System Memory And Clock Memory Provide Standard Functionality - Siemens SIMATIC S7-1200 Manual

The CPU performs a warm restart before going to RUN mode. Warm restart resets all non-
retentive memory to the default start values, but the CPU retains the current values stored in
the retentive memory.
Note
The CPU always performs a restart after a download
Whenever you download an element of your project (such as a program block, data block, or
hardware configuration), the CPU performs a restart on the next transition to RUN mode. In
addition to clearing the inputs, initializing the outputs and initializing the non-retentive
memory, the restart also initializes the retentive memory areas.
After the restart that follows a download, all subsequent STOP-to-RUN transitions perform a
warm restart (that does not initialize the retentive memory).
5.5.1

System memory and clock memory provide standard functionality

You use the CPU properties to enable bytes for "system memory" and "clock memory". Your
program logic can reference the individual bits of these functions by their tag names.
● You can assign one byte in M memory for system memory. The byte of system memory
● You can assign one byte in M memory for clock memory. Each bit of the byte configured
Easy Book
Manual, 03/2014, A5E02486774-AF
provides the following four bits that can be referenced by your user program by the
following tag names:
– First cycle: (Tag name "FirstScan") bit is set to1 for the duration of the first scan after
the startup OB finishes. (After the execution of the first scan, the "first scan" bit is set
to 0.)
– Diagnostics status changed (Tag name: "DiagStatusUpdate") is set to 1 for one scan
after the CPU logs a diagnostic event. Because the CPU does not set the "diagnostic
graph changed" bit until the end of the first execution of the program cycle OBs, your
user program cannot detect if there has been a diagnostic change either during the
execution of the startup OBs or the first execution of the program cycle OBs.
– Always 1 (high): (Tag name "AlwaysTRUE") bit is always set to 1.
– Always 0 (low): (Tag name "AlwaysFALSE") bit is always set to 0.
as clock memory generates a square wave pulse. The byte of clock memory provides 8
different frequencies, from 0.5 Hz (slow) to 10 Hz (fast). You can use these bits as control
bits, especially when combined with edge instructions, to trigger actions in the user
program on a cyclic basis.
Easy to create the device configuration
5.5 Configuring the operation of the CPU and modules
• In STOP mode
• In RUN mode
• In the previous
mode (prior to the
power cycle)
77