PLC concepts
5.1 Execution of the user program
S7-1200 Programmable controller
88 System Manual, V4.2, 09/2016, A5E02486680-AK
You can configure the "startup after POWER ON" setting of the CPU. This configuration item
appears under the "Device configuration" for the CPU under "Startup". Upon powering up,
the CPU performs a sequence of power-up diagnostic checks and system initialization.
During system initialization the CPU deletes all non-retentive bit (M) memory and resets all
non-retentive DB contents to the initial values from load memory. The CPU retains retentive
bit (M) memory and retentive DB contents and then enters the appropriate operating mode.
Certain detected errors prevent the CPU from entering the RUN mode. The CPU supports
the following configuration choices:
● No restart (stay in STOP mode)
● Warm restart - RUN
● Warm restart - mode prior to POWER OFF
Repairable faults can cause the CPU to enter STOP mode.
The CPU can enter STOP mode due to repairable faults, such as failure of a
replaceable signal module, or temporary faults, such as power line disturbance or erratic
power up event. Such conditions could result in property damage.
If you have configured the CPU to "Warm restart - mode prior to POWER OFF", the
CPU goes to the operating mode it was in prior to the loss of power or fault. If the CPU
was in STOP mode at the time of power loss or fault, the CPU goes to STOP mode on
power up and stays in STOP mode until it receives a command to go to RUN mode. If
the CPU was in RUN mode at the time of power loss or fault, the CPU goes to RUN
mode on the next power up providing it detects no errors that would inhibit a transition to
RUN mode.
Configure CPUs that are intended to operate independently of a STEP 7 connection to
"Warm restart - RUN" so that the CPU can return to RUN mode on the next power cycle.
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