No MIDI Splitter is necessary, the S90 XS has a built-in MIDI interface that can do the job for you.

You set the S90 XS to MIDI IN/OUT = USB
You'll be using the Yamaha USB-MIDI Driver for you particular computer type and operating system
http://download.yamaha.com

You don't tell us what computer application you will be using but the S90 XS will appear on the computer with five MIDI ports

Port 1 is for the tone generator of the XS itself
Port 2 is available as a Remote Control surface for Cubase, Logic, Sonar, Digital Performer
Port 3 is available for In and Out for any MIDI device you connect to the 5-pin MIDI jacks on the back panel of the XS
Port 4 is reserved for the S90 XS/S70 XS EDITOR VST
Port 5 is for the Remote Editor used to control VST plugins.

You will be using Port 1 and Port 3.

Thanks for your reply. I'm using Sonar Platinum.

I've installed the USB driver and connected the S90XS however I'm not getting any MIDI signal into the PC from the keyboard. Nor is my volume meter receiving any data from the S90XS's MIDI out.

I've noticed that when I set the S90 XS to MIDI IN/OUT to MIDI, my volume meter receives data but not when I set it to USB.

Any ideas?

We are not sure what you are using as your audio interface, you don't say. You did not mention a DAW software in your initial post. If you wish to record to a computer software DAW like Sonar Platinum - the Yamaha USB-MIDI driver will allow you to record MIDI only from your S90 XS.

In Sonar you must select the driver in the MIDI OPTIONS area, the keyboard communicates MIDI Note-on, Controller and clock data on Port 1. Until you enable the MIDI IN in Sonar you will not see the data arriving.

You'll want to set the S90 XS to MIDI IN/OUT = USB
You'll want to connect the Main L/R output of the keyboard to a stereo Input of your audio interface.

It is the audio interface's task to send audio into Sonar.