I don't own an RS-9 myself, but looking at the front panel it seems to be a sequencer specifically designed for drums, i.e. it has trigger and accent outputs. Both triggers and accents are just little spikes of voltage that can tell another module WHEN something happens, but not necessarily WHAT happens. To drive a synth voice like the Behringer 110, you want a trigger to tell an envelope generator when to fire. The envelope generator generates the signal that can open the VCF and VCA of your synth voice. Both VCF and VCA are part of the 110, but there doesn't seem to be an envelope generator on board. You'll need to invest in one of those (Behringer 140, if we're sticking with their System 100 series).
The second issue is that you'll want to tell your VCO what note to play. This information should come from your sequencer, but the RS-9 doesn't offer control voltage (CV) outputs. A Behringer 182 would do the trick. You connect the trigger output from the sequencer to your envelope generator and the CV output from the sequencer to the MOD IN on the 110.
Hope that helps!
