Data collection

The DNP3 adapter can collect two different types of data from DNP3 compliant outstations: static data and event data.

Static data

The DNP3 specification defines static data as the current value of a DNP point at the time of a request. The DNP3 adapter can be configured to request static data by polling the outstation for a range of points that share an object group and variation, which are configured in the data selection configuration. Static data is reported with the current time of the adapter machine. When polling for static data at predefined rate, it is possible that quickly changing data can be missed by the adapter. It is also possible that the adapter receives multiple values representing the same event but with different timestamps.

The adapter makes a request for the current value of specific points as configured in the data selection configuration file. The adapter requests this data through a range scan, which requests the static data for a range of point indices that share a group and variation, as configured in the data selection configuration. The outstation should report the current value of each point, but the outstation is not required to report using the requested object variation. If the outstation responds with a different variation than what the adapter has requested, the adapter still sends the data it receives. This could lead to differences in the reported value versus what is expected in terms of precision and status.

Support for requesting static data for DNP3 points through a range scan is at least a Level 2 functionality, and some groups and variations can be considered Level 3 or Level 4 functionality. Because of this, it is important to verify that your outstation supports this functionality before configuring the adapter to collect data in this manner. Some DNP3-L1 compliant outstations can optionally support this type of scan.

Event data

The DNP3 specification defines event data as the information that is retained regarding an event. An event is defined by the DNP3 specification as the occurrence of something significant happening. What constitutes an event can vary depending on the implementation of the outstation. Typically, an event results in a value change for a DNP point, although it is possible for an event to occur that does not change the value of any point on the outstation. Event data is saved at the outstations and should be kept until the adapter confirms that it has received the event object. The event object is the description of the event that the adapter should receive from the outstations. The event object can contain the value, time, and status code relating to the event and corresponding point. The exact information contained in the object is dependent on both the point type and variation defined in the DNP3 specification.

DNP3 events belong to one of three different classes of data: Class 1, Class 2, or Class 3. These event classes can be used to group events by priority, though neither the DNP3 adapter nor the DNP3 specification assign significance to the three event classes. According to the DNP3 specification, all DNP3 - Level 1 compliant outstations accept read requests for event class data.

Event scans

The DNP3 adapter can be configured to request event data through an event scan. During an event scan, the adapter polls the outstation(s) for the event data from each of the configured event classes. The event classes and the polling interval can be configured in the data source configuration. For more information, see Configure data source.

Event scans offer several advantages over polling for static data. When polling for static data, it is possible to miss value changes for points on the outstation; however, event scans ensure that each outstation reports every event to the adapter. Similarly, even when polling quickly, some events could be missed if they do not change the value of the DNP point. If bandwidth is a concern, the adapter can make efficient use of the network by only requesting event data. When polling for static data, the outstation can report unchanging data unnecessarily, whereas event scans should only return new events. It is important that the adapter is configured to perform event scans at an interval that is not long enough to allow the outstation event buffers to become full. To determine what constitutes an event and how much time it takes before the buffer becomes full, refer to the documentation for a specific outstation.

Unsolicited events

In addition to the event scans above, the DNP3 adapter can be configured to receive unsolicited responses containing event data. An unsolicited response is a message sent from an outstation that the adapter did not explicitly request. If the adapter is configured to receive unsolicited data, outstations that support sending unsolicited data should report event data to the adapter as it occurs. This could eliminate the need for the adapter to poll the outstation(s) for data. The decision to configure the adapter to receive unsolicited data or to perform event scans should be carefully considered.

Integrity scans

The DNP3 adapter can be configured to perform an integrity scan on startup, when the outstation's event buffer overflows, at a defined interval, or any combination of the three. An integrity scan is also used during the discovery process.

During an integrity scan, the adapter polls the outstations for events and then the current value of all points that are assigned to one of the event classes (or Class 0). The adapter performs an integrity scan by polling Object Group 60. If bandwidth or outstation performance is a concern, carefully consider the value of an integrity scan, as the outstation can respond with data for many more points than the adapter is configured to collect data for. The adapter simply discards any data that it receives without a corresponding data selection item. To retrieve the current value of any points not assigned to an event class, the adapter needs to perform a static scan.

If a startup integrity scan is configured, it runs whenever the adapter is started or whenever a shutdown is indicated by the outstation's IIN bits. It may also be used for discovery. Alternatively, if no startup integrity scan is configured, but an integrity scan at a set interval is defined, then the first integrity scan is performed when the adapter is started.