User`s manual

74 Application Hints
PCIS-DASK
DAQ Event Message Programming Hints
4.5 DAQ Event Message Programming Hints
DAQ Event Message functions are efficient ways to monitor your
background data acquisition processes without dedicating your
foreground process for status checking. There are two kinds of
events: AI/DI/DO operation completed notification event and half
buffer ready notification event.
To receive notification from the PCIS-DASK data acquisition pro-
cess in case of special events, you can call AI_EventCallBack,
DI_EventCallBack, or DO_EventCallBack to specify an event.
Event notification is done through user-defined callbacks. When a
user-specified DAQ event occurs, PCIS-DASK calls the user-
defined callback. After receiving the message, the user's applica-
tion carries out the appropriate task.
The event message mechanism is easy and safe in Windows
®
98
and Windows
®
NT systems. However, the time delay between the
event and notification is highly variable and depends largely on
how your system is loaded. In addition, if a callback function is
called, succeeding events will not be handled until your callback
has returned. If the time interval between events is smaller than
the time taken for callback function processing, the succeeding
events will not be handled. Therefore this mechanism is not suit-
able for the frequent events occurrence condition.