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Does anyone know of a way to get modifier keys to work across USB devices in OSX?

I have a USB footswitch that sends a left control key. It sends the signal as expected but does not register shortcuts. Pressing it and then pressing "left" does not trigger the appropriate shortcut, but when I hit control and left on built in macbook pro keyboard it works as expected.

I found the following on the kinesis website:

Note: Modifier actions from one USB device cannot modify the input of a second USB device due to limitations designed into the Apple operating system. Example: Shift, Control, Command, or Option keystrokes programmed into the footswitch cannot modify the input of a separate USB keyboard or mouse. However, a key sequence like ‘Cmd-W’ or ‘Cmd-Shift-left arrow’ will work on a Macintosh if the entire sequence of keystrokes has been pre-programmed into the footswitch. (Footswitch can only be programmed on a Windows PC).

This explains what I am experiencing. I tested it by trying the same thing with an external usb keyboard. I would press the left control on the external usb keyboard and the "left" arrow on the built in keyboard and it would not register. BUt when hitting the "left" arrow and the control key on the usb keyboard it registered as expected.

Does anyone know how to fix this issue?

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2 Answers 2

Reset to default
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You can install Karabiner, which should be enough to aggregate USB inputs.

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  • Thanks, this solved the problem for me too. To anyone curious- you just need to install Karabiner and run it (giving it the necessary permissions the first time it runs). No configuration required. Cross-device modifiers are basically a side effect of the app.
    – Kip
    Commented Nov 4, 2021 at 19:11
  • On Karabiner-Elements 14.11.0. After install and run it. It is necessary to open Karabiner-Elements Settings, in the Devices tab, find your USB devices(keyboards), and turn on the "Modify events" switch for each device/(keyboard) you want to use for modifier keys. Otherwise the modifier key won't work across multiple keyboards.
    – frantic1048
    Commented Jan 16, 2023 at 2:29
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I ended up creating my own "keyboard combiner". It lets me use the Kinesis Savant Elite Triple Foot Pedals for shift, alt, and command modifiers on a Mac.

https://github.com/davidbkemp/KeyboardCombinerArduinoSketch/wiki

It is quite easy to assemble, but I cannot guarantee that it will work for you. It certainly does not work with some pedals and some keyboards. For the Kinesis pedals, you will initially need to configure them to act as shift, alt, and command keys, and that means using a PC to do the initial configuration.

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  • very impressed, but i don't think this will work for me. :( i have a gaming mouse with a button mapped to enter, and sometimes i hold cmd on the keyboard and hit enter on the mouse. i assume if i plugged the mouse into the arduino it would stop functioning as a mouse
    – Kip
    Commented Nov 4, 2021 at 19:00

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