Siemens S7-1200 System Manual page 102

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.
Table 4- 16 System memory
7 6 5 4
Reserved
Value 0
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.
102
CAUTION
3 2
Always off Always on
Value 0 Value 1
1
Diagnostic status
indicator
1: Change
•
0: No change
•
System Manual, 03/2014, A5E02486680-AG
0
First scan indicator
1: First scan after
•
startup
0: Not first scan
•
S7-1200 Programmable controller