---
title: Use the Write Topic to Send Data to a PLC
slug: bhdo-v3/use-the-write-topic-to-send-data-to-a-plc
createdAt: 2023-09-27T17:07:41.846Z
---

:::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. 

# 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\:AVTs_R7RjIdM9g7LSawjm). For this use case, a Simulator Generator device is used. Click the checkbox for **Enable alias topics**. 
2. Follow the steps to [Add a Tag](docId\:CsrurTOACnmmAMyN_uC5o) 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/vAqK-046vdh4I0e1y9cfX/jx1rHgXJfu6oXqZbuAWXp_bhdocopytag.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/vAqK-046vdh4I0e1y9cfX/XRQQW8edJHIit1wUMt3aV_bhdoviewwritetopic.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/vAqK-046vdh4I0e1y9cfX/D_wPRpI-T3QSQbHY4pHD1_bhdocopywritetopic.png "Copy icon for Write topic")
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/DTraaEexmGJNFjyI4mlDh_go-to-flow-definitionbhdo.png" size="80" width="931" height="481" position="center" caption="Go To Flow Definition icon" 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/NVB_Ibwtx1yNe01-rswWG_inject-and-publish-nodes.png)
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 Subscribe connection. See the *Step 3: Configure Connector Nodes* section in [Create a Flow](docId\:CrMXrOqTwtycjCddtoaHD) 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/vAqK-046vdh4I0e1y9cfX/cmBKkfGMlw3271N_Rfjt2_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\:CrMXrOqTwtycjCddtoaHD) to learn more. 
    - Click **Done**. 
12. Click **Deploy**. 
13. Expand the message window beneath the flow. 

    ![](https://api.archbee.com/api/optimize/vAqK-046vdh4I0e1y9cfX/woRka6vsjqXH6vyk3rb_M_flow3.png)
14. Click the **Debug **icon to view messages from the debug node. 

    ![](https://api.archbee.com/api/optimize/vAqK-046vdh4I0e1y9cfX/uONqtt_ITQGccj5-6bc-2_flow4.png "Debug icon")
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/vAqK-046vdh4I0e1y9cfX/IpqI394J_yHe8wJ7CWJeq_flow5.png)