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Entry type: Application example Entry ID: 21626316, Entry date: 06/02/2005

Influence of the Communication on the Interval Between two Watchdog Interrupts of an S7 CPU.

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Closed-loop controllers are required in many areas of automation. Numerous closed-loop control tasks can be cost effectively implemented in the user program of the S7 CPU. For such purposes the S7 CPU provides a timed interrupt processing mechanism (Timed Interupt OB). The Timed Interupt OB is called continuously at a configurable interval. The precondition for optimal closed-loop control is an exactly equal interval between time interupts. An important question in the design of such configurations is thus:
  • How exactly will the configured interval between two time interupts be kept ?

In addition to its control task, a S7 CPU also has to perform many communication tasks. Very often a S7 CPU will communicate simultaneously with a Programming Device, Operator Panels and with other S7 CPUs over PB or IE subnetworks. Depending on the type of S7 CPU and the extent of the communication load, the interval between two time interupts will vary. This raises more questions:

  • What is the influence of the communication?
  • What will be the variation of the interval between two time interupts ?
  • Are there differences between the S7 CPUs?

The figure below illustrates the task:

We have performed extensive measurements on real configurations with SIMATIC components to answer the questions listed above. Different S7-CPUs are loaded as heavily as possible with different communication tasks:

  • communication load on the integrated DP interface and MPI interface of the S7-CPU
  • communication load on CPs

The results of the measurements are available as two downloads:

  • Calculator: An interactive user interface enables you to quickly find the required performance data. By mouse click you can simultaneously select two different configurations and compare them directly.
  • Documentation: If you are used to working with diagrams, you can refer to the detailed separate documentation. There you will find many informative diagrams and valuable information on the subject of measuring. It also includes a detailed description of the measurement and the simulator.

Overview of the involved SIMATIC components:

Technological component

Solution component

S7-CPU S7-300, S7-400
communication load on integrated interfaces 32 ET200M, up to 15 operator devices and 1 programming device
communication load on CP up to 8 S7-400

Overview of the measured variables:

Measured variable


Interval between watchdog interrupts Interval of two OB35 calls
Cycle time Time between two process image transfers in the S7 CPU.


Contents of the downloads


Calculator (German) 21626316_weckalarm_KALK_v30_d.xls ( 253 KB )
Calculator (English) 21626316_weckalarm_KALK_v30_e.xls ( 255 KB )
Documentation (German) 21626316_weckalarm_DOKU_v30_d.pdf ( 699 KB )
Documentation (Englisch) 21626316_weckalarm_DOKU_v30_e.pdf ( 687 KB )

To install the calculator, proceed as follows:

  • Load the calculator (Excel file) to the PC and open it.
  • Acknowledge the Excel input prompt by clicking "Enable macros".

Please make sure that the security level in Excel under "Tools -> Macro -> Security" is set to "medium" or "low":




06/2004 First edition (V3.0)


CPU314C, CPU315, CPU317, CPU318, CPU416, OB35, interrupt

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