User`s guide

Continuous Data Transfer in PCIS-DASK • 67
6.3 Trigger Mode Data Acquisition for Analog Input
A trigger is an event that occurs based on a specified set of conditions. An
interrupt mode or DMA-mode analog input operation can use a trigger to
determinate when acquisition stops or starts.
PCIS-DASK also provides two buffering methods for trigger mode AI – double-
buffering and single-buffering. However, the single buffer in trigger mode AI is
different from that in non-trigger mode AI. It is a circular buffer just like that in
double buffer mode but the data stored in the buffer can be processed only
when the continuous data reading is completed. The buffer will be reused until
the data acquisition operation is completed. Therefore, to protect the data you
want to get from being overwritten, the size of the single buffer should be the
same as or larger than the amount of data you wish to access. For example, if
you want to perform single-buffered middle-trigger AI with PCI-9812, and the
amount of data you want to collect before and after the trigger event are 1000
and 3000 respectively, the size of single buffer is at least 4000 in order to get
all the data you want to collect. Since the data are handled after the input
operation is completed, the desired data loss problem hardly occurs.
Since PCIS-DASK uses asynchronous AI to perform trigger mode data
acquisition, the SyncMode of continuous AI should be set as ASYNCH_OP.