Thanks Richard, appreciate the effort but (always a but)....
Problem with the ZCD and my signal is that the bus idles at 35VDC for "long" periods of time and swings +0/-6V from there, not +/-3V around 35VDC.
If I'm not reading the sim setup wrong (which I might) you have a +/-3V signal sitting on a DC-bias of 32V when, in reality, it is a +0/-6V signal sitting at that 32VDC bias (or 35 but it doesn't matter).

Try keeping that 32VDC bias but have the signal only swing down from there, and then make bursts, 25 cycles followed by 5 seconds of silence. What happens then?

Also, and I'm not sure this matters but I think so, it doesn't look like Protues is simulating the true behaviour of the ZCD-pin. If it did you wouldn't be able to measure a signal there (which looks to be Ch. C on the scope) because as long as you have the correct (enough) series impedance the ZCD-peripheral will drive the input pin to a voltage of zero. Weather the signal is above or below GND is determined by if the internal current source or current sink is active.

/Henrik.