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({
  where: { name: { eq: "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. When a frame has more commands than the server accepts in one request, the Driver splits the write into smaller requests.

Channel reference

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

Data types

The Driver reads the type of each node when it connects, and converts each command to that type. For example, a float64 channel can write 3 to an Int16 node, or 1 to a Boolean node.

The Driver does not send a command that does not fit the node, such as 300 for a Byte node or 1.5 for an Int16 node. The task shows “Failed to write commands” and writes the other channels.

When the server does not give the type of a node, the Driver writes the value with the OPC UA type of the command channel:

A write task cannot write to a field of a structure node.

Troubleshooting

• ... returned BadTypeMismatch. The value supplied for the attribute is not of the same type as the attribute's value.

  Cause

  The server did not give the type of the node, so the Driver wrote the type of the command channel, and the type does not match the node.

  Fix

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

• ... returned BadNodeIdUnknown. The node id refers to a node that does not exist in the server address space.

  Cause

  The server has no node with that ID.

  Fix

  Drag the node from the Browser again.

• ... the server has no namespace ...

  Cause

  The server has no namespace with the URI in the node ID of the channel. The server can add the namespace later, for example when a PLC loads its program.

  Fix

  Check that the server loaded its program, or drag the node from the Browser again.

• ... returned BadNotWritable. 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 a form that the Driver accepts.

  Fix

  Write the node ID in one of the forms in node IDs.

• Failed to write commands

  Cause

  The task could not write a command. The description names each channel and the reason. The task writes the other channels of the command and keeps running. When the connection is lost, the Driver connects again once and repeats the write, so a value that the server already got can arrive twice. If that fails, the task drops the command. It does not keep commands for later, because a late command could move hardware at the wrong time.

  Fix

  Fix the cause in the description. For a lost connection, check the server connection. The next command connects again.

For other problems, see Troubleshoot hardware.