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OPC transfers

The trigger you select for an OPC transfer determines whether the transferred values are technically compatible. The available triggers and which of them work with OPC Classic and OPC UA are listed in the trigger overview for the OPC client at ; there you’ll also find a signal diagram, dialog table, and timeout handling for each trigger.

Data-Change and Threshold Triggers for Analog Values​

For a single analog value, use the Data-Change trigger or the Threshold trigger. In this case, the trigger data point and the transfer data point are identical, and the transfer begins immediately after the trigger condition is met. If the OPC transfer object contains additional data points to be transferred, it is therefore possible that values may be transferred that do not logically belong together.

Bit and message triggers for OPC telegrams​

If you need to read multiple related data points consistently, use the bit trigger or the message trigger . The OPC Router then waits for another read cycle before reading the data points of the OPC transfer object, so that the PLC has had time to write all values. On the PLC side, however, the trigger should still be written last.

Always use bit triggers with response bits​

Always use the bit trigger with a response bit. The reason: If the trigger data point in the PLC changes twice within a read cycle, the OPC Router does not detect the second change—the corresponding transfer does not occur, and thus the response bit is also omitted. The PLC must respond to this missing response; for details on the flow, see “: Timeout Handling” at .

A handshake bit should only be written by one side: The PLC resets the trigger data point after receiving the response. The OPC Router resets the response data point itself after the time interval you set in the bit trigger under “Reset response bit after.”

Recording message trigger values​

Record the value of the message trigger. This allows you to track whether data records were not transferred or were transferred twice. On the PLC side, the data point for the message trigger should be incremented by 1 whenever new data is received. A detailed example can be found at Message-Trigger .

Many data points with the same structure​

For many data points with the same structure, instead of using many individual triggers, it is worth using the Multi-Datachange Trigger , which monitors an entire OPC UA data point group, or the transfer object OPC Batch Read , which generates many addresses from a base path. Both are available only with OPC UA. The throughput performance of these two methods is measured at Performance Analysis .