@DavidSimons: The signals aren't especially compatible. I usually use 1 or 2 degrees of separation between my gear. I have a few types of translator circuits that allow my gear to talk. Nothing high tech. In some cases I just use a transistor. A pulse from my modular synth applied to the "gate" leg of a transistor will allow current to flow across the "collector" and "emitter" legs. Relays can be really useful too. Like in my relay step sequencer  
http://casperelectronics.com/finished-pieces/sequencers/step-sequencer/

I also use an envelope follower module which translates incoming audio signals into modular standard voltages.