Detect discrepancy fault of failsafe IO in user program

Hello,

I did a little research in the manuals. Here what I would check.

If you configure an input as 1oo2, a discrepancy will cause the passivation of that input, and I expect that you will get a global QBAD in the F I/O DB of that module.

I also expect an F-DI to generate a diagnostic interrupt. The specific manual of that bF-DI lists what events causes an diagnostic event.  For the ET200 SP 4-FDI for instance (6ES7136-6BA00-0CA0) discrepancy is part of that list. The CPU calls the interrupt OB for diagnostic interrupt (OB82).
in this FAQChannel diagnostic in the user program
they show how to get diagnostic info in general.

In this manual: Safety configuration
you will see in chapter 6.2 that you some modules have an extra value status information.
Extract: The value status is additional binary information for the channel value of an F-I/O. The value status is entered in the process image input (PII).
The value status is supported by S7-1500/ET 200MP, ET 200SP, ET 200S, ET 200iSP, ET 200pro, S7-1200 fail-safe modules or S7-300 F-SMs fail-safe GSD based I/O devices as well as fail-safe GSD based DP slaves which support the "RIOforFA-Safety" profile.
The value status bits come straight after the channel values in the PII.
The location of the channel values in the PII can be found in the device manual for the F-I/O.
So in case of discrepancy, I expect that you will get a failsafe value (value status=0)  instead of a valid value (value status = 1) for that input.
Note: 1oo2 evaluation of the sensors combines the two channels. With 1oo2 evaluation of the
sensors you may only access the low order channel in the safety program.