---
title: Use the Poll-Once Topic to Manually Retrieve Register Values
slug: manufacturing-connect-edge/use-the-poll-once-topic-to-manually-retrieve-register-values
docTags: 
createdAt: 2023-09-27T15:19:35.851Z
---

:::hint{type="info"}
**Notes**:

- You need to follow the following guidelines for topic names: [Subject-Based Messaging](https://docs.nats.io/nats-concepts/subjects). 
- 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: **

1. Follow the steps to [Connect a Device](docId\:nm1lQfefyA-DSIfFitIty). For this user scenario, a Simulator Generator device is used. 
2. Follow the steps to [Add a Tag](docId\:h5heqIcXrCY3NCH9KBg9I) 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
3. Click the **Copy **icon for the tag's RAW Topic. You will refer to this later. 
   ![](https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/s5T1XTlGkhdDd4prbnkM1_copy-poll-once-tagmce.png "Copy RAW Topic icon")
4. Click the **Action menu** for the tag and select **View Poll Topic**. 
   The *Poll Once dialog* box displays. 
   ::Image[]{src="https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/Ko-UTuj4v4A4E19dTb_0e_view-poll-topicmce.png" size="80" width="1592" height="669" caption="The View Poll Topic option" position="center" showCaption="true"}
5. Click the **Copy **icon for the tag. You will refer to this later. 
   ::Image[]{src="https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/Ccnt5LEZFsec1_ZI5ITSS_poll-once-icon.png" size="80" width="786" height="316" position="center" showCaption="false"}
6. Navigate to **Flows Manager**. 
7. For a Flows Manager, click the** Go To Flow Definition** icon. 
   The Flow canvas opens in a new browser tab.
   ::Image[]{src="https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/SK07rlGyJmpNYWUSXPwIX_go-to-flow-definitionmce.png" size="80" width="937" height="548" caption="Go To Flow Definition icon" position="center" showCaption="true"}
8. On the Flow canvas, drag the **inject **node and **DataHub Publish** node onto the canvas. Then, connect the two nodes. 
   ::Image[]{src="https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/DezwjDs5jwEmTUqko9Bqq_inject-and-publish-nodes.png" size="80" width="862" height="555" caption="The inject and DataHub Publish nodes" position="center" showCaption="true"}
9. 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\:VeeOD3N3SLCAsdLEedZBr) to learn more. 
   - Click **Done**. 
10. Drag the **DataHub Subscribe** node and **debug **node onto the canvas. 
    ::Image[]{src="https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/E9uIt7zH_PdKJpsS1HOYl_subscribe-and-debug-nodes.png" size="80" width="801" height="462" caption="The DataHub Subscribe and debug nodes" position="center" showCaption="true"}
11. 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**. 
12. Expand the message window beneath the flow. 
    ::Image[]{src="https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/Zh___1Jnu2LMvcPHcAmSQ_poll-once-message-bar.png" size="80" width="1713" height="892" caption="The message window" position="center" showCaption="true"}
13. Click the **Debug **icon to view messages from the debug node.
    ::Image[]{src="https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/tnKgBHcMjkJX1PYETh0Yt_poll-once-debug-icon.png" size="80" width="1712" height="625" caption="The debug icon" position="center" showCaption="true"}
14. Click the **Inject **button. This triggers the timestamp message, which will then trigger the poll once tag to be activated. 
    ::Image[]{src="https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/DNQ7i4TPmQUvsbGOiS2O4_poll-once-inject-button.png" size="80" width="820" height="500" caption="The nject button" position="center" showCaption="true"}
15. Confirm the **Debug **node is activated.  
    ::Image[]{src="https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/KCp1K7jEKyfaQioiTaG-x_poll-once-debug-button.png" size="80" width="801" height="462" caption="The Activate Debug button" position="center" showCaption="true"}
16. 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.       
    ::Image[]{src="https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/cO5Ut4NUShhaFKWyEG-uZ_poll-once-success-message.png" size="80" width="1920" height="937" position="center" showCaption="false"}

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.