--- title: Use the Write Topic to Send Data to a PLC slug: litmusedge-v2/how-to-guides/devicehub-guides/write-topic-guide description: Learn how to use the Write topic. docTags: createdAt: 2023-09-07T17:25:50.922Z --- :::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. ::: The Write Topic allows you to write data to a PLC device. # Video :::Iframe{iframeHeight="0" code=""} ::: # User Scenario In your production environment one of the sensors that a PLC uses malfunctions, causing the PLC to receive inaccurate data. While waiting for the sensor to be repaired, you input a value manually to simulate the correct sensor data. # Use the Write Topic Complete the following steps to use the write topic based on the user scenario. **To use the Write topic: ** 1. Follow the steps to [Connect a Device](docId\:iSH7BQHZxsWtDx8vbNSZb). For this use case, a Simulator Generator device is used. 2. Follow the steps to [Add Tags](docId\:XgWOkQbTPevII7OR82LL0) and create one tag with the following parameters. - **Name**: M - Memory Register - **Value Type**: int64 - **Polling Interval**: 5 - **Tag Name**: Enter a name for the tag. - **Address**: 1 3. Copy the topic name under the *RAW Topic* column. You will refer to this later. ![](https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/wWKrvxaYnXWk2WotSwg-L_copytag.png "Copy tag icon") 4. Click the** Action menu **for the tag and select **View Write Topic**. The *Write *dialog box displays. ![](https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/iE3BapB7FgBQXOhgXH8b4_viewwritetopic.png "View Write Topic option") 5. Click the **Copy **icon for the tag. You will refer to this later. ![](https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/v6fuUYdfgGvY1AjiX7df9_copywritetopic.png "Copy Write topic icon") 6. Navigate to **Flows Manager**. 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/7V-lMFoDbKs0kJVZDJlgK_go-to-flow-definition.png" size="80" width="1010" height="490" caption="Go To Flow Definition icon" position="center" showCaption="true"} 7. On the Flow canvas, drag the **inject **node and **DataHub Publish** node onto the canvas. Then, connect the two nodes. ![](https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/DezwjDs5jwEmTUqko9Bqq_inject-and-publish-nodes.png "The inject and DataHub Publish nodes") 8. Double-click the **DataHub Publish **node. The *Edit DataHub Publish* node dialog box displays. - In the **Topic **field, paste the Write topic you copied previously (number 5). - If needed, configure the Datahub Publish connection. See the *Step 3: Configure Connector Nodes* section in [Create a Flow](docId\:veYYTE7xdcANgGLzBqya0) to learn more. - Click **Done**. 9. Double-click the **inject **node. The *Edit inject node* dialog box displays. - For **msg.payload**, update the **timestamp **selection and select **string**. - In the **string **field, enter **\{"value":114}**. This will insert the desired value into the tag payload. - Click **Done**. 10. Drag the **DataHub Subscribe** node and **debug **node to the canvas and connect them. ![](https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/Q_SPD2vMbkMTTo5s307ns_flow2.png) 11. Double-click the **DataHub Subscribe** node. The *Edit DataHub Subscribe* node dialog box displays. - In the **Topic **field, paste the topic name you copied previously (number 3). - If needed, configure the Datahub Subscribe connection. See the *Step 3: Configure Connector Nodes* section in [Create a Flow](docId\:veYYTE7xdcANgGLzBqya0) to learn more. - Click **Done**. 12. Click **Deploy**. 13. Expand the message window beneath the flow. ![](https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/CQPkR4GFHYaySoxYE7d0E_flow3.png) 14. Click the **Debug **icon to view messages from the debug node. ![](https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/RNP1NwQzoi08nJXNo02j7_flow4.png) 15. Click the **inject **node button. This will insert the value (114) into the payload of the tag. Confirm you see the value in the debug window. ![](https://api.archbee.com/api/optimize/SSUUxKZUk9bFTEPNn_6Zo/1-GGT23fek8_YAY1APX2K_flow5.png)