Wincc event-question/problem

I have folowing code in Plc(Simatic S7)
SP_Change is reset after 3 seconds.
Frequency of SP change is 25-30 seconds.

Folowing massage configured in WinCC:
"Set point change Old_Value/New_Value".
Message is triggered by #SP_Cange bit.
Usually everything works as expected and i get messages like
"Set point change 40/41".
But sometimes i get messages with 0/0 or same value like 40/40.
I fail to understand why this is happening.

Hello

WinCC update his tags asynchronously from PLC's Cycle.

It means that WinCC can catch the rising edge in "SP_Change" when the first 2 lines have just been ejecuted.

So what you are saying is that trigger bit of a message is updated faster then "process value" blocks tags?

I'm saying that WinCC detects changes in the bit just in the moment that PLC is executing the next cycle (or laters) but before the increment of the SP