Siemens SIMATIC S7-300 Series Reference Manual page 39

CPU Reaction on Missing Error OB
If you have not programmed an error OB, the CPU reacts as follows:
CPU goes into STOP on missing ...
OB 80 (Runtime error)
OB 85 (Program cycle error)
OB 86 (Station failure in the PROFIBUS-
DP subnet)
OB 87 (Communication error)
OB 121 (Programming error)
OB 122 (Peripheral direct access
error)
CPU Behavior When There Is No Interrupt OB
If you have not programmed an interrupt OB, the CPU reacts as follows:
CPU goes into STOP on missing ...
OB 10/11 (TOD interrupt)
OB 20/21 (Delay interrupt)
OB 40/41 (Process interrupt)
OB 55 (TOD interrupt)
OB 56 (Delay interrupt)
OB 57 (for manufacturer-specific
interrupts)
OB 82 (Diagnostic interrupt)
OB 83 (Insertion/Removal interrupt)
Tip on OB35 (CPU 318-2: also OB32)
For the watchdog interrupt OB 35/32, you can specify times starting from 1 ms.
Note: The smaller the selected watchdog interrupt period, the more likely
watchdog interrupt errors will occur. You must take into account the operating
system times of the CPU in question, the runtime of the user program and the
extension of the cycle by active programming device functions, for example.
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01
CPU Remains in RUN with Missing ...
OB 81 (Power break)
CPU Remains in RUN with Missing ...
OB 32/35 (Watchdog interrupt)
CPUs
1-23