Please help me also to understand the below file from diagnostic buffer of CPU 315 . and where's the problem as the PLC stop and SF led become on  

Event 9 of 10:  Event ID 16# 38C4
Distributed I/Os: station return
Address of the affected DP slave: station number:    4
DP master system ID:      1
Log. base address of the DP slave: Input address:  1022
Log. base address of the DP master:   1023
Requested OB: Rack failure OB (OB86)
OB not found, or disabled, or cannot be started in the current operating mode
External error, Outgoing event
03:44:58.990 PM  12/21/1997
(Coding: 16# 38C4  FE56  C054  03FF  03FE  0104)

Event 10 of 10:  Event ID 16# 456F
STOP caused by rack failure (OB not loaded or not possible, or no FRB)
Breakpoint in user program: Cyclic program (OB1)
Priority class:    1
FC number:      3
Module address:    244
Previous operating mode: RUN
Requested operating mode: STOP (internal)
Internal error, Incoming event
03:44:58.878 PM  12/21/1997
(Coding: 16# 456F  FF84  8C70  0101  0003  00F4)

----------------------------------------------------------------------------------------------------
Split from Fault OB82 in CPU diagnostic buffer.

 Hello amr;

Your SF LED comes on because there is an internal (or external) fault encountered while processing the PLC; the fault could be hardware or software. The diagnostic buffer messages you included point to Profibus slave no.4, with diagnostic addresses 1022 (and others0 for the affected modules; please refer to your hardware configuration to see what modules these addresses refer to.

The PLC goes to Stop mode because, when encountering the fault that caused the appearance of the SF LED, the operating system of the CPU could not locate in the PLC program a specifically needed diagnostic OB. In your case, the messages from the diagnostic buffer mention OB 86 is required but missing.

So go to your program blocs, create a new block of type OB, name it OB 86, save (without any programming inside the block for the moment) and download it to the CPU.

Next time the SF fault appears (the presence of the diagnostic OB 86 does not prevent the fault, it helps the CPU work around it and introduces more complete diagnostic information to the buffer) your CPU should keep running its program, and the diagnostic buffer should contain a lot more information on the source of the problem you encountered. That is what the diagnostic OBs are used for.

Now, open the diagnostic buffer again, read the new messages, locate the error(s) on your Profibus network (OB 86 is concerned with errors on the network), repair them and the SF LED should go off.

Hope this helps,

Daniel Chartier

Thanks for valuable information