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Output Selection Behavior


The output behavior allows selecting whether keyboard output is sent to the USB or bluetooth connection when both are connected. This allows connecting a keyboard to USB for power but outputting to a different device over bluetooth.

By default, output is sent to USB when both USB and BLE are connected. Once you select a different output, it will be remembered until you change it again.

Powering the keyboard via USB

ZMK is not always able to detect if the other end of a USB connection accepts keyboard input or not. So if you are using USB only to power your keyboard (for example with a charger or a portable power bank), you will want to select the BLE output through below behavior to be able to send keystrokes to the selected bluetooth profile.

Output Command Defines#

Output command defines are provided through the dt-bindings/zmk/outputs.h header, which is added at the top of the keymap file:

#include <dt-bindings/zmk/outputs.h>

This allows you to reference the actions defined in this header:

OUT_USBPrefer sending to USB
OUT_BLEPrefer sending to the current bluetooth profile
OUT_TOGToggle between USB and BLE

Output Selection Behavior#

The output selection behavior changes the preferred output on press.

Behavior Binding#

  • Reference: &out
  • Parameter #1: Command, e.g. OUT_BLE


  1. Behavior binding to prefer sending keyboard output to USB

    &out OUT_USB
  2. Behavior binding to prefer sending keyboard output to the current bluetooth profile

    &out OUT_BLE
  3. Behavior binding to toggle between preferring USB and BLE

    &out OUT_TOG