Megasquirt 1 clock phase

[AGDrake57]

Is reversed when tach signal is routed to 4N25 cathode, (tach pulse connected to XG1) instead of anode (tach signal to COIL net through D5 R10 D8 ) -

With cathode coupling, 4N25 photo emitter led current flows when tach signal is grounded, as opposed to when tach signal is routed to anode, led current in 4N25 flows when tach signal is high.

I have many miles on SBC 350 w 8 pin hei using cathode routed tach config. I'm helping someone set up his V8 w dizzy, with unilite distributor, and would like to use cathode coupled configuration. My question is this: will this tach signal inversion foul the system, or is it allowable?

Unilite seems to have proportional dwell, as long as the slot is open, tach signal is low.

Is there a MSnS setting related to clock inversion?

Thanks in advance!

[dontz125]

While the MS2 and 3 processors can select rising or falling edge, the MS1 ONLY triggers on Falling edge.

[AGDrake57]

Oh, that's just right: Tach signal from unilite looks just like points, where critical edge is rising (coil charges when coil minus is grounded, fires at rising edge on coil minus signal), so phase reversal at cathode coupled 4N25 configuration provides falling edge at CPU.

Unilite green wire pulled up to +5V with 1KΩ scopes a clean 5V square wave for tach input.

Thanks again.