Technical data

Diagnostics
23.9 Program Measures for Handling Errors
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 513
23.9.7 Power Supply Error (OB81)
Description
The operating system of the CPU calls OB81 if one of the following fails in a CPU or an expansion
unit
• The 24-V voltage supply
• A battery
• The complete backup
This OB is also called when the problem has been eliminated (the OB is called when an event
comes and goes).
Programming OB81
You must create OB81 as an object in your S7 program using STEP 7. Write the program to be
executed in OB81 in the generated block and download it to the CPU as part of your user program.
You can, for example, use OB81 for the following purposes:
• To evaluate the start information of OB81 and determine which power supply error has
occurred.
• To find out the number of the rack with the defective power supply.
• To activate a lamp on an operator station to indicate that maintenance personnel should
replace a battery.
If you do not program OB81, the CPU does not change to STOP mode if a power supply error is
detected. The error is, however, entered in the diagnostic buffer and the corresponding LED on the
front panel indicates the error.
You can find detailed information on OBs, SFBs, and SFCs in the corresponding Help on Blocks.