Sandra,
You are correct, the NI 6035E does not support hardware analog triggering. The PFI lines can be used for digital triggering (TTL logic). A digital stop (reference) trigger does support both pre-trigger scans and post-trigger scans. The example Acquire N Scans Digital Trig.vi demonstrates this programming procedure in LabVIEW. If you do need to perform analog triggering, you should use an analog software trigger. The example Acquire N Scans Analog Software Trig.vi demonstrates this programming procedure.
Good luck with your application.
Spencer S.