Other Features To Make Programming Easy; System Memory And Clock Memory Provide Standard Functionality - Siemens SIMATIC S7-1200 Easy Book Manual

5.4

Other features to make programming easy

5.4.1

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.
● 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
The CPU initializes these bytes on the transition from STOP mode to STARTUP mode, and
the bits of the clock memory change synchronously to the CPU clock throughout STARTUP
and RUN modes.
Because both the clock memory and system memory are unreserved M memory,
instructions or communications can write to these locations and corrupt the data. Overwriting
the system memory or clock memory bytes can corrupt the data in these functions and
cause your user program to operate incorrectly. Always configure the system memory and
clock memory for a memory address that is not accessed by other elements of your user
program.
Easy Book
Manual, 05/2009, A5E02486774-01
provides the following four bits that can be referenced by your user program:
– "Always off" bit is always set to 0.
– "Always on" bit is always set to 1.
– "Diagnostic event changed" is set to 1 for one scan after the CPU logs a diagnostic
event.
The CPU does not set the "Diagnostic event changed" bit until the end of the first
execution of the of the program cycle OBs. Your 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.
– "First scan" 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.)
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.
5.4 Other features to make programming easy
The system memory byte turns bits on
(value = 1) for the following conditions:
• First scan: Turns on for the first
scan cycle after a power cycle
• Diagnostic graph changed.
• Always 1 (high): Always turned on
• Always 0 (low): Always turned off
Programming options
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