• Within the CPU every time when there is a problem with a Slave the "Error" OBs (OB85 / OB86 and so on) are called. There you will find detailed informations about the I/O adresses of the slaves and reason why the OB is called.
• In the diagnostic buffer of the CPU a short information is given too.
• Have you seen the online view of the hardware configuration? The only problem using this functionality is, that it show a "frozen picture" of the actual situation.
  
• Maybe you can use the socalled Diagnostic Repeater. In a lot of cases he can give you more informations about your PROFIBUS network
  Here you will find more informations about this Repeater:
  SIMATIC Diagnostic Repeater for PROFIBUS-DP
  Diagnostic Repeater Stage 2 for PROFIBUS-DP Now Available
  

Hello

You can try FB125

Best regards
Shejun