Hi Marco,
The explanation in the user manual is not complete concerning how you may control the EXTSTROBE signal through software. This functionality is not available in LabVIEW or NI-DAQ. This signal is used for controlling a SCXI chassis, and it is not a general-purpose signal. If you want to use/control this signal, you will have to perform register-level programming.
Check out a Knowledge Base concerning this signal and a link to register-level programming resources
here.
regards,
Eric
Applications Engineer
National Instruments