If I want to open some directory by the following way:

PROGRAM PLC_PRG
VAR
xDirOpen:BOOL;
DirOpen:FILE.DirOpen;
END_VAR

===================
IF xDirOpen THEN
REPEAT
DirOpen(xExecute:=xDirOpen,sDirName:='D:\myTestDir';
IF DirOpen.xDone OR DirOpen.xError THEN
xDirOpen:=FALSE;
DirOpen(xExecute:=FALSE);
END_IF
UNTIL NOT xDirOpen

  END_REPEAT

END_IF

When the program is running,I change the value of xDirOpen from FALSE to TURE,but the result is endless loop!
Can someone give some explaination?

Remove the repeat / untill loop
It could take some scans before the OS returns a result.
Wait for the done/error and then set a condition to handle the next step.

TimvH hat geschrieben:
Remove the repeat / untill loop
It could take some scans before the OS returns a result.
Wait for the done/error and then set a condition to handle the next step.

====================================================
Do you mean that the scan cannot be excute during a PLC cycle even if the PLC cycle takes a long time? but must be execute during the intervals between two PLC cycles?

Yes, that is my experience.
It is made like this to make sure it is non-blocking.
With the SysFile library the PLC execution halted untill the OS returned a result, but this was not good for time critical applications.

I don't know the exact reason behind it (internal operation) why it takes at least 2 scans.