Cycle and Response Times of the S7-400
9.9 Example: Calculating the Interrupt Reaction Time
S7-400 Automation System, CPU Specifications
9-24 Manual, 10/2006, 6ES7498-8AA04-8BA0
9.9 Example: Calculating the Interrupt Reaction Time
Parts of the Interrupt Response Time
As a reminder: The hardware interrupt response time comprises the following:
● Hardware interrupt response time of the CPU
● Hardware interrupt response time of the signal module.
● 2 x DP cycle time on PROFIBUS-DP
Example: You have an S7-400 consisting of a CPU 416-2 and 4 digital modules in the
central rack. One digital input module is the SM 421; DI 16×UC 24/60 V; with hardware and
diagnostic interrupts. In the parameter assignment of the CPU and the SM, you have only
enabled the hardware interrupt. You do not require time-driven processing, diagnostics and
error handling. You have set an input delay of 0.5 ms for the digital input module. No
activities are necessary at the cycle checkpoint. You have set a cycle load caused by
communication of 20%.
Calculation
The hardware interrupt response time for the example results from the following times:
● Hardware interrupt response time of the CPU 416-2: Approx. 0.23 ms
● Extension by communication according to the equation in the table "Hardware interrupt
and diagnostic interrupt response times; maximum interrupt response time without
communication":
100 µs + 1000 µs x 20 % = 300 µs = 0.3 ms
● Hardware interrupt response time of the SM 421; DI 16xUC 24/60 V:
– Internal interrupt processing time: 0.5 ms
– Input delay: 0.5 ms
● Since the signal modules are plugged into the central rack, the DP cycle time on the
PROFIBUS-DP is not relevant.
The hardware interrupt response time results from the sum of the listed times:
Hardware interrupt response time = 0.23 ms +0.3 ms + 0.5 ms + 0.5 ms = approx. 1.53 ms.
This calculated hardware interrupt response time is the time from a signal being applied
across the digital input to the first instruction in OB 40.
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