Technical data

Diagnostics
23.9 Program Measures for Handling Errors
Programming with STEP 7
516 Manual, 05/2010, A5E02789666-01
23.9.10 CPU Hardware Fault (OB84)
Description
The operating system of the CPU calls OB84 when an error is detected on the interface to the MPI
network, to the communication bus, or to the network card for the distributed I/Os; for example, if
an incorrect signal level is detected on the line. The OB is also called when the error is eliminated
(the OB is called when the event comes and goes).
Programming OB84
You must create OB84 as an object in your S7 program using STEP 7. Write the program to be
executed in OB84 in the generated block and download it to the CPU as part of your user program.
You can use OB84, for example, for the following purposes:
• To evaluate the start information of OB84.
• By including system function SFC52 WRUSMSG to send a message to the diagnostic buffer.
If you do not program OB84, the CPU changes to STOP mode when a CPU hardware fault is
detected.
You can find detailed information on OBs, SFBs, and SFCs in the corresponding Help on Blocks.
23.9.11 Program Sequence Error (OB85)
Description
The operating system of the CPU calls OB85:
• When a start event for an interrupt OB exists but the OB cannot be executed because it has
not been downloaded to the CPU.
• When an error occurs accessing the instance data block of a system function block.
• When an error occurs updating the process image table (module configured but does not exist
or module configured but defective).
Programming OB85
You must create OB85 as an object in your S7 program using STEP 7. Write the program to be
executed in OB85 in the generated block and download it to the CPU as part of your user program.
You can use OB85, for example, for the following purposes:
• To evaluate the start information of OB85 and determine which module is defective or not
inserted (the module start address is specified).
• By including SFC49 LGCGADR to find out the slot of the module involved.
If you do not program OB85, the CPU changes to STOP mode when a priority class error is
detected.