Different Cycle Times - Siemens SIMATIC S7-400 Reference Manual

Cycle and Reaction Times of the S7-400
3.3

Different Cycle Times

The length of the cycle time (T
figure shows different cycle times, T
because the cyclically scanned OB 1 is interrupted by a time-of-day interrupt OB
(in this instance, OB 10).
Current cycle
T
ting
PIQ PII
Upda– Upda–
OB1
ting ting
Figure 3-2

Different Cycle Times

A further reason for cycle times of different length is the fact that the execution
time of blocks (for example, OB 1) can vary on account of:
• Conditional instructions
• Conditional block calls
• Different program paths
• Loops, etc.
Maximum Cycle Time
You can modify the default maximum cycle time in STEP 7 (cycle monitoring time).
If this time has expired, OB 80 is called, and in it you can define how you want the
CPU to respond to the 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 goes
to STOP at the second call of OB 80.
If there is no OB 80 in the CPU memory, the CPU goes to STOP.
3-8
) is not identical in each cycle. The following
cyc
cyc1
Next cycle
T
cyc 2
PIQ
PII
Upda–
SCC Upda–
OB1
ting
ting
and T . T is longer than T
cyc2 cyc2
Next cycle but one
OB10
PIQ
Upda–
OB1
SCC
ting
Automation System S7-400 CPU Specifications
,
cyc1
PII
Upda–
ting
A5E00165965-01