I am reviewing a PLC code which have been reported that PLC trips and the message is ""Data corruption in the safety program prior to output to F-I/O".

have been through the code and I believe that all the Siemens guidelines for this have been checked, including

- overflow

- indirect addressing

- checking the compilation error

the only item which I am a bit suspicious is that there are two OB's:

OB33 for safety

and OB35 which is by a higher priority.

there is a memory bit inside OB35 which is used in OB33. But I could not simulate the trip for this.

If I use this bit directly to drive a safety output and modify the offset phase to have OB35 with less phase (OB35 interrupts OB33). So this way I could simulate the trip but not from the way the current PLC code is.

If I move the contents of OB35 to a lower priority OB, the CPU does not go to stop (as tested above)

All the test have been done with PLCSim

Please see attached for details. they will be attached in the following posts

So any idea?

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Split from Data corruption in safety program before output to F I/O..