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Appendix A.20. Multiple requests in a single poll.
To provide level 2 compliance a DNP 3.0 device must support the ability to serve responses to single polls that contain requests for multiple object types. Outstations commonly use this technique to send a single message requesting class 1, 2 and 3 data. From version 1.03j onwards the driver acting as a client can send requests which contain multiple objects.
To have the driver request multiple object types in a single poll it is necessary to associate multiple Map Descriptors for this common purpose. This is done by specifying the 'dnpMultiMsg' parameter in the Map Descriptor. Assign the same positive whole number to all Map Descriptors to be associated. When the driver processes an active Map Descriptor with a non zero dnpMultiMsg parameter value it locates other Map Descriptors with he same dnpMultiMsg parameter value and uses all of these to form the poll.
When configuring the Client side Map Descriptors for this purpose, only one of the associated Map Descriptors may be set to active. The other Map Descriptors must be set as Server/Passive.
In the example below the driver finds the Map Descriptor called Class0. It sees that dnpMultiMsg is non zero and has a value of 1. It searches through the remaining Map Descriptors to find any other with dnpMultiMsg =1.and finds the Map Descriptor called Class1. Now the driver sends a single message that requests class 0 and class 1 info.
Map_Descriptors |
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Map_Descriptor_Name | ,Data_Array_Name | ,Data_Array_Offset | ,Function | ,Node_Name | ,Address | ,Length | ,dnpDataType | ,dnpDataVari | ,dnpAssociate | ,Scan_Interval | ,dnpMultiMsg |
Class0 | ,DA_Class0 | ,0 | ,Rdbc | ,Node_A | ,0 | ,1 | ,60 | ,1 | ,1 | ,2.0s | ,1 |
Class1 | ,DA_Class1 | ,0 | ,Server | ,Node_A | ,0 | ,1 | ,60 | ,2 | ,1 | ,2.0s | ,1 |
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If the driver is configured as a Server then no action is required. Simply ensure that that there is a Server Map Descriptor for each object requested. No special actions are required to configure the Server to respond to requests for multiple object types.
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