Siemens S7-300 appendix Sample calculations, Example of cycle time calculation

Cycle and reaction times 5.6 Sample calculations

5.6Sample calculations

5.6.1Example of cycle time calculation

Installation

You have configured an S7­300 and equipped it with following modules in rack "0":

•a CPU 314C-2

•2 digital input modules SM 321; DI 32 x 24 VDC (4 bytes each in the PI)

•2 digital output modules SM 322; DO 32 x 24 VDC/0.5 A (4 bytes each in the PI)

User program

According to the Instruction List, the user program runtime is 5 ms. There is no active communication.

Calculating the cycle time

In this example, the cycle time is equivalent to the sum of the following times:

•User program execution time:

approx. 5 ms x CPU-specific factor 1.10 = approx. 5.5 ms

•Process image transfer time

Process image of inputs: 100 μs + 8 Byte x 37 μs = approx. 0.4 ms

Process image of outputs: 100 μs + 8 Byte x 37 μs = approx. 0.4 ms

•Operating system runtime at the scan cycle checkpoint: approx. 0.5 ms

Cycle time = 5.5 ms + 0.4 ms + 0.4 ms + 0.5 ms = 6.8 ms.

Calculating the physical cycle time

•There is no active communication.

•Interrupts are not processed. Hence, the physical cycle time is 6 ms.

5-24	CPU 31xC and CPU 31x, Technical data
	Manual, Edition 08/2004, A5E00105475-05