Hi chitarrone,
please make sure the following things:
Make sure to use a USB type "AB" cable (USB 2.0 certified), connected to the USB-to-Host terminal of the S975.
Make sure to use the MIDI templates in the "PRESET" tab. (In the "USER" tab, changed templates may be saved, but with the same name as the preset templates.)
If "All Parts" is activated, the keyboard parts are transmitted on the following MIDI channels:
Right1: Port1 Ch1
Right2: Port1 Ch2
Left: Port1 Ch4
Important:
At least one keyboard part must be switched on via PART ON / OFF, preferably Right 1. In addition, the Part Volume in the Balance display or in the Mixing Console should not be set too low.
If "KBD & STYLE" is activated, the area above the left split point (right hand) is treated as an "Upper" part and the left area as a "Lower" part. These two "special" parts are transmitted as follows:
Upper: Port1 Ch1
Lower: Port1 Ch2
Note:
The keyboard data is sent here even if all keyboard parts are switched off. In addition, no Program Changes are sent to external MIDI devices when Voices on the internal keyboard parts are switched.
"Master KBD" works in the same way as "KBD & STYLE", except that the Multi Pad and Style parts are not sent externally via MIDI.
Of course, it is also important that the app used on the iPad can receive on the channels transmitted by the keyboard (at least on Ch1).
Hope this helps!
Best regards,
Chris