I'm using a tunable laser source of ANDO called AQ4321 A/D with the DAQ 6527 the AQ4321 has TTL connector which is used to trigger other instruments/devices this TTL connector is sending a digital signal once, when it starts an execution
my question is: how can I use this trigger to start running a loop in a VI ?
Your question should be posted under "Measurement Devices >> Digital I/O" instead of "Real-Time DAQ" because the 6527 is a digital board. The category "Real-Time DAQ" is reserved for 7030 Real-Time devices.
To answer your question, you can use DAQ Occurences in LabVIEW to 'wake up' code when certain hardware events occur, such as a change in polarity of the digital lines on the 6527. If you are using a non-LabVIEW compiler, the functions are DIG_Change_Message_Config and DIG_Change_Message_Control. For more information on these functions, please read the NI-DAQ Functions Reference Manual, which can be found at www.ni.com/manuals.