Siemens SIMATIC S7-300 Series Reference Manual page 136

Cycle and Reaction times
Parts of the Response Time
As a reminder, the response time is formed by the sum of:
S 2
Process image transfer time of inputs +
S 2
Process image transfer time of outputs +
S 2
operating system cycle time+
S 2
program cycle time+
S Processing time of the S7 timers +
S Delay times of the inputs and outputs
Tip: Simple calculation: calculated cycle time
Thus, for example one applies: 3.34 ms
the I/O modules.
Sample Configuration 2
You have configured an S7-300 with the following modules on two racks:
S 1 CPU 314
S 4 SM 321 DI 32DC 24 V digital input modules (4 bytes each in the PI)
S 3 SM 322 DO 16 DC 24 V/0.5A (2 bytes each in the PI)
S 2 SM 331 AI 8 12 bits (not in the process image)
S 2 SM 332 AI 4 12 bits (not in the process image)
User program
According to the Instruction List, the user program has a runtime of 2 ms. By
taking into account the CPU-specific factor of 1.15, the resulting runtime is approx.
2.3 ms. The user program operates up to 56 S7 timers simultaneously. No
activities are required at the scan cycle checkpoint.
Calculation
In this example, the response time is calculated from the following times:
S Process image transfer time
Input process image: 147 ms + 16 bytes 13.6 ms = approx. 0.36 ms
Output process image: 147 ms + 6 bytes 13.6 ms = approx. 0.23 ms
S Operating system execution time
Cycle control: approx. 1 ms
S User program processing time: 2.3 ms
3-12
2 + delay times.
2 + timers
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01