Hi Casaet
1. The connection to the outside world of the SD2 is via Midi.
2. Apart from a volume control, it has no other controls on it.
3. If your keyboard has good flexible Midi control facilities on-board, then you can use the SD2 as if it was just another inbuilt voice.
4. To get to the more advanced features of the SD2, you will need to use the Midi Implementation chart at the back of the manual, and match it with your Midi Controller/Keyboard
5. If you just want to access the sounds, then providing your keyboard can send Bank and Program Change messages, all the sounds are available to you.
6. The SD4 is essentially two SD2 modules in a box, with on-board controls and programming, thus is ideal for keyboards with limited Midi facilities. (More expensive though)
Hope this helps

Bill