Yamaha UX16 Owner's Manual page 19

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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 Windows 2000/XP and Macintosh computers, this function is valid only when the
NOTE
MIDI Patch screen or sequence software is using the UX16.
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