Siemens SIMATIC S7-400H, Системное руководство

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S7-400 cycle and response times
19.7 Examples of calculating the cycle and response times
S7-400H
System Manual, 03/2012, A5E00267695-11
377
Example II
You have installed an S7-400 with the following modules:
●
a CPU 414–5H in redundant mode
● 4 digital input modules SM 421; DI 32×DC 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 8x13 bit (not in the PI)
● 2 analog output modules SM 432; AO 8x13 bit (not in the PI)
CPU parameters
The CPU was parameterized as follows:
●
Cycle load due to communication: 40 %
User program
According to the instruction list, the user program runtime is 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:
approx. 12.0 ms
● Process image transfer time (4 x double-word access and 3 x word access)
Process image: 9 µs + 7 ×25 µs = approx. 0.184 ms
● Operating system runtime at scan cycle checkpoint:
approx. 0.31 ms
The total of the listed times is equivalent to the cycle time:
Cycle time = 12.0 ms + 0.184 ms + 0.31 ms = 12.494 ms.
Calculation of the actual cycle time
● Allowance for communication load:
12.494 ms * 100 / (100–40) = 20.823 ms.
● A time-of-day interrupt with a runtime of 0.5 ms is triggered every 100 ms.
The interrupt can be triggered a maximum of one time during a cycle:
0.5 ms + 0.490 ms (from table 16-9) = 0.99 ms.
Allowing for communication load:
0.99 ms * 100 / (100–40) = 1.65 ms.
● 20.823 ms + 1.65 ms = 22.473 ms.
Taking into account the time slices, the actual rounded up cycle time is 22.5 ms.