Examples Of Calculating The Cycle And Reaction Times - Siemens SIMATIC S7-400H System Manual

S7-400 cycle and reaction times

16.7 Examples of calculating the cycle and reaction times

16.7 Examples of calculating the cycle and reaction times
Example I
You have installed an S7-400 with the following modules in the central unit
● a 414-4H CPU 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 operation list, your user program has a runtime of 15 ms.
Calculating the cycle time
The 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)
● OS execution time at the cycle checkpoint:
The cycle time is obtained from the sum of the listed times:
Cycle time = 18.0 ms + 0.112 ms + 0.609 ms = 18.721 ms
Calculating the actual cycle time
● Allowance for communication load (default value: 20 %):
● There is no interrupt handling.
So the actual cycle time rounded up is 23.5 ms
Calculating the longest reaction time
● Longest reaction time
● The delay of the inputs and outputs is negligible.
● All the components are installed in the central rack, so DP cycle times can be ignored.
● There is no interrupt handling.
So the longest reaction time rounded up is = 47 ms
278
approx. 18.0 ms
Process image: 9 µs + 4 ×25.7 µs = approx. 0.112 ms
approx. 0.609 ms
18.721 ms * 100 / (100-20) = 23.401 ms.
23.5 ms * 2 = 47.0 ms.
System Manual, 09/2007, A5E00267695-03
S7-400H