Siemens SIMATIC S7-300 Series Reference Manual page 135

Sample Configuration 1
You have configured an S7-300 with the following modules on one rack:
S 1 CPU 314
S 2 SM 321 DI 32DC 24 V digital input modules (4 bytes each in the PI)
S 2 SM 322 DO 32 DC 24 V/0.5A (4 bytes each in the PI)
According to the Instruction List, the user program has a runtime of 1.5 ms.
There is no communication.
Calculation
In this example, the cycle time is calculated from the following times:
S Process image transfer time
Input process image: 147 ms + 8 bytes 13.6 ms = approx. 0.26 ms
Output process image: 147 ms + 8 bytes 13.6 ms = approx. 0.26 ms
S Operating system cycle time
Cycle control: approx. 1 ms
S User program processing time:
approx. 1.5 ms CPU specific factor 1.15 = approx. 1.8 ms
S Processing time of S7 timers
Assumption: 30 S7 timers are running.
For 30 S7 timers, the one-off update takes
8 ms = 240 ms.
30
Process image transfer time + operating system processing time + user
program cycle time results in this time interval:
0.26 ms + 0.26 ms + 1 ms + 1.8 ms = 3.32 ms.
Since the S7 timers are called every 10 ms, a maximum of one call can be
made in this time interval, i.e. the S7 timers might increase cycle time by a
maximum of 240 ms (= 0.24 ms)
The cycle time is calculated from the sum of the listed times:
Cycle time = 0.26 ms + 0.26 ms + 1 ms + 1.8 ms + 0.24 ms = 3.56 ms.
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01
Cycle and Reaction times
3-11