The Siemens driver provides support for Siemens unsolicited messages, allowing a trigger's execution to be initiated from the ladder logic in a Siemens PLC. A trigger can be defined with an event type of Siemens (TCP Send), Siemens (AG_SEND/TSEND), or Siemens (BSEND). This means that the trigger will execute when one of these unsolicited message types is received from the ladder logic running in a Siemens 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 polling is replaced with a solution in which the ladder logic running within a PLC detects the data change condition and sends the data to the node.
The Siemens unsolicited message is used to push data to the node. The message block in the ladder is defined to send any data buffer available within the Siemens 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 in a different PLC or 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.
Triggers defined to receive the Siemens (AG_SEND/TSEND) or Siemens (BSEND) messages must be associated with an S7 Listener device defined to use the ISO on TCP (RFC1006) protocol. Using this protocol, the S7 Listener device is receiving messages on port 102. Triggers that utilize the Siemens (TCP Send) message will be associated with S7 Listener devices that are using the TCP protocol. Using this protocol allows the user to specify the port on which messages will arrive. See the Using the Workbench to define a Siemens S7 Listener device page for information on creating a S7 Listener device.
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. The network traffic will be dramatically reduced. The PLC no longer has to constantly field read requests from the node, more often than not on data values that are not changing. This allows the PLC to focus on performing the functions defined in the ladder, rather than fielding data read requests. 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.