Use the Poll-Once Topic to Manually Retrieve Register Values

notes you need to follow the following guidelines for topic names subject based messaging when publishing structured data such as json, you must convert to a byte array (string) first to do this in a flow, you can use the json node you can use the poll once topic to retrieve a register value when an event occurs the poll once topic will manually trigger the polling of register values user scenario you don’t need to have real time monitoring for a plc, and you only need to get register values when a specific event is triggered, thereby reducing unnecessary network traffic you therefore use the poll once topic to retrieve these event triggered register values set up and use the poll once topic to set up and use the poll once topic follow the steps to connect a device docid\ rfvijdxz7dbad8mwbisma for this user scenario, a simulator generator device is used follow the steps to add tags docid\ ioanzd2awqnkuppgee3eh and create one tag with the following parameters name s random value generator value type int64 polling interval 0 tag name enter a name for the tag address 1 click the copy icon for the tag's raw topic you will refer to this later click the action menu for the tag and select view poll topic the poll once dialog box displays click the copy icon for the tag you will refer to this later navigate to flows manager for a flows manager, click the go to flow definition icon the flow canvas opens in a new browser tab on the flow canvas, drag the inject node and datahub publish node onto the canvas then, connect the two nodes double click the datahub publish node the edit datahub publish node dialog box displays in the topic field, paste the poll once topic you copied in step 5 if needed, configure the datahub subscribe connection see the step 3 configure connector nodes section in create a flow docid ol2gwhtg2gkdjlbr6cai to learn more click done drag the datahub subscribe node and debug node onto the canvas double click the datahub subscribe node the edit datahub subscribe node dialog box displays in the topic field, paste the raw topic you copied in step 2 click done , and then deploy expand the message window beneath the flow click the debug icon to view messages from the debug node click the inject button this triggers the timestamp message, which will then trigger the poll once tag to be activated confirm the debug node is activated the debug window displays a "success" \ true message, which means the poll once tag successfully polled the device and displayed the data for the tag the rest of the payload displays the following information for the tag data type the data type configured for the tag you created in step 1 timestamp date and time data for the poll once message register id the register id for the tag generated in manufacturing connect edge value the value generated for the tag when the poll was triggered since the tag was configured with s random value generator , a random value is generated device id the device id for the datahub device generated in manufacturing connect edge tag name the name you configured for the tag device name the name you configured for the device description the description you entered for the tag since no description was entered for the tag in this use case, no value was displayed metadata the meta data configured for the tag since no meta data was configured for the tag in this use case, no value was displayed