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

16.9

Example of calculation of the interrupt reaction time

Elements of the interrupt reaction time
As a reminder: The process interrupt reaction time is made up of:
● the process interrupt reaction time of the CPU and
● the process interrupt reaction time of the signal module
● 2 × DP cycle time on PROFIBUS DP
Example: You have installed a 417-4H CPU and four digital modules in the central unit. One
digital input module is the SM 421; DI 16xUC 24/60 V; with process and diagnostic interrupts.
In the CPU and SM parameters, you have only enabled the process interrupt. You have no
time-driven processing, diagnostics or error handling. For the digital input module you have
configured an input delay of 0.5 ms. No actions at the cycle checkpoint are required. You
have set a communication load of 20 % for the cycle.
Calculation
The process interrupt reaction time for the example is derived from the following times:
● Process interrupt reaction time of CPU 417-4H: Approx. 0.6 ms (mean value in
● Extension due to communication according to the description in Section Interrupt reaction
100 µs + 1000 µs × 20 % = 300 µs = 0.3 ms
● Process interrupt reaction time of SM 421; DI 16 x UC 24/60 V:
● The DP cycle time on the PROFIBUS-DP is irrelevant, because the signal modules are
The process interrupt reaction time is produced from the sum of the listed times:
Hardware interrupt reaction time = 0.6 ms +0.3 ms + 0.5 ms + 0.5 ms = approx. 1.9 ms
This calculated process interrupt reaction 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, 09/2007, A5E00267695-03
redundant mode)
time (Page 281):
– Internal interrupt processing time: 0.5 ms
– Input delay: 0.5 ms
installed in the central unit.
16.9 Example of calculation of the interrupt reaction time
S7-400 cycle and reaction times
283