Cycle and response times
7.6 Sample calculations
User program
According to the Instruction List, the user program has a runtime of 10.0 ms.
Cycle time calculation
The cycle time for the example results from the following times:
• User program execution time:
approx. 10 ms x CPU-specific factor 1.10 = approx. 11.0 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 + 6 bytes x 37 μs = approx. 0.322 ms
• Operating system runtime at scan cycle checkpoint:
approx. 0.5 ms
The cycle time for the example results from the sum of the times listed:
Cycle time = 11.0 ms +0.396 ms +0.322 ms + 0.5 ms = 12.218 ms.
Calculation of the actual cycle time
Allowance for communication load:
12.218 ms * 100 / (100-40) = 20.36 ms.
Thus, under consideration of time slots, the
actual cycle time is approx. 20 ms.
Calculation of the longest response time
• Longest response time = 20 ms * 2 = 40 ms.
• Delays in the inputs and outputs
– The maximum input delay of the digital input module SM 321; DI 32 x 24 VDC is
4.8
ms
per channel.
– The output delay of the digital output module SM 322; DO 16 x 24 VDC / 0.5 A
can be
neglected
.
– The analog input module SM 331; AI 8 x 12 bit was configured for an interference
suppression at 50 Hz. This results in a conversion time of 22 ms per channel. Since 8
channels are active, a cycle time of
176 ms results for the analog input module.
– The analog output module SM 332; AO 4 x 12-bit was programmed for the measuring
range of 0 ...10 Hz. This gives a conversion time of 0.8 ms per channel. Since 4
channels are active, a cycle time of 3.2 ms results. The settling time for the resistive
load of 0.1 ms must still be added. The result is a response time of
3.3 ms for an
analog output.
• All the components are plugged into the central rack; DP cycle times do not therefore
have to be taken into account.
S7-300 CPU Data: CPU 315T-2 DP
Manual, 12/2005, A5E00427933-02
7-19
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