Remind me how to use the 909's arpeggiator

[Numina]

I thought I've run the arpeggio from a remote synth via midi in to the 909 before, but I can't seem to get this to work. When the arpeggio light is on, I can only get an arp pattern to run if I play the velocity pads, but when playing from my controller (a JP-8000 in this case) I only play the sound in normal state.

Does the 909 play an arp from a controller? If so what setting am I missing?

Thanks!

J.