I just build this weekend the PPM2PC interface from Eric Brouwer and it works fine with my old HITEC PRISM 7 (not trainer port) and FMS simulator using the "Serial PIC interface".
That's great !!!
I have used a pic16f876 and I am waiting for digital potentiometers (MAX DS1803) in order to test it with other simulators. I will have to modify Eric code (the DS1803 uses I2C protocol).
Here is a PRISM 7 connection scheme (you have to remove the HF module):