Description:
Before a S7-300 CPU starts processing the user program after switch-on, a startup program is processed. In the startup program, you can define specific presettings for your cyclic program by programming the startup OB accordingly.

All S7-300 CPUs always execute a restart (warm start).
(Only the CPU 318-2 can also execute a cold restart, see Entry ID: 34053758)

In operating mode "STARTUP":

• The program is processed in the startup OB 100 for restart (warm start).
• Time- and alarm-controlled program processing is not possible.
• Times are updated.
• The runtime meter runs.
• The digital outputs on signal modules are blocked, but can be set via direct access.

Restart (warm restart)

In a restart (warm restart), program processing restarts at the beginning of the program with a "basic setting" of the system data and user address areas.

• The process image and the non-retentive markers, timers and counters are reset. Retentive markers, timers and counters each retain their last valid value. All data blocks parameterized with the property "Non Retain" are reset to the loaded values. The other data blocks each retain their last valid value.
• Program processing starts again at the beginning (startup OB or OB 1).

Order of operations for restart (warm restart):

You can trigger a manual restart (warm restart):

• Via the mode selector switch.
• Via menu command from the PG or via communication functions (if the mode selector switch is set to RUN or RUN-P).

An automatic restart (warm restart) can be triggered at POWER ON if:

• The CPU was not in STOP at POWER OFF.
• The mode selector switch is set to RUN or RUN-P.
• There is no automatic hot restart parameterized for POWER ON.
• The restart (warm restart) of the CPU has not been interrupted by a power failure (independent of the startup parameterized).