17 (GPIO 13)

• Check ADuM1201 pin 2 connected to pico B pin 22 (GPIO 17)
• Check ADuM1201 pin 7 connected to pico A pin 16 (GPIO 12)

• Check both 1 and 2
• If none of this worked, check the remaining pins on the ADuM1201 (3.3v, GND) on both sides.

Make sure you have release v0.4+ and plug in the keyboard to board A. If no LEDs flash:

• Check if Pico pads marked GP14, GP15, VBUS have been soldered.
• Check if all 4 USB-A pins are soldered correctly.

Repeat for board B.

Try setting ENFORCE_PORTS to 1 in user_config.h and rebuilding.

It seems Microsoft KB5003637 update to Win 10 brok^H^H^H^H changed the HID system behavior.

There are workarounds in latest firmware release. You need to park your mouse on the affected screen and press

RIGHT SHIFT + BACKSPACE + 2 first (to inform the box you have two screens on that output)

and then RIGHT SHIFT + BACKSPACE + 7, 8 or 9 depending if your OS on that output is Linux, Windows or MacOS.

Mac workaround is better, Windows one is still rather crappy.

Try this test_fw.zip, as it seems your device's descriptor might haven't been parsed/interpreted correctly. It would be helpful if you could share the descriptor using usbhid-dump if on Linux, or find an alternative on your OS.

Right Shift + F12 + D resets saved settings which might end in resolving your problem if it's due to the wrong config being saved in flash.