OPC UA read task

Read the values of OPC UA nodes at a fixed rate and record them in channels.

An OPC UA read task reads the values of nodes on a server at a fixed rate and records them in channels. For example, it can record the tank pressures and pump speeds of a PLC 50 times a second. The Driver times each read in software.

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

Node IDs

Each channel of the task reads one node. A node ID has the form NS=<namespace>;<type>=<identifier>:

Write NS and the type in uppercase. The Driver rejects ns=2;s=Tank1.Pressure. The browser in the Console fills the node IDs for you.

Create an OPC UA read task

import synnax as sy
from synnax import opcua

server = client.devices.retrieve(name="Line 1 PLC")
time = client.channels.create(
    name="plc_time", is_index=True, data_type=sy.DataType.TIMESTAMP
)
pressure = client.channels.create(
    name="tank_1_pressure", index=time.key, data_type=sy.DataType.FLOAT64
)

task = opcua.ReadTask(
    name="Line 1 PLC read",
    device=server.key,
    sample_rate=50,
    stream_rate=25,
    channels=[
        opcua.ReadChannel(channel=pressure.key, node_id="NS=2;S=Tank1.Pressure")
    ],
)
client.tasks.configure(task)
task.start()

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

const [server] = await client.devices.retrieve({ names: ["Line 1 PLC"] });
const time = await client.channels.create({
  name: "plc_time",
  isIndex: true,
  dataType: "timestamp",
});
const pressure = await client.channels.create({
  name: "tank_1_pressure",
  index: time.key,
  dataType: "float64",
});

const task = await client.tasks.create({
  name: "Line 1 PLC read",
  rack: server.rack,
  type: "opc_read",
  config: opcua.readConfigZ.parse({
    device: server.key,
    sampleRate: 50,
    streamRate: 25,
    channels: [{ channel: pressure.key, nodeId: "NS=2;S=Tank1.Pressure" }],
  }),
});
await task.start();

Read arrays

Some servers buffer fast samples in an array node. To read the arrays:

1. Turn on Array sampling.
2. Set Array size to the number of values in each array.

The Driver then reads each node Sample rate / Array size times a second, and spreads the timestamps of the values evenly over the read period. Sample rate must be equal to or more than Array size. Each array must have exactly Array size values, or the Driver drops the frame and shows a warning. Stream rate does not apply in array mode.

Use server timestamps

By default, the Driver gives the values of each read the time halfway through the read request. To use the time from the server instead:

1. Add a node with a DateTime value, such as the source time of the PLC, to the task.
2. Turn on Use as index for that channel.

The Driver then uses the value of that node as the timestamp of every channel in the task. Only one channel can be the index.

Task options

TypeScript uses the same fields in camel case, such as sampleRate. See timing.

Channel reference

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

Data types

The Console creates each channel with the data type of its node:

Troubleshooting

• Invalid NodeId format

  Cause

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

  Fix

  Write NS and the type in uppercase.

• ... (NS=...): 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.

• OPC UA array for ... is too small... is too large

  Cause

  An array node has a different number of values than Array size.

  Fix

  Set Array size to the length of the array.

• Skipped invalid data

  Cause

  The server sent a value that the Driver cannot convert. The Driver skips the frame.

  Fix

  Check the data type of the node.

• Channel ... already exists as ...

  Cause

  Another task already reads the node into that channel.

  Fix

  Move the channels to one task.

• Failed to browse OPC UA nodes

  Cause

  The Driver could not browse the server.

  Fix

  Check the server connection.

For other problems, see Troubleshoot hardware.