Siemens S7-1200 - Page 83

PLC concepts
4.1 Execution of the user program
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG
83
● In STOP mode, the CPU handles any communication requests (as appropriate) and
performs self-diagnostics. The CPU does not execute the user program, and the
automatic updates of the process image do not occur.
● In STARTUP and RUN modes, the CPU performs the tasks shown in the following figure.
STARTUP
RUN
A
Clears the I (image) memory area
①
Writes Q memory to the physical outputs
B Initializes the Q output (image)
memory area with either zero, the last
value, or the substitute value, as
configured, and zeroes PB, PN, and
AS-i outputs
②
Copies the state of the physical inputs to I
memory
C Initializes non-retentive M memory and
data blocks to their initial value and
enables configured cyclic interrupt and
time of day events.
Executes the startup OBs.
③
Executes the program cycle OBs
D Copies the state of the physical inputs
to I memory
④
Performs self-test diagnostics
E Stores any interrupt events into the
queue to be processed after entering
RUN mode
⑤
Processes interrupts and communications
during any part of the scan cycle
F Enables the writing of Q memory to the
physical outputs