Siemens SIMATIC S7-1200 - Page 113

PLC concepts
5.1 Execution of the user program
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
113
System memory configures a byte with bits that turn on (value = 1) for a specific event.
Table 5- 19 System memory
7
6
5
4
3
2
1
0
Reserved
Value 0
Always off
Value 0
Always on
Value 1
Diagnostic status indica-
tor
• 1: Change
• 0: No change
First scan indicator
• 1: First scan after
startup
• 0: Not first scan
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- 20 Clock memory
Bit number
7
6
5
4
3
2
1
0
Tag name
Period (s)
2.0
1.6
1.0
0.8
0.5
0.4
0.2
0.1
Frequency (Hz)
0.5
0.625
1
1.25
2
2.5
5
10
Because clock memory runs asynchronously to the CPU cycle, the status of the clock memory can
change several times during a long cycle.