Hi dreamhawk,
There are some examples online that demonstrate software triggering with a High-Speed Digitizer such as the NI 5911. Because you didn't specify which programming language you're using, I'll assume you're using LabVIEW. I suggest you take a look at the example program "Software Triggering with NI-SCOPE and a High-Speed Digitizer" at
http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=B45EACE3E89156A4E034080020E74861&p_node=175383&p_source=External.
Regards,
David Shatila
Applications Engineer
National Instruments