Siemens Simatic S7-300 CPU 31xC and CPU 31x S7-300 - Manual (Page 109)

Cycle and reaction times
5.6 Sample calculations
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05
5-27
5.6.3 Example of interrupt response time calculation
Installation
You have assembled an S7-300, consisting of one CPU 314C-2 and four digital modules in
the CPU rack. One of the digital input modules is an SM 321; DI 16 x 24 VDC; with
process/diagnostic interrupt function.
You have enabled only the process interrupt in your CPU and SM parameter configuration.
You decided not to use time-controlled processing, diagnostics or error handling. You have
configured a 20% communication load on the cycle.
You have configured a delay of 0.5 ms for the inputs of the DI module.
No activities are required at the scan cycle checkpoint.
Calculation
In this example, the process interrupt response time is based on following time factors:
•
Process interrupt response time of CPU 314C-2: approx. 0,7 ms
• Extension by communication according to the formula:
200 μs + 1000 μs x 20 % = 400 μs = 0.4 ms
• Process interrupt response time of SM 321; DI 16 x 24 VDC:
–
Internal interrupt preparation time: 0.25 ms
– Input delay: 0.5 ms
• Neither PROFIBUS DP, nor PROFINET IO are being used, so you do not have to make
allowances for any DP cycle times on PROFIBUS DP or for PROFINET IO update times.
The process interrupt response time is equivalent to the sum of the listed time factors:
Process interrupt response time = 0.7 ms + 0.4 ms + 0.25 ms + 0.5 ms = approx. 1.85 ms.
This calculated process interrupt response time expires between the time a signal is
received at the digital input and the call of the first instruction in OB40.