What do you have to watch out for in STEP 7 V5.x for the S7-300/S7-400 CPUs if the F CPU goes into STOP due to internal error 16#75D1?

Description
The F CPU goes into STOP mode due to internal error 16#75D1. The information about this error given in the Help does not apply.

Entry (16#75D1) is displayed when the CPU's diagnostic buffer is read out. 

Fig. 1: Diagnostic buffer display with the current error highlighted

The Online Help for this error mentions the following causes and specifies the corresponding measures for clearing the errors (Help on Event 7xD1):

A safety-critical error has been detected in the safety program. Possible causes of the errors:

• The standard user program describes illegal safety program data.
• Safety program data has been changed illegally via "Monitor/Control Variable".
• Automatically generated parts of the program have been changed or deleted.
• Blocks reserved for the F CPU have been changed or deleted in the HW Config in the "CPU Properties" dialog, in the "F Parameters" tab. 
• An internal CPU error has occurred.

Remedy:

1. Check whether the standard user program affects the safety program.
2. Check whether safety program data has been changed illegally via "Monitor/Control Variable".
3. Copy F blocks used in the F library into the safety program again. 
4. Generate the safety program again.
   or 
5. Replace the F CPU.

However, the cause might also be as follows.
You have read access to a temporary variable in an F block without having initialized it beforehand. This means that you are processing a signal with an indefinite status in the safety program. The F control block detects this and sets the F CPU to the safe STOP mode.

Remedy

• Initialize all the temporary variables through a write access before you read access them or 
• Use safe global addresses from an F DB or 
• Use static variables in an F FB.