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Subscribing to Device Tags
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A device only produces values for tags you subscribe to. A subscription tells the driver which paths to read and gives the tag a display name, a data type and its history, scaling, deadband and OPC-UA settings.
Open the device from Devices. The device page has the tabs Live Data (N), Browse Tags, Manage Tags, Tag Locator and Settings. (A Simulator device's Manage Tags tab is a simulation editor instead; see Built-in Devices.)
Subscribe by browsing
Use this when the driver can list the device's tags (OPC-UA, ControlLogix, MTConnect and others).
- Open the Browse Tags tab. The driver queries the device and shows a tree. Filter tags... narrows it.
- Tick the tags you want. Already-subscribed tags are marked and cannot be ticked again.
- Click Subscribe Selected.
The display name comes from the browsed name and the data type from what the driver reported. If the device can't be browsed you see Unable to Browse Tags and a Retry button; use Add Tag instead.
Subscribe by typing a path
Use this for Modbus registers, Siemens addresses and any device that can't be browsed.
- Open Manage Tags and click Add Tag (tooltip Add a new tag subscription by path).
- Fill in the Add Tag Subscription form (below) and click Subscribe.
| Field | What it does | Default / rules |
|---|---|---|
| Tag Path (required) | The address in the driver's syntax, e.g. Spindle/Speed or HoldingRegister/0. See your protocol's help page for the format. | Required. Not validated: a wrong path subscribes fine and then reads bad quality. |
| Data Type | float, int, bool or string. | float. |
| Display Name | Friendly label shown in lists, pickers and dashboards. | Blank uses the last path segment (Spindle/Speed becomes Speed), or the whole path when that segment is a bare number or two letters (HoldingRegister/0, T4:0/DN). |
If the path is already subscribed on that device, nothing happens and no error is shown.
Note: For most drivers the data type you pick is a label. The value's type comes from what the driver reads. The subscription's type decides how the tag is published on the OPC-UA server (float = Double, int = Int32, bool = Boolean, anything else = String) and filters the Tag Locator. You cannot change the type after subscribing; unsubscribe and add the tag again.
Other ways to create subscriptions
- Tag Locator tab: find tags by behavior and subscribe the matches in bulk. See Tag Locator.
- Tag UDTs: an instance creates one subscription per field. See Tag UDTs.
- API:
POST /api/tags/subscribetakes a whole array of tags in one call (see Tag API Reference). - Configuration bundles: export
tag_subscriptionsfrom one appliance and import them on another. See Configuration API.
There is no CSV import or export of tags on the device page.
The Manage Tags table
The count above the table reads e.g. 12 subscribed tags.
| Column | Meaning |
|---|---|
| Checkbox | Select for bulk unsubscribe. The header box selects every non-system tag. A Unsubscribe N button appears once something is selected. |
| Tag Path | The path. System tags show a system badge. |
| Display Name | Label, or — when blank. |
| Type | The subscription's data type. |
| Poll (ms) | The tag's own polling rate, Device when it follows the device, or the name of its scan class when it has one. |
| Last Value | Live value with its quality dot, or the last saved value. |
| Enabled | on / off. Disabled tags are removed from the driver's poll list. |
| History | Checkbox. Ticked = every change is written to the historian. Takes effect immediately. |
| OPC-UA / paired | R (shared read-only), R/W (shared and writable remotely) or —. Hover for the server name. Shared means OPC-UA clients and paired appliances can read it. |
| Actions | Pencil (Edit), shield (Security levels for writing and editing this tag, admins only) and trash (Unsubscribe). System tags show managed instead. |
Editing a subscription
Click the pencil. The row turns into inline fields; click the save icon to apply or the X to cancel.
| Field | What it does | Rules |
|---|---|---|
| Display Name | Rename the label. | Blank falls back to the last path segment in lists. |
| Poll (ms) | Per-tag polling rate. Device means the tag polls at its device's rate. | Whole number, 100 or more; empty returns the tag to the device's rate. Only sent if you changed it. Used by devices that talk to real equipment and by simulator devices; demo and Local Tags devices ignore it. |
| Scan class (below the row) | A scan class for this tag. | None: Poll (ms), or the device's rate by default. A scan class takes precedence over Poll (ms). Applied to the running poll loop without reconnecting. |
| Change script (below the row) | The Tag Change scripts that watch this tag, each linking to it, and Create change script. | Creating one needs the admin or editor role. |
| Enabled | Stop or resume reading this tag. | Re-sends the device's tag list to the driver. |
| Expose to OPC-UA clients and paired appliances (OPC-UA / paired column) | Publish the tag on the appliance's OPC-UA server and to paired appliances. | Reveals Server name and Writable remotely (OPC-UA and paired appliances). See Writing Tag Values & OPC-UA Exposure. |
The tag path and data type cannot be edited.
Note: A tag's rate comes from, in order: its own scan class, its own Poll (ms), its device's scan class, the device's Polling Rate (ms) (device Settings tab). The Tag Browser's details show the rate a tag is polling at right now under Polling now.
Recording history
Tick the History checkbox in the tag's row in Manage Tags. There is no history option on the add form, the edit row, the Tag Browser or the Local Tags page. For local tags and UDT members on the local device, open Devices → Local Tags → Manage Tags and tick it there.
- A value is recorded each time the tag changes (after scaling and deadband), not on a fixed interval. A tag that holds steady for an hour writes nothing for that hour.
- Numbers are stored as numbers;
true/falseare stored as 1/0 so they chart; other text is stored as text. - Bad-quality readings are recorded too, with an empty value.
- Where history goes (InfluxDB, SQL or both) and how long it is kept is set on the historian. See Historian.
Tip: On the appliance, a noisy analog tag with History ticked and no deadband can write several points a second. Add a deadband (see Scaling & Deadband) before you tick History on analog values.
Unsubscribing
Click the trash icon, the × on a Live Data card, or select several rows and click Unsubscribe N. It happens immediately, with no confirmation. The subscription's history setting, scaling, deadband, OPC-UA exposure, and any alarm rules and tag permissions tied to it are removed with it; data already in the historian stays. Dashboards, scripts and workflows that reference the path are not updated and will show no value.
Deleting a device deletes all its subscriptions.
Live Data tab
One card per subscribed tag: quality dot, display name, path, value with scaling units, and the time of the last change. Badges show SCALED and DB: (deadband). Card icons: deadband settings (only when the Deadbanding feature is licensed), Engineering-unit scaling, and × to unsubscribe. With nothing subscribed it shows No tags subscribed yet. and a Browse Tags button.
The header Simulate button (admins only) replaces the device's values with random numbers from 0 to 100 for every enabled, non-system subscription so you can test screens without the machine. Stop ends it.