Siemens SIMATIC S7-400H System Manual page 279

Example II
You have installed an S7-400 with the following modules:
● a 414-4H CPU in redundant mode
● 4 digital input modules SM 421; DI 32xDC 24 V (each with 4 bytes in the PI)
● 3 digital output modules SM 422; DO 16xDC 24 V /2 (each with 2 bytes in the PI)
● 2 analog input modules SM 431; AI 8x3 bit (not in the PI)
● 2 analog output modules SM 432; AO 8x13 bit (not in the PI)
CPU parameters
The CPU has been assigned parameters as follows:
● Cycle load due to communication: 40 %
User program
According to the operation list, your user program has a runtime of 10.0 ms.
Calculating the cycle time
The theoretical cycle time for the example is derived from the following times:
● As the CPU-specific factor is 1.2, the user program execution time is:
● Process image transfer time (4 x double-word access and 3 x word access)
● OS runtime at the cycle checkpoint:
The cycle time is obtained from the sum of the listed times:
Cycle time = 12.0 ms + 0.189 ms + 0.609 ms = 12.789 ms
Calculating the actual cycle time
● Allowance for communication load:
● A time-of-day interrupt with a runtime of 0.5 ms is triggered every 100 ms.
● 21.33 ms + 1.813 ms = 23.143 ms
Taking into account the time slices, the actual cycle time rounded up is 23.2 ms.
S7-400H
System Manual, 09/2007, A5E00267695-03
approx. 12.0 ms
Process image: 9 µs + 7 ×25.7 µs = approx. 0.189 ms
approx. 0.609 ms
12.789 ms * 100 / (100-40) = 21.33 ms.
The interrupt can be triggered a maximum of one time during a cycle:
0.5 ms + 0.588 ms (from table 15-9) = 1.088 ms
Allowing for communication load:
1.088 ms * 100 / (100–40) = 1.813 ms
16.7 Examples of calculating the cycle and reaction times
S7-400 cycle and reaction times
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