Cycle and response times
7.6 Sample calculations
Calculation of the cycle time for the CPU 315T-2 DP
The cycle time for the example results from 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 bytes x 37 μs = approx. 0.396 ms
Process image of outputs: 100 μs +8 bytes x 37 μs = approx. 0.396 ms
• Operating system runtime at scan cycle checkpoint:
approx. 0.5 ms
Cycle time = 5.5 ms +0.396 ms +0.396 ms + 0.5 ms = 6.792 ms.
Calculation of the actual cycle time
• There is no active communication.
• There is no interrupt handling.
Therefore, the
actual cycle time is also 6.792 ms.
Calculation of the longest response time
Longest response time:
6.792 ms x 2 = 13.584 ms.
• The delay in the inputs and outputs is negligible.
• All the components are plugged into rack 0; DP cycle times do not therefore have to be
taken into account.
• There is no interrupt handling.
7.6.2 Calculation example for the response time of the CPU 315T-2 DP
Structure
You have configured an S7300 and equipped it with the following modules on a rack:
• One CPU 315T-2 DP
Configuration of the cycle load as a result of communication: 40 %
• 2 digital input modules SM 321; DI 32 x 24 VDC (4 bytes each in the PI)
• 3 digital output modules SM 322; DO 16 x 24 VDC/0.5 A (2 bytes each in the PI)
• 1 analog input module SM 331; AI 8 x 12-bit (not in the PI)
• 1 analog output module SM 332; AO 4 x 12-bit (not in the PI)
S7-300 CPU Data: CPU 315T-2 DP
7-18 Manual, 12/2005, A5E00427933-02
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