Configuring TAG_PREFIX on WinCC OnlineTrendControl via VBS

Configuring TAG_PREFIX on WinCC OnlineTrendControl via VBS

The WinCC OnlineTrendControl (formerly distributed as the ActiveX AxOnlineTrendControl in classic WinCC) does not automatically inherit the TagPrefix property of its parent picture window. Engineers who embed a trend view inside a picture window expecting the standard {prefix} ag substitution — the same behavior that I/O fields, text fields, and bar graphs receive — find that the trend control silently ignores the prefix and shows no data, no error, and no warning in the compiler. This article documents the root cause, the official object-model documentation path, and the VBS workarounds that allow runtime control of trend tag names with a configurable prefix.

1. Overview of the Limitation

Picture windows in TIA Portal (WinCC Comfort, WinCC RT Advanced, WinCC RT Professional, and the corresponding HMI panel families) accept a TagPrefix string property. Tags referenced inside the embedded screen are rewritten at compile/runtime with the prefix prepended, so the same screen can be reused for many different data contexts by changing only the picture-window prefix. According to the TIA Portal V20 VBS object model documentation, this property:

"Specifies the tag prefix that is prefixed for all tags contained in the screen window. In this way, a screen that is embedded in a screen window retains access ..." — TIA Portal V20 VBS Reference — TagPrefix property

The OnlineTrendControl violates this implicit contract. The control's curve TrendTagName field is treated as a literal qualified path: either a fully qualified PLC tag (e.g., "S7-1500S/PLC_1::DB1.HMI_Data.OilTemp") or a tag-logging archive path (e.g., "OIL_LOG\OilTemp"). The picture-window prefix substitution engine does not perform the rewrite for this control. The visible symptom is:

• The trend window opens, axes render, time grid renders, but no curve line is drawn.
• The Configuration > Select Trends dialog shows the curve entry with the unresolved tag name (e.g., {prefix}\OilTemp).
• No compiler error, no runtime error code — the tag simply cannot be resolved against the underlying S7 connection or tag-logging archive.

2. Prerequisites

Before applying any VBS workaround, verify the following:

3. Understanding the WinCC VBS Object Model for Trends

The OnlineTrendControl exposes a hierarchical object model that mirrors the visual structure: a control object (the widget itself) and a collection of trend-curve objects, each with its own data source, color, and visibility state. The two most important access patterns are listed below.

3.1 Control-Level Object

The control is reached through the ScreenItems collection by the name assigned at configuration time (default Control1, but the engineer may rename it). Its top-level properties are read/write from VBS.

3.2 Curve-Level Object via GetTrend()

The GetTrend(Index) method returns a trend-curve object. Indices are 0-based. If a trend control was configured with 3 curves in the configuration dialog, valid indices are 0, 1, and 2 regardless of the curve's position in the configuration list.

The following inline diagram shows the object hierarchy:

The properties that control curve content and visibility are documented in the scripting manual A5E50503017-AA, section 1.14, pages 577–585 of the 11/2020 release.

4. Root Cause: Why TAG_PREFIX is Ignored

The picture-window tag-prefix substitution is implemented in the screen compiler for properties that are registered as "tag-evaluable" — IO fields, text fields, bar graphs, gauges, and similar dynamic widgets. The OnlineTrendControl is registered as an ActiveX / OCX-based control with its own internal storage for curve configuration. The compiler hands the entire curve data structure to the control as a binary blob, and the prefix substitution engine treats it as an opaque payload because the TrendTagName field can legitimately contain non-tag content (archive paths, formula references, server prefixes, etc.).

The same blind spot applies to:

• AxBarChartControl — value column tag source
• AxFunctionTrendControl — f(x) tag source
• AxOnlineTableControl — column tag source
• AxAlarmControl — server name and message filter

The control is unaware that a prefix was expected. The screen compiler will not refuse to download the project; the runtime will not raise an alarm; the trend window will simply draw an empty canvas.

5. Solution 1 — Embed the Prefix Manually in TrendTagName

The cleanest solution is to construct the full tag name at configuration time, including whatever prefix the picture window would have applied. The TrendTagName field accepts both formats described below.

5.1 Tag-Logging Archive Path

For tags stored in a WinCC tag-logging archive, the syntax is "<ArchiveName>\<TagName>". The prefix is inserted before the tag name:

TAGGLOGGING_ARCHIVE\Line1.OilTemp

If the picture-window prefix is Line1, the value above is what you would type directly into the Tag Name column of the Configuration > Trends dialog at design time. The control does not see the prefix — only the resolved path.

5.2 Direct PLC Connection Path

For a live (non-archived) tag, use the connection-qualified path:

S7-1500S/PLC_1::DB1.HMI_Data.OilTemp

This bypasses the archive entirely and reads the live PLC value. Useful for short-term trending on tags that are not logged.

5.3 When the Prefix Varies at Runtime

If the prefix is a runtime variable (for example, the operator selects which production line to view from a dropdown), the configuration-time manual approach fails. Use Solution 2 below.

6. Solution 2 — Dynamic Trend Curve Configuration via VBS

Build a single OnlineTrendControl with the maximum number of curves you will ever display. At runtime, use VBS to assign each curve's TrendTagName from a calculated string that includes the active prefix, and toggle the TrendVisible flag per curve.

6.1 Initialize the Control with N Placeholder Curves

1. Open the trend control's configuration dialog.
2. In the Trends tab, add the maximum number of curves you will ever need (e.g., 8).
3. For each curve, give a descriptive Object name (e.g., Curve_OilTemp, Curve_Pressure). The Object name is the title shown in the legend; the Tag name is left blank or set to a placeholder.
4. Set the curve's Display property to Off so it does not draw before VBS sets it up.

6.2 Attach Tags and Visibility via VBS

The following script demonstrates the canonical pattern: read the active prefix from an HMI tag, loop over the configured curves, and set both the data source and the visibility flag.

' VBScript — Configure trends on Open event of the screen
' Place this on the "Open" event of the screen that contains Control1

Dim objCtrl
Dim objCurve
Dim strPrefix
Dim i

Set objCtrl = ScreenItems.Item("Control1")

' Read the active prefix from an HMI tag; default is empty
strPrefix = SmartTags("ActiveTagPrefix")

' Configure 3 curves — adjust count to match the design-time configuration
For i = 0 To 2
    Set objCurve = objCtrl.GetTrend(i)
    Select Case i
        Case 0
            objCurve.TrendTagName = strPrefix & "\OilTemp"
            objCurve.TrendName    = "Oil Temperature"
            objCurve.TrendColor   = RGB(255, 0, 0)
            objCurve.TrendVisible = True
        Case 1
            objCurve.TrendTagName = strPrefix & "\Pressure"
            objCurve.TrendName    = "Pressure"
            objCurve.TrendColor   = RGB(0, 200, 0)
            objCurve.TrendVisible = True
        Case 2
            objCurve.TrendTagName = strPrefix & "\FlowRate"
            objCurve.TrendName    = "Flow Rate"
            objCurve.TrendColor   = RGB(0, 0, 255)
            objCurve.TrendVisible = False   ' Hidden by default; toggled later
    End Select
Next

6.3 React to Prefix Changes

If the operator changes the active line from the HMI, fire a tag-prefix change event (e.g., on a button click) that re-runs the configuration script. The control rebuilds its internal curve table on each assignment, so the script is idempotent and safe to call repeatedly.

' Sub called from a button's Click event
Sub OnPrefixChanged
    Dim strNewPrefix
    strNewPrefix = SmartTags("ActiveTagPrefix")

    Dim objCtrl
    Set objCtrl = ScreenItems.Item("Control1")

    objCtrl.GetTrend(0).TrendTagName = strNewPrefix & "\OilTemp"
    objCtrl.GetTrend(1).TrendTagName = strNewPrefix & "\Pressure"
    objCtrl.GetTrend(2).TrendTagName = strNewPrefix & "\FlowRate"
End Sub

6.4 Use the Built-In Toolbar for Show/Hide

WinCC OnlineTrendControl exposes a default toolbar (the "Select Trends" icon) that calls a configurable event handler. The OnToolbarButtonClicked event receives the button ID; the application can then show a popup that toggles per-curve visibility:

' Handle toolbar button click — show/hide curve 2
Sub OnToolbarButtonClicked(ByVal item, ByVal buttonId)
    If buttonId = 5 Then  ' Example: assume button 5 is "Select Trends"
        Dim objCurve
        Set objCurve = ScreenItems.Item("Control1").GetTrend(2)
        objCurve.TrendVisible = Not objCurve.TrendVisible
    End If
End Sub

The button-ID mapping is documented in the same scripting manual (A5E50503017-AA), section 1.14.4. Note that some IDs are reserved by the runtime and must not be reassigned.

7. Complete Parameter Reference

The following table lists the most commonly used properties of the OnlineTrendControl and its GetTrend() curve objects. All read/write properties are accessible from VBS; read-only properties are listed for completeness.

8. Common Errors and Field-Recovered Fixes

9. Verification and Commissioning Steps

After applying either solution, perform the following checks before sign-off:

1. Compile and download the project to the target. Watch the TIA Portal output panel for warnings related to the trend control.
2. Open the screen that contains the trend. Verify the trend control renders axes and a time grid.
3. Trigger the VBS script (e.g., the Open event of the screen). Use a temporary HMIRuntime.Trace call to log each TrendTagName assignment to the diagnostic trace buffer.
4. Open the configuration dialog of the trend control at runtime (right-click → Configuration). Confirm the Tag name column shows the resolved full path, not a placeholder.
5. Toggle the curve visibility via the built-in Select Trends toolbar. Each curve should appear/disappear without disturbing the others.
6. Change the prefix at runtime (e.g., select a different production line from a dropdown). The trend should repopulate with the new tag within one polling cycle.
7. Cross-check the tag values in the trend against the live PLC value (use a watch table or an HMI tag display) to confirm the prefix substitution is correct.
8. Disconnect the PLC and confirm the trend shows "No connection" or grayed axes rather than a runtime error. This is the correct behavior for a valid tag with a missing PLC.

10. Best Practices for Production Code

• Centralize the prefix string in a single HMI tag (e.g., ActiveTagPrefix) so the entire application reads the same value. Avoid hard-coding the prefix in multiple places.
• Use a wrapper sub that takes the control name, curve index, and tag suffix as parameters. This eliminates the "Not DRY" (Don't Repeat Yourself) criticism of duplicating the prefix concatenation in every script.
• Validate the tag name before assigning it: check for empty prefix, leading/trailing backslashes, and illegal characters. Assigning a malformed string silently fails — the trend simply shows no data.
• Document the tag-logging archive name in the project notes. Archive names are case-sensitive on Comfort Panels and RT Advanced; a mismatch is one of the most common commissioning delays.
• Test with picture windows if you intend to use the same trend screen inside multiple picture windows. The trend control's data sources are evaluated at the screen level, not the picture-window level.
• Limit curve count to 8 on Comfort Panels — higher counts degrade redraw performance. RT Professional handles more, but 8–12 is a practical ceiling for readability.

11. Differences from Classic WinCC V7

Engineers maintaining mixed fleets (WinCC V7 panels and TIA Portal panels) should note the following differences:

12. Reference: Siemens Official Documentation

• TIA Portal V20 VBS Reference — TagPrefix property for picture windows
• WinCC: Scripting (VBS, ANSI-C, VBA) — System Manual, 11/2020, A5E50503017-AA (Siemens entry-level help portal)
• WinCC V20 Help: Working with trends > Configuring trends via VBS