Siemens SIMATIC S7-400H, Системное руководство

Страница: 376 / 504

S7-400 cycle and response times
19.7 Examples of calculating the cycle and response times
S7-400H
376
System Manual, 03/2012, A5E00267695-11
19.7 Examples of calculating the cycle and response times
Example I
You have installed an S7-400 with the following modules in the central unit:
●
a CPU 414–5H in redundant mode
● 2 digital input modules SM 421; DI 32xDC 24 V (each with 4 bytes in the PI)
● 2 digital output modules SM 422; DO 32xDC 24 V/0.5 (each with 4 bytes in the PI)
User program
According to the instruction list, the user program runtime is 15 ms.
Calculating the cycle time
The cycle time for the example results from the following times:
●
As the CPU-specific factor is 1.2, the user program execution time is:
approx. 18.0 ms
● Process image transfer time (4 double-word accesses)
Process image: 9 µs + 4 ×25 µs = approx. 0.109 ms
● OS execution time at the scan cycle checkpoint:
approx. 0.31 ms
The total of the listed times is equivalent to the cycle time:
Cycle time = 18.0 ms + 0.109 ms + 0.31 ms = 18.419 ms.
Calculation of the actual cycle time
● Allowance for communication load (default value: 20%):
18.419 ms * 100 / (100–20) = 23.024 ms.
● There is no interrupt processing.
So the actual, cycle time is approx. 23 ms.
Calculating the longest response time
● Longest response time
23.024 ms * 2 = 46.048 ms.
● The delay of 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 processing.
So the longest, rounded up response time is = 46.1 ms.