Objects declared in a system or device implementing DNP protocol in Slave mode can be grouped in classes. DNP protocol defines the standardized classes described next.

•Class 0: This means all objects, that is, during initialization the Driver on the Master side can perform a Class 0 (zero) request and in the response the Slave side sends the current value of all declared objects

•Classes 1 to 3: These are entities that temporarily store lists of events or changes in objects. Each object must be configured on the Slave side to generate events when there are changes, and usually there is a pattern among DNP users to reserve Class 1 (one) for digital events, Class 2 (two) for analog events, and Class 3 (three) for counters