How can I connect the Motif Rack ES to the Yamaha PSR s900 keyboard so that I can use the Motif voices and still maintain the use of the Styles on the s900 keyboard?

Not clear on what you want to do.

MIDI is the answer but it seems you would know this. Connect the MIDI OUT of the keyboard to the MIDI IN of the Rack ES>
Set the Motif-Rack ES so that it is set to MIDI IN/OUT = MIDI

The question remains what do you want to accomplish.

Thank you for your reply.
I hope this makes it more clear: I want to be able to play the Motif Rack sounds with the RIGHT part of the s900 keyboard while playing the s900 keyboard styles with the LEFT part of the keyboard.  With the keyboard split at F#2 I want the Motif Rack to only the recognize control from the G2 upward and ignore control from F#2 downward.  Is it possible to make this setup?

I guess the answer has to be: It depends.

Your post does not use the word “accompaniment” - but as you know, the PSR-S900 is all about accompaniment. I’m sure you realize that although you control the accompaniment on the PSR-S900 with your left hand (say below F#2), many of the PARTS that are responding respond with notes that are both above and below the split point.

The left hand typically triggers Drums, Bass, a chordal part, a pad part and a couple of phrase to play from the what you enter below F#2 for example… The responding PARTS can be all over the keyboard both above that split point.

The drum pattern that plays can include notes that are above or below that split point. Same for the Bass, the Chordal part, the Pad Part, the Phrases, etc.

I don’t know whether to answer your question literally - about the trigger notes or about the notes generated by the accompaniment parts. So I ask again what do you really want to happen.

You are sending MIDI information with the accompaniment - and it is on several different channels simultaneously. The PSR’s use channels 9-16 for their accompaniment PARTS. The notes you use to trigger them typically are simply outlining CHORD Voicings that tell each PART what to do. When the Chord is recognized the SYTLE Part alters to follow particular rules when creating the result.

The physical split point F#2 / G2 is not really important to the resulting data.

Thanks for sharing the infor.  It gave me some points to contemplate.  Along with some study and some trial and error, I finally have it working.  I can now play my s900 styles (accompaniment) with lower split KB at F#2 and Motif Rack voices with upper split KB.  As I study it more I am sure there is other options that can be performed.  Both instruments seem to be very versatile.