Yamaha USB-MIDI INTERFACE UX16 Owner's Manual page 23

THRU output of MIDI signals (MIDI IN MIDI OUT)
You can route MIDI signals received at the input port of the UX16 directly to the
output port of the UX16 (this is called THRU output). This routing is useful when
you wish to route MIDI signals from a MIDI keyboard directly to a tone generator.
The left column shows the UX16's output ports (1), and the top column shows
the UX16's input port (1). To enable this function, check the "Thru ON/OFF"
check box in the lower right corner of the screen.
For Macintosh, this function is valid only when the MIDI Patch screen or sequence
NOTE
software is running.
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