Example Of Calculation Of The Interrupt Response Time - Siemens SIMATIC PCS 7 System Manual

Supplementary information
16.15 Cycle and response times of the CPU 410-5H
16.15.8

Example of calculation of the interrupt response time

Elements of the interrupt response time
As a reminder: The hardware interrupt response time is made up of the following:
● Hardware interrupt response time of the CPU
● Hardware interrupt response time of the signal module
● 2 × DP cycle time on PROFIBUS DP
Example
You have installed a CPU 410-5H and four digital modules in the central controller. One
digital input module is the SM 421; DI 16×UC 24/60 V; with hardware and diagnostic
interrupts. You have enabled only the hardware interrupt in your CPU and SM parameter
assignment. You decided not to use time-driven processing, diagnostics or error handling.
You have assigned an input delay of 0.5 ms for the digital input modules. No activities are
required at the scan cycle checkpoint. You have set the communication load of the cycle as
20%.
Calculation
In this example, the hardware interrupt response time is based on following time factors:
● Process interrupt response time of CPU 410-5H: Approx. 0.3 ms (mean value in
redundant operation)
● Extension due to communication according to the description in Chapter Interrupt
response time (Page 304):
100 µs + 1000 µs × 20% = 300 µs = 0.3 ms
● Hardware interrupt response time of SM 421; DI 16×UC 24/60 V:
– Internal interrupt processing time: 0.5 ms
– Input delay: 0.5 ms
● The DP cycle time on the PROFIBUS DP is irrelevant, because the signal modules are
installed in the central controller.
The hardware interrupt response time is equivalent to the sum of the listed time factors:
Hardware interrupt response time = 0.3 ms + 0.3 ms + 0.5 ms + 0.5 ms = approx. 1.6 ms.
This calculated hardware 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.
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CPU 410-5H Process Automation/CPU 410 SMART
System Manual, 09/2014, A5E32631667-AB