Technical data

Basics of Designing a Program Structure
4.2 Blocks in the User Program
Programming with STEP 7
110 Manual, 05/2010, A5E02789666-01
4.2.4.7 Error Handling Organization Blocks (OB70 to OB87 / OB121 to OB122)
Types of Errors
The errors that can be detected by the S7 CPUs and to which you can react with the help of
organization blocks can be divided into two basic categories:
• Synchronous errors: these errors can be assigned to a specific part of the user program. The
error occurs during the execution of a particular instruction. If the corresponding synchronous
error OB is not loaded, the CPU changes to STOP mode when the error occurs.
• Asynchronous errors: these errors cannot be directly assigned to the user program being
executed. These are priority class errors, faults on the programmable logic controller (for
example, a defective module), or redundancy errors. If the corresponding asynchronous error
OB is not loaded, the CPU changes to STOP mode when the error occurs (exceptions: OB70,
OB72, OB81, OB 87).
The following table shows the types of errors that can occur, divided up into the categories of the
error OBs.
Asynchronous Errors/Redundancy Errors Synchronous Errors
OB70 I/O Redundancy Error (only H CPUs) OB121 Programming Error (for example, DB is not
loaded)
OB72 CPU Redundancy Error (only in H CPUs, for
example, failure of a CPU)
OB122 I/O Access Error (for example, access to a
signal module that does not exist)
OB80 Time Error (for example, scan cycle time
exceeded)
OB81 Power Supply Error (for example, battery
failure)
OB82 Diagnostic Interrupt (for example, short circuit in
the input module)
OB83 Remove/Insert Interrupt (for example, removing
an input module)
OB84 CPU Hardware Fault (fault at the interface to
the MPI network)
OB85 Priority Class Error (for example, OB is not
loaded)
OB86 Rack Failure
OB87 Communication Error (for example, incorrect
message frame ID for global data communication)