2 Visual Components Manuals

esponsible for managing access.
4.11.11.2 Requirements
There are a few rules concerning the VISAPI that must absolutely be observed.
4.11.11.2.1 1. Screen Access
Each VISAPI function which draws something on the screen must be called between the VA_Saccess and
VA_Srelease functions. Several VISAPI drawing functions may be called in one block.
VA_Saccess attempts to reserve access to the screen for the task. If successful, the function returns 0.
Otherwise, it returns a corresponding error code. Drawing functions can only be used if VA_Saccess was called
successfully.
VA_Srelease gives up access to the screen again so it can be used by other tasks or the Visual Components
interpreter.
Warning:
If VA_Srelease is not called after VA_Saccess was successfully called, then other tasks (or the Visual
Components interpreter) cannot access the screen.
If your visualization project uses VISAPI and suddenly 'freezes', then make sure that you have correctly called
VA_Srelease!
Example in the C Programming Language:
if (!VA_Saccess(TRUE, handle))
{
VA_Line(......);
VA_Rect(......);
VA_Srelease(TRUE, handle);
}
4.11.11.2.2
Cyclic Operation
The VISAPI library is designed to operate in cyclic tasks. In particular, this should be considered when using the
VA_Saccess function. If this function returns an error stating that access to the screen is not possible at the
moment, then a new attempt is made in the next cycle.
It usually doesn't make sense to try reserving the screen in the same cycle. An exception might be if VISAPI
drawing functions are carried out in a higher priority task class.
Example in the C Programming Language:
_CYCLIC void main(void)
{
if (!VA_Saccess(TRUE, handle))
{
VA_Line(......);
VA_Rect(......);
VA_Srelease(TRUE, handle);
}
}
4.11.11.2.3 Cycle Times
Like every other control task, a VISAPI task runs during a fixed cycle time. This should be considered when
programming with the VISAPI. Functions which require longer times to carry out drawing functions (i.e. ellipse)
can cause cycle time violations under certain circumstances if they are placed in faster task classes.
If necessary, place the task in a slower task class of use fewer functions in one cycle.
Introduction to Visual Components 2.3
424 4.11.11.2 Requirements