Hi Cassp
I have the p121 connected to my sx900, BUT, p121 doesn’t have dedicated midi sockets.
You need some sort of a midi interface.
You can buy dedicated little boxes apparently, ( I haven’t checked into them) because I just use my laptop since I only use it at home.
USB out sx900, USB out p121 both outs plug into my laptop.
Again there’s probably dedicated software available but I find it easy to just use my xg works pc sequencer. I just set the midi in , to p121, midi out, sx900 press the record icon ( doesn’t actually record anything,just allows the 2 to communicate with each other)

Might sound complex , but really quite simple.

A little box would probably be easier, just haven’t bothered to date. Figured why spend the extra money , if I don’t need to.

I use the p121 as controller for the sx900 hence I need the set up above Ie I use the p121 keys to conrol the sx900, the sx supplies the styles and all the sounds, splits etc.

I can play the sx900 on its own. I can play the p121 on its own.
I can play r/h melody p121, chords l/h on sx900 , then I I don’t need the midi set up above.