Paul,
It sounds like WFTRIG is exactly what you will want to use. To output this signal on PFI6, you will need to use the Select_Signal() function. Below, is the complete function call you will need to use:
Select_Signal (1, ND_PFI_6, ND_OUT_START_TRIGGER, ND_LOW_TO_HIGH);
Good luck with your application.
Spencer S.