Creating an OPC-UA Tag

After creating a tag on an OPC-UA device, you need to tell Koios which node in the OPC-UA server this tag represents. You can configure these fields during creation or afterwards on the tag's Configuration tab.

Use the node browser instead of typing manually

On the Configuration tab, click Browse to connect to the OPC-UA server and navigate its node tree. When you select a node, all four fields below are auto-populated. No manual entry needed.

Protocol Fields

Namespace

The namespace URI published by the server for the part of its address space the node belongs to. Not an index.

Example (standard OPC-UA namespace): http://opcfoundation.org/UA/
Example (a server's own application namespace): urn:MyPLC:Server or http://examples.freeopcua.github.io

The value must match one of the URIs in the server's namespace array exactly. Koios looks the string up in that array when it reads the tag, so a value the server does not publish fails with "Namespace not found on the OPC-UA server".

The node browser fills this in automatically, which is the safest way to get it right.

A namespace index is not accepted

Entering a number such as 2 creates a tag that never reads. Enter the full URI, or use Browse to select the node.

Identifier

The node's identifier within its namespace. This is the specific address of the data point you want to read or write.

The format depends on the Identifier Type. It could be a number, a string path, a GUID, or a byte string
Example (numeric): 1001
Example (string): Boiler.Temperature.PV

Identifier Type

The format of the node's identifier.

Type	Description	Example
Numeric	An integer node ID, most common for device data points	`1001`
String	A human-readable string path, common in simulation servers and some PLCs	`Boiler.Temperature.PV`
GUID	A globally unique identifier	`72962B91-FA75-4AE6-8D28-B404DC7DAF63`
ByteString	A raw byte string identifier, rare in practice	—

Default: String

Data Type

The expected data type of the node's value. Koios uses this to correctly interpret the raw bytes returned by the server.

Data Type	Description
Boolean	True/false
Int16 / UInt16	16-bit signed/unsigned integer
Int32 / UInt32	32-bit signed/unsigned integer
Int64 / UInt64	64-bit signed/unsigned integer
Float	32-bit floating point
Double	64-bit floating point
String	Text value
DateTime	Timestamp

These are the types you will use for almost every data point. The dropdown on the Configuration tab offers the full set of OPC-UA variant types — including Null, SByte, Byte, GUID, ByteString, XmlElement, NodeId, ExpandedNodeId and StatusCode — for nodes that need them. The create form offers a shorter list still — everything in the table above except Int64 and UInt64, which are only available on the Configuration tab.

Data type must match the server

If the data type doesn't match what the OPC-UA server returns, Koios may misinterpret the value or fail to read it. The node browser auto-detects the correct data type, including the less common ones. Use it when possible.

Using the Node Browser

The OPC-UA node browser connects to the server in real time and lets you navigate the address space visually.

Open the tag's Configuration tab and click Browse
The browser displays the server's node tree as expandable folders
Navigate to the node you want and select it
The Namespace, Identifier, Identifier Type, and Data Type are all populated automatically
Click Apply to save the selection

The device must be reachable to browse

The browser connects directly to the OPC-UA server. The device must be powered on and reachable on the network, but it does not need to be enabled in Koios.

Timestamp Source

By default, Koios records each reading with the time the data collector read the value. Some OPC-UA servers also publish a source timestamp (the time the value was actually produced or sampled), which can be earlier than when Koios reads it. This matters for backdated signals such as lab results that are reported hours after the sample was taken.

Option	Timestamp recorded
Collector Time	When Koios read the value. The default, and the behavior of every existing tag.
Source Timestamp	The value's OPC-UA source timestamp, its true production or sample time.

With Source Timestamp, a value that arrives late but carries an older source time is stored in history at that original time instead of the moment it was read. This keeps late-arriving lab or batch results aligned with the process data they describe. This setting appears only on OPC-UA tags, and if the server does not provide a valid source timestamp for a value, Koios records the collector time for that reading instead.

When a reading on such a tag fails, Koios keeps the time of the last reading the tag took rather than recording the time of the failed attempt. A tag on Source Timestamp therefore stays on the server's clock whether it is reading or failing, and a tag that has stopped reading never appears more recently updated than one that is working. A tag that has not yet read successfully has no earlier time to keep, so its first failure is recorded at the collector time.

Reading backdated values over history

Because a source-timestamped value is stored in the past, anything that reads this tag's history over a short recent window may not see it. For example, a component asking for "the last few minutes." Read history over a window wide enough to cover the longest expected reporting delay. The tag's live value also shows its source time (e.g. "Updated 3 hours ago") even while it is read every scan. This is expected.

Read Status and Quality

Every OPC-UA reading arrives with a status saying how far the server vouches for the value. Koios applies one rule to every OPC-UA tag, with no per-tag setting:

Server status	Tag	What the tag reports
Good	Running	The reading, with the status name as its quality
Uncertain	Running	The reading, plus a warning naming the status and any low or high limit the value has reached
Bad, reporting the data itself bad: a sensor or device failure, a point out of service, a value out of range, lost communication between the server and its data source, or a configuration problem on the server or its data source	Failed	Error code `107`, with the status name, limit, and code in the error detail
Bad, for any other reason, such as a node that does not exist or a read the server does not permit	Failed	Error code `106`, with the status name and code in the error detail
Good or Uncertain, but no value sent	Failed	Error code `105`

A bad status fails the tag even when the server sends a value with it, because a bad status carries no usable value; the error detail quotes the value that was sent. Test on the Configuration tab applies the same rule, so an uncertain reading passes and names its status. See Bad, Missing, or Frozen Tag Values for what each code means and how to fix it.

After Configuration

Save the tag configuration
Test the tag: click
Test
to verify Koios can read a value from this node (see Testing a Tag)
Enable the tag: flip the enable switch to start collecting data

Enable the device first

Tags only collect data when their parent device is also enabled and running.

Creating a Tag Creating a Modbus TCP Tag