[sdiy] What is the function of this diode in the Serge DUSG?

[Paul Glass-Steel]

Hello all,

I'm trying to improve my understanding of the trigger and EG logic in the
Serge DUSG (https://sdiy.info/wiki/CGS_Serge_dual_universal_slope_generator).
I've read over Tim Stinchcombe's analysis (
https://www.timstinchcombe.co.uk/index.php?pge=vcs) many times but there's
one component he does not touch on and I'm struggling to understand its
roll.

At the trigger input A, after the usual series of conditioning resistors
and capacitors, there is a diode conducting the pulse into the LM3900 pin
13. And there is a *second* diode just below it that leads to (from?) the
output of the end-of-cycle comparator - this is the one I'd like to
understand.

Thanks!

-Paul

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