I think you are looking for the SCAN_Start() function. As stated in the NI-DAQ Function Reference Help, the purpose of SCAN_Start() is the following:
"Initiates a multiple-channel scanned data acquisition operation, with or without interval scanning, and stores its input in an array (MIO, AI, and DSA devices only)."
As an additional note, if you are not already aware of it, documentation concerning all NI-DAQ functions is present in the NI-DAQ Function Reference Help. This is available in the following location:
Start >> Programs >> National Instruments >> NI-DAQ >> NI-DAQ Help
Good luck with your application.