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Diagnostics
23.9 Program Measures for Handling Errors
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 517
You can find detailed information on OBs, SFBs, and SFCs in the corresponding Help on Blocks.
23.9.12 Rack Failure (OB86)
Description
The operating system of the CPU calls OB86 when it has detected one of the following events:
• Failure of a central expansion rack (not for S7-300) such as a broken connecting line,
distributed power failure on a rack
• Failure of a master system or a slave (PROFIBUS DP) or failure of an IO system or an IO
device (PROFINET IO)
OB86 is also called when the error is eliminated (the OB is called when the event comes and
goes).
Programming OB86
You must create OB86 as an object in your S7 program using STEP 7. Write the program to be
executed in OB86 in the generated block and download it to the CPU as part of your user program.
You can use OB86, for example, for the following purposes:
• To evaluate the start information of OB86 and determine which rack is defective or missing.
• To enter a message in the diagnostic buffer with system function SFC 52 WRUSMSG and to
send the message to a monitoring device.
If you do not program OB86, the CPU changes to STOP mode when a rack failure is detected.
You can find detailed information on OBs, SFBs, and SFCs in the corresponding Help on Blocks.
23.9.13 Communication Error (OB87)
Description
The operating system of the CPU calls OB87 when a communication error occurs in data exchange
using communication function blocks or in global data communication, for example:
• When receiving global data, an incorrect frame ID was detected
• The data block for the status information of the global data does not exist or is too short.
Programming OB87
You must create OB87 as an object in your S7 program using STEP 7. Write the program to be
executed in OB87 in the generated block and download it to the CPU as part of your user program.
You can use OB87, for example, for the following purposes:
• To evaluate the start information of OB87.