Where are you starting from? As in, do you know how to read parallel port pins in PsychoPy? Off the top of my head and assuming you have the correct pport driver installed, I’d write a routine that checks the pin corresponding to the scan trigger that only proceeds when it reads a 1 from that pin (indicating high). Here’s an implementation from another PsychoPy user similar to what I described - their trigger signal is coming from pin 13.