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In accordance with the diagram above, we obtain the following picture:

  • parameter attribute, or the parent controller object, performs the periodic requests HardwarePLC::valueRequest() to get the value of signal or group of signals;
  • received signal values are located locally in the parameter attributes;
  • object of the SCADA-system sends the value request to the parameter attribute DAQParamAttribute::getVal() and gets locally saved value of the previous session of the data source polling.