Hey Schicki,
I have included two links below that should help you with your application. The first one is setup to perform digital pattern I/O with an external clock in CVI. The second shows how to incorporate start/stop triggers in digital pattern I/O operations. You should be able to combine parts of these two programs to complete your application.
NI 6534 Double-Buffered Pattern Generation, External Clock -- CVI
http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=B45EACE3E7FD56A4E034080020E74861&p_node=DZ52321&p_source=External
Output Digital Data from a Buffer to a Group of Two Digital Ports Upon a Digital Start Trigger
http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=B45EACE3EA4556A4E034080020E74861&p_node=DZ52321&p_s
ource=External
I hope this helps.
Regards,
Todd D.
NI Applications Engineer