The CTI driver provides support for unsolicited messages, allowing a trigger's execution to be initiated from the ladder logic in a CTI 2500 series programmable controller. A trigger can be defined with an event type of CTI 2572 Message. This means that the trigger will execute when an unsolicited message is received from the ladder logic in a CTI controller. The use of unsolicited messages can take the place of having a trigger constantly polling a PLC variable to see if a data condition has been met.
The unsolicited messages will include data that is sent to the node. This can be any data buffer available within the CTI controller. The trigger associated with the specific unsolicited message will have access to this data and will be able to utilize it as an input to any action defined within the trigger. For example, a trigger could use the data sent by the unsolicited message to write to a variable in a PLC or other device. The trigger could also use the unsolicited message data as input to a transaction that will update a table in an enterprise database. For more information on trigger event types and actions, see Trigger event type reference and Trigger actions reference.
The CTI controller that sent the unsolicited messages will expect the trigger to supply it with a return code. The return code value must be negotiated between the person responsible for the controller's ladder logic and the person writing the trigger so that the ladder program will know if the message sent was successfully processed.
The use of unsolicited messages is an excellent strategy to reduce the processing load on both the node and the PLC. Rather than constantly polling a PLC and reading a data point to determine if a condition has been met to execute a trigger, the PLC can indicate when the condition has been met and send the unsolicited message. Likewise, the PLC no longer has to constantly field read requests from the node, more often than not on data values that are not changing. The PLC ladder logic can be modified to initiate the execution of a trigger based on criteria defined in the ladder, by sending the unsolicited message to the node. If the criteria needs to be modified, the changes are isolated to the PLC and the trigger logic is not impacted.