Siemens S7-1200 System Manual page 102

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Table of Contents

PLC concepts

4.1 Execution of the user program

The CPU initializes these bytes on the transition from STOP mode to STARTUP mode. The

bits of the clock memory change synchronously to the CPU clock throughout the STARTUP

and RUN modes.

Risks with overwriting the system memory or clock memory bits

Overwriting the system memory or clock memory bits can corrupt the data in these

functions and cause your user program to operate incorrectly, which can cause damage to

equipment and injury to personnel.

Because both the clock memory and system memory are unreserved in M memory,

instructions or communications can write to these locations and corrupt the data.

Avoid writing data to these locations to ensure the proper operation of these functions, and

always implement an emergency stop circuit for your process or machine.

System memory configures a byte with bits that turn on (value = 1) for a specific event.

System memory

Clock memory configures a byte that cycles the individual bits on and off at fixed intervals.

Each clock bit generates a square wave pulse on the corresponding M memory bit. These

bits can be used as control bits, especially when combined with edge instructions, to trigger

actions in the user code on a cyclic basis.

Diagnostic status

0: No change

System Manual, 03/2014, A5E02486680-AG

First scan indicator

1: First scan after

0: Not first scan

S7-1200 Programmable controller