Frank,
From the sounds of your application, you want to trigger directly fromt he IMAQ board. To do this, I would suggest wiring directly to the 4 trigger lines located on pins 95-98. These are programable digital I/O lines. These lines should be accessible through the RTSI, but this is only useful if you are transferring the signal directly to another NI board in your system. For more information, here is a link to the 1424 manual, I suggest looking into the trigger functionality.
http://www.ni.com/pdf/manuals/321869a.pdf
I hope this helps,
Chris D
NI Applications Engineer