Example Of Calculation Of The Interrupt Response Time - Siemens SIMATIC S7-400H System Manual

19.9

Example of calculation of the interrupt response time

Elements of the interrupt response time
As a reminder: The process interrupt response time is made up of:
● The process interrupt response time of the CPU
● The process interrupt response time of the signal module
● Twice the DP cycle time on PROFIBUS DP
Example: You have installed a 417-5H CPU and four digital modules in the central controller.
One digital input module is the SM 421; DI 16×UC 24/60 V; with process and diagnostic
interrupts. You have enabled only the process interrupt in your CPU and SM
parameterization. You decided not to use time-driven processing, diagnostics or error
handling. You have parameterized an input delay of 0.5 ms for the digital input modules. No
activities are required at the scan cycle checkpoint. You have set a cycle load of 20% due to
communication.
Calculation
In this example, the process interrupt response time is based on following time factors:
● Process interrupt response time of CPU 417-5H: Approx. 0.3 ms (mean value in
● Extension due to communication according to the description in section Interrupt
100 µs + 1000 µs × 20% = 300 µs = 0.3 ms
● Process interrupt response time of SM 421; DI 16×UC 24/60 V:
● The DP cycle time on the PROFIBUS DP is irrelevant, because the signal modules are
The process interrupt response time is equivalent to the sum of the listed time factors:
Process interrupt response time = 0.3 ms + 0.3 ms + 0.5 ms + 0.5 ms = approx. 1.6 ms.
This calculated process interrupt response time is the time between detection of a signal at
the digital input and the call of the first instruction in OB 4x.
S7-400H
System Manual, 03/2012, A5E00267695-11
redundant system mode)
response time (Page 379):
– Internal interrupt processing time: 0.5 ms
– Input delay: 0.5 ms
installed in the central unit.
19.9 Example of calculation of the interrupt response time
S7-400 cycle and response times
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