Description
The messages "PLC fault synchronous" and "PLC fault asynchronous" are triggered when a synchronous or asynchronous fault is triggered in the PLC.

These can be the following:
 

AS errors, synchronous:

OB 121: Programming error

OB 122: I/O access error

AS errors, asynchronous:

OB 80: Time error (e.g. exceeding cycle time)

OB 81: Power supply error (e.g. battery error)

OB 82: Diagnostics interrupt (e.g. defective fuse on a signal module)

OB 83: Remove/plug interrupt

OB 84: CPU hardware error (e.g. interface error)

OB 85: Program runtime error

OB 86: Subrack failure

OB 87: Communications error

You can delimit these faults by proceeding as follows:

1. Read out the diagnostics buffer of the CPU.
   (Select CPU in the SIMATIC Manager, right click "Target System" to open the "Module Properties", select the "Diagnostics Buffer" tab).
2. Evaluate the local variables of the fault OBs.
   Double-click the relevant OB in the block container. With a small STL program transfer the contents of the required local variables into a marker byte or a data block that you have created beforehand. The relevant commands are as follows, for example:

   	L	#OB80_EV_CLASS
   	T	MB80
   	or
   	L	#OB80_FLT_ID
   	T	DB1.DBB0

3. Finally, you must load the modified OB or data block into the PLC.

If you can't clear the cause of the fault message, you have the option of suppressing the message in WinCC Runtime. For this refer to the FAQ "Inhibit messages in Runtime", Entry ID: 25288354.