Cycle and Response Times of the S7-400
9.7 Examples of Calculating the Cycle Time and Reaction Time
S7-400 Automation System, CPU Specifications
Manual, 10/2006, 6ES7498-8AA04-8BA0
9-19
9.7 Examples of Calculating the Cycle Time and Reaction Time
Example I
You have installed an S7-400 with the following modules in the central rack:
● One CPU 414-2
● 2 digital input modules SM 421; DI 32xDC 24 V (4 bytes each in the PI)
● 2 digital output modules SM 422; DO 32xDC 24 V/0.5A (4 bytes each in the PI)
User Program
According to the Instruction List, your user program has a runtime of 12 ms.
Cycle Time Calculation
The cycle time for the example results from the following times:
● Process image transfer time
Process image: 7 µs + 16 bytes×1.5 µs = approx. 0.042 ms
● Operating system runtime at scan cycle checkpoint:
approx. 0.17 ms
The cycle time for the example results from the sum of the times listed:
Cycle time = 12.00 ms + 0.042 ms + 0.17 ms = 12.21 ms.
Calculation of the Actual Cycle Time
● Allowance of communication load (default value: 20 %):
12.21 ms x 100 / (100-20) = 15.26 ms.
● There is no interrupt handling.
The actual rounded cycle time is therefore 15.3 ms.
Calculation of the Longest Response Time
● Longest response time
15.3 ms * 2 = 30.6 ms.
● The delay in the inputs and outputs is negligible.
● All the components are plugged into the central rack; DP cycle times do not therefore
have to be taken into account.
● There is no interrupt handling.
The longest rounded response time is therefore 31 ms.
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