Siemens SIMATIC S7-400H - Different Cycle Times

S7-400 cycle and response times
19.3 Different cycle times
S7-400H
364 System Manual, 03/2012, A5E00267695-11
19.3 Different cycle times
The cycle time (T
cyc
) length is not the same in every cycle. The figure below shows different
cycle times T
cyc1
and T
cyc2
. T
cyc2
is longer than T
cyc1
because the cyclically executed OB 1 is
interrupted by a TOD interrupt OB (here: OB 10).
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2%
2%
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6&&
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F\F
1H[WF\FOH7F\F
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3,2
8SGDWH
3,2
8SGDWH
3,,
8SGDWH
3,2
8SGDWH
3,,
8SGDWH
3,,
Figure 19-2 Different cycle times
Fluctuation of the block processing time (e.g. OB 1) may also be a factor causing cycle time
fluctuation, due to:
● conditional instructions
● conditional block calls
● different program paths
● loops, etc.
Maximum cycle time
In STEP 7 you can modify the default maximum cycle time (scan cycle monitoring time). OB
80 is called when this time expires. In this block you can specify the CPUs response to this
time error. If you do not retrigger the cycle time with SFC 43, OB 80 doubles the cycle time
at the first call. In this case the CPU switches to STOP mode when OB 80 is called a second
time.
The CPU switches to STOP mode if OB 80 does not exist in its memory.