Trailing cois, select signal, feedback??
 

Hi guys,
I am just starting to work on converting my 84 FB 12A to run megasquirt. I got the CAS from the second gen. Looking into the CAS, something occurred to me that I never thought of before(rotaries have been absent from my life maybe 15 years). Anyway, the cas produces an engine speed signal and a TDC-ish signal to have a reference on when to fire the primaries. Two teeth - leading coils fire both at the same time, wasted spark on one rotor, that makes all sense. Now the trailing coils can not fire together, they have to fire selectively. This is where I am puzzled on the process how the ecm decides which one to fire, since the CAS signals do not contain the information about which rotor is in firing position. I did run into someone's article discussing a select signal and feedback signal between the igniter and the control module. I grasped that the select signal is either high or low, thats how the igniter decides which one to fire, but how does the igniter provide the information on the feedback line to the ECM which one just fired? That is what I struggle to grasp. Also, if I get the OEM FC trailing coils with the igniter, will the megasquirt retain the function of trailing coils firing correctly?
Thanks!

With a 2-rotor engine, there is a firing event every 180 degrees since each rotor has 3 faces but spins at 1/3 of the e-shaft speed, so each rotor face does a compression and passes the spark plug each revolution (360 deg). Since there are two rotors, phased 180 deg apart, there is a firing event every 180 deg. The CAS or distributor is geared to the eccentric shaft to spin at 1/2 the e-shaft speed (what would be cam speed on a piston engine), so only a single one of the teeth on the 2-tooth wheel passes the pick-up each revolution. Since this is timed to the e-shaft in a specific spot, the ECU knows that the next rotor to fire after that tooth will be Rotor 1, followed by Rotor 2, then followed by the next tooth.

Thanks topless!
I was confused about the relationship of the e-shaft speed and cas shaft speed. I was for some reason thinking they spun at the same speed and that the ECU got 2 bumps from the "G" pickup per one revolution of the e-shaft. Brainfart...