THRU/OUT OUTPUT THRU/OUT OUTPUT K770-KBD interface has both MIDI input and output. However, only MIDI input is necessary for basic operation of the interface (i.e controlling the instrument by MIDI commands). MIDI THRU/OUT output connector need not to be installed at all. If you don’t require transfer of MIDI data to another MIDI devices (THRU function ) or reverse communication of the interface with host system (OUT function).