System And Clock Memory - Siemens S7-1200 System Manual

The CPU supports a total of 10240 bytes of retentive data. To see how much is available,
from the PLC tag table or the assignment list, click on the "Retain" toolbar icon. Although this
is where the retentive range is specified for M memory, the second row indicates the total
remaining memory available for M and DB combined. Note that for this value to be accurate,
you must compile all data blocks with retentive tags.
Note
Downloading a program does not clear or make any changes to existing values in retentive
memory. If you want to clear retentive memory before a download, then reset your CPU to
factory settings prior to downloading the program.
4.1.5.1

System and clock memory

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
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG
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.
PLC concepts
4.1 Execution of the user program
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