Look for "Station Failure" or "Return of Module" messages which might point to a specific faulty slave or module. Best Practices to Prevent Recurrence
Faulty PROFINET or PROFIBUS cables and connectors. siemens fault 0a7991 best
Verify that the "Tipp" (Input/Output) times are consistent across all nodes in the synchronous group. 4. Analyze the Diagnostic Buffer The TIA Portal Diagnostic Buffer is your best friend. Go to . Read the entries immediately preceding the 0A7991 fault. Look for "Station Failure" or "Return of Module"
Increase the time slightly to give the network more "breathing room." siemens fault 0a7991 best