Basically, the idea is simple:
Assign MIDI-IN-signals to trigger patterns. Example of how this would work: I press a C-4 key on the USB MIDI keyboard, which then triggers a pattern to play.
Possibilities of assigning multiple keys: If I press C-4, E-4, G-4 together – the assigned pattern is playing major C and if I press C-4, D#4, G-4 together the triggered pattern is playing a sequence in minor C.
In pseudo-code:
IF MIDI-IN = (C-4, D#4, G-4) THEN PLAY PATTERN A
IF MIDI-IN = (C-4, E-4, G-4) THEN PLAY PATTERN B
…
ELSE PLAY PATTERN C