ok, i think i figured it out
ecu has 3 signals for the igniter.
IGT, IGDA, IGDB
to fire, it sends signal IGT, then IGT+IGDA, then IGT+IGDB...... right?
so to fire IG1, IG2, IG3...
IG1 has been fired... which is IGDA (IGT+IGDA=IG1).... which fires cylinders ?
IG2 has been fired...