Siemens SIMATIC S7-400 Manual page 133

Minimum Cycle Time
You can set a minimum cycle time for a CPU in STEP 7. This is appropriate in the following
cases:
● you want the intervals of time between the start of program scanning of OB1 (free cycle)
● updating of the process images would be performed unnecessarily often with too short a
● You want to process a program with the OB 90 in the background.
Current cycle
PCl16
PCl07
Process image Process image
PCl01
updating updating
of the outputs of the inputs
PK29
(= PK0,29)
Tmin = the adjustable minimum cycle time
Tmax = the adjustable maximum cycle time
Tcyc = the cycle time
Twait = the difference between Tmin and the actual cycle time; in this time,
any interrupts that occur, the background OB and the SCC tasks can be processed.
PCI = priority class
Figure 9-3 Minimum cycle time
The actual cycle time is the sum of T
S7-400 Automation System, CPU Specifications
Manual, 10/2006, 6ES7498-8AA04-8BA0
to be roughly of the same length.
cycle time.
T
max
T
min
T
cyc
OB10
OB1
Cycle and Response Times of the S7-400
T
wait
OB40
OB1 SCC
OB90
OB90
and T . It is always greater than or equal to T
cyc wait
9.3 Different cycle times
Next cycle
Reserve
Process image
updating
. . .
of the outputs
min.
9-7