Siemens S7-1200 System Manual page 84

PLC concepts
4.1 Execution of the user program
STARTUP processing
Whenever the operating mode changes from STOP to RUN, the CPU clears the process
image inputs, initializes the process image outputs and processes the startup OBs. Any read
accesses to the process-image inputs by instructions in the startup OBs read zero rather
than the current physical input value. Therefore, to read the current state of a physical input
during the startup mode, you must perform an immediate read. The startup OBs and any
associated FCs and FBs are executed next. If more than one startup OB exists, each is
executed in order according to the OB number, with the lowest OB number executing first.
Each startup OB includes startup information that helps you determine the validity of
retentive data and the time-of-day clock. You can program instructions inside the startup
OBs to examine these startup values and to take appropriate action. The following startup
locations are supported by the Startup OBs:
Table 4- 1
Input
LostRetentive
LostRTC
The CPU also performs the following tasks during the startup processing.
● Interrupts are queued but not processed during the startup phase
● No cycle time monitoring is performed during the startup phase
● Configuration changes to HSC (high-speed counter), PWM (pulse-width modulation), and
PtP (point-to-point communication) modules can be made in startup
● Actual operation of HSC, PWM and point-to-point communication modules only occurs in
RUN
After the execution of the startup OBs finishes, the CPU goes to RUN mode and processes
the control tasks in a continuous scan cycle.
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Startup locations supported by the startup OB
Data Type Description
Bool This bit is true if the retentive data storage areas have been lost
Bool This bit is true if the time-of-day clock (Real time Clock) has been lost
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG