Rockwell unsolicited message support

The Rockwell driver provides support for Rockwell unsolicited messages, allowing a trigger's execution to be initiated from the ladder logic in a Rockwell PLC. A trigger is defined with an event type of:

  • Rockwell (CIP) or
  • Rockwell (PLC5/SLC).

This means that the trigger will execute when an unsolicited message is received from the ladder logic in a Rockwell PLC. 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 Rockwell driver supports two Logix PLC based unsolicited message types, the CIP Generic message type and the CIP Data Table Write message type. In addition, there are five different Service Types within the CIP Generic message type, namely Set Attribute Single, Get Attribute Single, Custom, DF1 PLC5 Typed Write, and DF1 PLC2 Unprotected Write. The Rockwell driver also supports the PLC5 Typed Write and the PLC2 Unprotected Write unsolicited message types that are sent from PLC-5 and SLC 500 PLCs.

Some unsolicited messages will send additional data to the node. This can be any data buffer available within a Logix, a PLC-5, or a SLC 500 PLC. 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 on 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 triggers event types and actions, see Trigger event type reference and Trigger actions reference.

Some unsolicited messages will expect the trigger to supply them with data. The Logix PLC will use the data supplied as values to be written to its controller tags. The data sent back to the Logix PLC can come from an enterprise application, other devices defined within the node, or any other data generated by the trigger.

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, reading a data point to determine if a condition has been met to execute a trigger, the PLC indicates 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 is 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.

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