In accordance with the diagram above, we obtain the following behavior of the package mechanism for asynchronous requests:
- parameter attribute, or the parent controller object, performs the periodic requests HardwarePLC::valueRequest() to get the value's packages of a signal or group of signals;
- received value packages of the signals are placed in the archive by the request DAQParamAttributeArch::setValues(), and the last value of the packages is located in the parameter attribute;
- object of SCADA-system sends the request of the archive's fragment to the parameter attribute DAQParamAttribute::getValues(), and he relays the request to the archive DAQParamAttributeArch::getValues(). As the result, the fragment of the archive, available after the previous session of the polling of the data source, is returned;
- object of the SCADA-system sends the request of the last value of the parameter attribute DAQParamAttribute::getVal() and gets the locally saved value of the previous session of the data source polling.