Unsolicited mode can be enabled in the relay by the UR mode parameter. The operating principle is that the server sends event data spontaneously to the client. When the unsolicited mode has been activated by the setting parameter, the client station must activate UR reporting by Enable unsolicited Application function (20). After activation the server begins sending Class 1/2/3 events spontaneously.
Client must acknowledge unsolicited events reported by the server to ensure the communication is fully operative. If the server does not receive acknowledgement in time, defined by the UR TO and App confirm TO parameters, it does a count of resends defined by the parameter UR retries. If the client station does not respond acknowledgement during the count of retries, the server goes to offline mode. This means that spontaneous events are not sent for a period of time. The length of this period is defined by the parameter UR offline interval, (default 15 minutes). If it is not wanted that the server goes to offline mode, limitless retries can be achieved by setting the UR retries to “65535”.
Class event reporting can be buffered in the unsolicited mode by setting the parameters UR Class x Min event and UR Class x TO. The event parameter UR Class x Min defines how many events must be buffered in the device before unsolicited report is sent out. The UR Class x TO parameter in turn defines the minimum waiting time the device buffers events before sending them out, after occurrence of an event. These parameters are useful for controlling the flow of events from the device and combining them into bigger reports.
Legacy master UR provides compatibility to some older DNP3 clients. When set to disabled, the server follows the DNP3 standard, sending its first unsolicited message after a connection has been established following relay reboot. The client is expected to send the Enable/Disable Unsolicited messages command to the relay. When Legacy master UR is enabled, the relay does not send the initial unsolicited message. Unsolicited responses are sent without the need of the Enable Unsolicited command. The client still needs to open a connection for the server to start sending unsolicited messages.
Unsolicited reporting mode is not recommended in half-duplex (RS-485 2-wire mode or optical ST mode) serial bus due to possibility of collisions in the serial bus. Especially in a system where event reporting is frequent, collisions can cause failed DNP3 requests/responses and thus retransmissions. If unsolicited mode is used, application level confirmations should be enabled to ensure that transmission is successful. In half-duplex serial bus, polled mode is recommended.