Understanding Siemens OB80, OB35 & OB40 relationships

[thelectric]

Hello there Wizards,
I spent sometime browsing older posts before posting these questions on what appears to be an old subject & classic problem. Level one S7-300 training at Siemens did not cover this issue. And we were told not to take level two without a year in the field.
Question here on the relationship of OB80, OB35 & OB40 in scan time over run faults. The diagnostic buffer posts show an OB80 scan time overrun fault with a causing OB80, followed by a second post of the OB80 scan time overrun fault with causing OB35. As I understand it, the initial scan time over run was not caused by the OB35 interrupt, but by something else. And the OB35 error stopping the processor, is because it could not be processed twice in a row. If this is true, could an OB40 hardware interrupt being processed too frequently, cause the increased scan time? Thereby a faulty input device? I struggle with this because our average scan time is 1-2ms. CPU scan time set to 150ms, OB35 is 100ms, and therefore OB40 is potentially the wild card.

 

[thelectric]

Is it possible that an undersized MMC card could cause erratic OB80 scan time overrun faults?

 

[JesperMP]

, could an OB40 hardware interrupt being processed too frequently, cause the increased scan time? Thereby a faulty input device?

Yes.
You could add a counter in OB40, to trace how often it is triggered.
Reset the counter every second or so.
Then you can monitor the counter in code and generate an alarm if the input triggers in an unrealistical manner.

Is it possible that an undersized MMC card could cause erratic OB80 scan time overrun faults?

No. If the MMC is too small, you would be unable to load the program on the card.
Do you write to the card from the PLC program ?