Plc Concepts Made Easy; Tasks Performed Every Scan Cycle - Siemens SIMATIC S7-1200 Manual

PLC concepts made easy

4.1

Tasks performed every scan cycle

Each scan cycle includes writing the outputs, reading the inputs, executing the user program
instructions, and performing system maintenance or background processing. The cycle is
referred to as a scan cycle or scan. Under default conditions, all digital and analog I/O points
are updated synchronously with the scan cycle using an internal memory area called the
process image. The process image contains a snapshot of the physical inputs and outputs
on the CPU, signal board, and signal modules.
STARTUP
A
B
C Executes the startup OBs
D Copies the state of the physical inputs to I
E
F
Easy Book
Manual, 11/2009, A5E02486774-01
Clears the input (or "I") memory
Initializes the outputs with either the last
value or the substitute value
memory
Stores any interrupt events into the queue to
be processed in RUN mode
Enables the writing of the output (or "Q")
memory to the physical outputs
The CPU reads the physical inputs just prior to the
execution of the user program and stores the input
values in the process image input area. This ensures
that these values remain consistent throughout the
execution of the user instructions.
The CPU executes the logic of the user instructions and
updates the output values in the process image output
area instead of writing to the actual physical outputs.
After executing the user program, the CPU writes the
resulting outputs from the process image output area to
the physical outputs.
RUN
① Writes Q memory to the physical outputs
② Copies the state of the physical inputs to I
memory
③ Executes the program cycle OBs
④ Performs self-test diagnostics
⑤ Processes interrupts and communications
during any part of the scan cycle
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