profibus system with CPU 315-2DP: Erratic BF-SF faults

Tej_BLR

Hello , 

We have a profibus system with base station with CPU 315-2DP + its I/O . 2 Remote racks with 1 containing IM153-1 ( ET200M)  and I/O . 

Am encounting Erratic BF-SF faults on base station . SF is steady ON and BF is blinking. 

This happens for 1-2 sec and then everything is back online and alarm is clear. 

Checked PLC diagnostics and Node 8 seams to be the issue . 

But Profibus cable was already replaced 

Power supply to the Remoter rack is new. 

Strange  only One function  on  remote rack seams to trigger this error , all other functions  seams  Normal . When this specific function is on a I/O module  is used ,randomly OB 86 Rack failure is encountered . 

Attached Diagnostic and Profibus HW config . How can i see beyond ET200M . Track specific module and I/O responsible for it ? 

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Split from S7-300 Rack failure OB (OB86) .