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. Before you start, you need a connected OPC UA 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.

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()

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.

Channel reference

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

Data types

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

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.

  Cause

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

  Fix

  Use the data type from the table. For example, a Double node needs a float64 channel.

• ... (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

  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.

  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.