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
9-20 Manual, 10/2006, 6ES7498-8AA04-8BA0
Example II
You have installed an S7-400 with the following modules:
● One CPU 414-2
● 4 digital input modules SM 421; DI 32xDC 24 V (4 bytes each in the PI)
● 3 digital output modules SM 422; DO 16xDC 24 V/2A (2 bytes each in the PI)
● 2 analog input modules SM 431; AI 8x13Bit (not in PI)
● 2 analog output modules SM 432; AO 8x13Bit (not in PI)
CPU Parameters
The CPU has been assigned parameters as follows:
● Cycle load due to communications: 40%
User Program
According to the Instruction List, the user program has a runtime of 10.0 ms.
Cycle Time Calculation
The theoretical cycle time for the example results from the following times:
● Process image transfer time
Process image: 7 µs + 22 bytes×1.5 µs = approx. 0.047 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 = 10.0 ms + 0.047 ms + 0.17 ms = 10.22 ms.
Calculation of the Actual Cycle Time
● Allowance of communication load:
10.22 ms x 100 / (100-40) = 17.0 ms.
Every 100 ms, a time-of-day interrupt is triggered with a runtime of 0.5 ms.
The interrupt can be triggered a maximum of once during a cycle:
0.5 ms + 0.24 ms (from table " Increase in cycle time by nesting interrupts") = 0.74 ms.
Allowance for communication load:
0.74 ms x 100 / (100-40) = 1.23 ms.
● 17.0 ms + 1.23 ms = 18.23 ms.
The actual cycle time is therefore 18.23 ms taking into account the time slices.
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