Unsolicited message handling

A PLC typically registers a critical event before the Communication Driver has had a chance to poll for that data. To mitigate the potential information lag, the PLC should have the capability to inform the Communication Driver immediately, without having to wait for the Communication Driver to poll it.

This is the role of an unsolicited message. Once a PLC has determined that a critical condition (such as an alarm event) exists, it can generate a message that is immediately sent to the Communication Driver without a prior request from the Communication Driver. Compared to polling the PLC for an irregular event at fixed intervals, sending unsolicited messages from the PLC to the Communication Driver when the event occurs not only reduces network traffic, but may also get the data to the server more promptly.