karabiner is absolutely critical to support external input devices, such as footswitches. OSX does not allow usb devices to modify input from other devices: N

karabiner is absolutely critical to support external input devices, such as footswitches. OSX does not allow usb devices to modify input from other devices: 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). http://apple.stackexchange.com/q/140732/8592 http://apple.stackexchange.com/q/140732/8592 While Karabiner supports this: Share state of modifier keys with all connected keyboards: When you are using multiple keyboards, modifier keys are shared with all keyboards. For example, pressing "shift key on keyboard1" and "space key on keyboard2" sends shift-space.