Well,
Then you are all set, and would be able to use Ketron with Alesis controller.
But it will be "computer only" setup if you are ok with that. My understanding is that SD2 was specifically made as a standalone unit.

If your computer has 2 available usb ports than it should fairly straight forward.
You will be connecting single 5 pin cable from Focusrite output to Ketron 5 pin input, and setting Alesis as input and Focusrite as your output for midi. To hear the sounds from Ketron you will connect headphones or speakers directly to SD unit.

Or
To record the audio (not midi) from SD to computer you will need cables: RCA to 1/4 plugs and those will go to your Focusrite inputs from outputs of SD.


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Try reading about Halion Sonic. It is a software synth that supports GM program changes if you are in this sort of a game. I believe there is a free version of Sonic too., but do not hold me to it. In any case, I would say it is a very (!) reasonable software alternative to SD2 unit some sounds are great, it has a decent choice of standard instruments, Pianos, guitars,strings etc.