# OPC UA write task

> Write a value to an OPC UA node each time a command channel gets a new value.

An OPC UA write task writes a value to a node each time you write a value to a command channel. For example, a valve on a schematic can set a PLC setpoint. If you write several values at once, the task writes only the last one. The task writes single values, not arrays.

The task has no state channels. To see the value on the server, read the node back with an [OPC UA read task](https://docs.synnaxlabs.com/reference/hardware/opc-ua/read-task). Before you start, you need a [connected OPC UA server](https://docs.synnaxlabs.com/reference/hardware/opc-ua/connect-server).

## Create an OPC UA write task

When you create command channels with a client library, give each one its own index channel. A writer that holds several commands on one index must write all of them in every frame.

[Enterprise](https://docs.synnaxlabs.com/reference/get-started/editions "Needs Synnax Enterprise")

**Console**

1. In the devices toolbar, right-click the server and select **Create write task**.
2. In the **Browser**, expand the folders of the server.
3. Drag the variables to write into **Channels**.
4. Click (**Start**).

The Console creates a command channel for each node, named `<node>_cmd`, and its index `<node>_cmd_time`. To choose another name, right-click the channel and select **Rename** before you start.

[](https://synnax.nyc3.cdn.digitaloceanspaces.com/docs/hardware/opc-ua/write-task-dark.mp4#t=0.001)

**Python**

Python does not create channels. Create the command channel and its index first.

```python
import synnax as sy
from synnax import opcua

server = client.devices.retrieve(name="Line 1 PLC")
cmd_time = client.channels.create(
    name="pump_setpoint_cmd_time", is_index=True, data_type=sy.DataType.TIMESTAMP
)
cmd = client.channels.create(
    name="pump_setpoint_cmd", index=cmd_time.key, data_type=sy.DataType.FLOAT64
)

task = opcua.WriteTask(
    name="Line 1 PLC write",
    device=server.key,
    channels=[
        opcua.WriteChannel(cmd_channel=cmd.key, node_id="NS=2;S=Pump1.Setpoint")
    ],
)
client.tasks.configure(task)
task.start()
```

**TypeScript**

TypeScript does not create channels. Create the command channel and its index first.

```typescript
import { opcua } from "@synnaxlabs/client";

const [server] = await client.devices.retrieve({ names: ["Line 1 PLC"] });
const cmdTime = await client.channels.create({
  name: "pump_setpoint_cmd_time",
  isIndex: true,
  dataType: "timestamp",
});
const cmd = await client.channels.create({
  name: "pump_setpoint_cmd",
  index: cmdTime.key,
  dataType: "float64",
});

const task = await client.tasks.create({
  name: "Line 1 PLC write",
  rack: server.rack,
  type: "opc_write",
  config: opcua.writeConfigZ.parse({
    device: server.key,
    channels: [{ cmdChannel: cmd.key, nodeId: "NS=2;S=Pump1.Setpoint" }],
  }),
});
await task.start();
```

The OPC UA write task has one option, **Auto start** (`auto_start`), which starts the task each time the Driver starts. For node IDs, see [node IDs](https://docs.synnaxlabs.com/reference/hardware/opc-ua/read-task#node-ids).

## Channel reference

The table shows the Console label of each setting, with its Python field under it.

| Setting               | Default   | Notes                                                                                     |
| --------------------- | --------- | ----------------------------------------------------------------------------------------- |
| Channel name `name`   | Empty     | The name of the command channel that the Console creates.                                 |
| `cmd_channel`         |           | The key of the command channel.                                                           |
| Node ID `node_id`     |           | See [node IDs](https://docs.synnaxlabs.com/reference/hardware/opc-ua/read-task#node-ids). |
| Node name `node_name` | Empty     | The browse name. For display only.                                                        |
| `data_type`           | `float32` | The data type of the channel that the Console creates.                                    |
| Enabled `disabled`    | Off       | A disabled channel is not written.                                                        |

### Data types

The Driver writes each value with the OPC UA type that matches the data type of the command channel:

| Synnax data type | OPC UA type |
| ---------------- | ----------- |
| `uint8`          | Boolean     |
| `int8`           | SByte       |
| `int16`          | Int16       |
| `uint16`         | UInt16      |
| `int32`          | Int32       |
| `uint32`         | UInt32      |
| `int64`          | Int64       |
| `uint64`         | UInt64      |
| `float32`        | Float       |
| `float64`        | Double      |
| `string`         | String      |
| `timestamp`      | DateTime    |
| `uuid`           | Guid        |

A `uint8` channel writes a Boolean, so the task cannot write to a Byte node.

## Troubleshooting

* [... (NS=...): The value supplied for the attribute is not of the same type as the attribute's value.](https://docs.synnaxlabs.com/reference/hardware/opc-ua/write-task#ns-the-value-supplied-for-the-attribute-is-not-of-the-same-type-)

  * Cause

    The data type of the command channel does not match the type of the node.

  * Fix

    Use the data type from the [table](https://docs.synnaxlabs.com/reference/hardware/opc-ua/write-task#channel-reference). For example, a Double node needs a `float64` channel.

* [... (NS=...): The access level does not allow writing to the Node.](https://docs.synnaxlabs.com/reference/hardware/opc-ua/write-task#ns-the-access-level-does-not-allow-writing-to-the-node)

  * Cause

    The node is read only on the server, or the user of the connection cannot write to it.

  * Fix

    Make the node writable on the server, or connect with a user that can write to it.

* [Invalid NodeId format](https://docs.synnaxlabs.com/reference/hardware/opc-ua/write-task#invalid-nodeid-format)

  * Cause

    The node ID is not in the form `NS=<namespace>;<type>=<identifier>`.

  * Fix

    Write `NS` and the type in uppercase.

* [The server disconnects during a write.](https://docs.synnaxlabs.com/reference/hardware/opc-ua/write-task#the-server-disconnects-during-a-write)

  * Cause

    The Driver connects again once and repeats the write. If the second write fails, the task shows the error.

For other problems, see [Troubleshoot hardware](https://docs.synnaxlabs.com/reference/hardware/troubleshooting).