The safety function of the safety CPU is to correctly process signal information. It processes safety input signals and internal data storage to generate signals to safety output modules and set a new state of its internal data storage.

If this function cannot be correctly executed, the safety CPU goes to a SAFE STOP state, in which no valid safety telegrams are generated and, as a result, all safety output module channels are de-energized (‘0’ state) after watchdog time is expired.

Faults in the cyclic communication between the safety CPU and safety I/O modules or other F-Devices, e.g., SM560-S-FD-1 or SM560-S-FD-4 safety CPUs, are detected by the safety CPU and, as a result, ‘0’ values are handed to the safety application program.

The application program developer must implement a specific fault reaction, e.g., setting safety output channels to de-energized (‘0’ state), when required.