User's Manual

Chapter 2 Function Reference — Lab_ISCAN_Start
© National Instruments Corporation 2-273 NI-DAQ FRM for PC Compatibles
For the Lab and 1200 Series devices, if you enable pretrigger mode, Lab_ISCAN_Start
initiates a cyclical acquisition that continually fills the buffer with data, wrapping around to
the start of the buffer once NI-DAQ has written to the entire buffer. When you apply the signal
at the stop trigger input,
Lab_ISCAN_Start acquires an additional number of samples
specified by the ptsAfterStoptrig parameter in
DAQ_StopTrigger_Config and then
terminates.
Because the trigger can occur at any point in the scan sequence, the scanning operation can
end in the middle of a scan sequence. See the description for
Lab_ISCAN_Check to
determine how NI-DAQ rearranges the buffer after the acquisition ends. When you enable
pretrigger mode, the length of the buffer, which is greater than or equal to count, should be
an integral multiple of numChans.
If you have selected external start triggering of the data acquisition operation, a low-to-high
edge at the EXTTRIG of the Lab and 1200 Series device I/O connector input initiates the data
acquisition operation after the
Lab_ISCAN_Start call is complete. Otherwise,
Lab_ISCAN_Start issues a software trigger to initiate the data acquisition operation before
returning.
Note If your application calls Lab_ISCAN_Start, always make sure that you call
DAQ_Clear before your application terminates and returns control to the
operating system. Unless you make this call (either directly, or indirectly through
Lab_ISCAN_Check or DAQ_DB_Transfer), unpredictable behavior might result.