Cycle and reaction times
5.6 Sample calculations
5.6
Sample calculations
5.6.1
Example 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:
• Process image transfer time
• Operating system runtime at the scan cycle checkpoint:
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
approx. 5 ms x CPU-specific factor 1.10 = approx. 5.5 ms
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
approx. 0.5 ms
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05