Siemens SIMATIC S7-1200 Manual page 79

System memory configures a byte with bits that turn on (value = 1) for a specific event.
Table 5- 2 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.
Table 5- 3 Clock memory
Bit number
Tag name
Period (s)
Frequency (Hz)
Because clock memory runs asynchronously to the CPU cycle, the status of the clock memory can change several times
during a long cycle.
Easy Book
Manual, 11/2011, A5E02486774-04
3 2
Always off Always on
Value 0 Value 1
7 6
2.0 1.6
0.5 0.625
5.4 Configuring the operation of the CPU and modules
1
Diagnostic status
indicator
1: Change

0: No change

5 4 3
1.0 0.8 0.5
1 1.25 2
Easy to create the device configuration
0
First scan indicator
1: First scan after

startup
0: Not first scan

2 1
0.4 0.2
2.5 5
0
0.1
10
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