Still a work in progress, lots of tuning to do. The arduino sends midi data via serial,

ttymidi makes virtual midi in/out ports and reads data from arduino.http://www.varal.org/ttymidi/

jackd connects the ttymidi  to hydrogen drum machine

Firmware and host program

Hardware

http://www.youtube.com/watch?v=ZbPdxk_DV1k&feature=plcp