Activating HMI Screens by Tag in WinCC Flexible 2008 and TIA Portal V20 Unified

Dynamic screen activation by tag value gives an HMI application a single, scalable navigation entry point instead of dozens of hard-wired buttons. In plants with 50, 100, or several hundred screens, fixed navigation buttons become impractical to engineer and maintain. By binding a screen name to a string tag and routing that name through a script, you keep navigation logic in one place and localize it for translation and authorization.

This reference covers three working implementations:

1. WinCC Flexible 2008 / WinCC VBScript — using HmiRuntime.BaseScreenName assignment to change the base screen.
2. WinCC Flexible ActivateScreen system function — with conditional or Select Case logic.
3. TIA Portal V20 Unified Runtime — screen dynamization via a script trigger bound to a PLC tag.

Each approach trades off compatibility, runtime overhead, and engineering effort. Field engineers retrofitting legacy MP277 10" panels use method 1; new installations on TP1500 / TP2200 Comfort or Unified Comfort Panels use method 3.

Prerequisites

Method 1 — WinCC Flexible 2008: HmiRuntime.BaseScreenName Assignment

This is the canonical solution for legacy MP277 panels. The script runs on a button Press event and reassigns the base screen property to the value of a string tag bound to an IO field.

Tag Configuration

Step-by-Step Procedure

1. Open the project in WinCC flexible 2008.
2. Create the internal tag ScreenTag of type String, length 30.
3. Place an IO field on the "Navigation" screen. Configure: process tag ScreenTag, mode Input / Output, format String, length 30.
4. Place a "Go" button below the IO field.
5. Configure the button event Press → CallScript.
6. Enter the script body:

   ' WinCC flexible 2008 — Activate screen by tag value
   ' Trigger: Button "Go" — Press event
   ' Tag: ScreenTag (String, 30 chars)
   
   Dim sScreenName
   sScreenName = SmartTags("ScreenTag")
   
   ' Validate input — reject empty or oversized values
   If Len(sScreenName) = 0 Then
       ShowSystemAlarm "Screen name is empty. Enter a valid screen name."
       Exit Sub
   End If
   
   If Len(sScreenName) > 30 Then
       ShowSystemAlarm "Screen name exceeds 30 characters."
       Exit Sub
   End If
   
   ' Guard against recursion: skip if already on target screen
   If HmiRuntime.BaseScreenName = sScreenName Then
       Exit Sub
   End If
   
   ' Optional: log navigation event to alarm buffer
   SmartTags("LastScreenTag") = sScreenName
   
   ' Activate the screen — primary method
   HmiRuntime.BaseScreenName = sScreenName
   

7. Compile the project (Project → Compiler → All).
8. Transfer the runtime to the MP277 using Ethernet (RFC 1006) or USB-PPI.
9. Reboot the panel to load the new runtime.

Verification

• Type Screen_Overview in the IO field and press the button. The base screen should swap within ~200 ms.
• Type a non-existent name (e.g., DoesNotExist). The runtime logs an error in the diagnostic buffer and remains on the current screen.
• Inspect the HMI diagnostic buffer via Start → Settings → OP → Diagnostics or remote via ProSave.
• Confirm LastScreenTag tag in the tag simulator reflects the last successful navigation.

Method 2 — WinCC Flexible: ActivateScreen System Function

For projects that cannot use VBScript (older panels, RT-only licenses) use the built-in ActivateScreen system function. Note that ActivateScreen requires a string literal, not a variable — so a direct call like ActivateScreen ScreenChoiceTag, 0 raises a syntax error. The recommended indirection is:

' ActivateScreen with parameter derived from string tag
' Note: ActivateScreen takes a screen NAME literal, not a variable
' So we use indirect assignment via HmiRuntime

Dim sTarget
sTarget = SmartTags("ScreenTag")

' ActivateScreen cannot accept variables directly, so we route through BaseScreenName
HmiRuntime.BaseScreenName = sTarget

If the application genuinely requires the ActivateScreen function (for example, to trigger pop-up screens with field-specific parameters), use a Select Case ladder:

Select Case SmartTags("ScreenTag")
    Case "Overview"        : ActivateScreen "Screen_Overview", 0
    Case "Diagnostics"     : ActivateScreen "Screen_Diagnostics", 0
    Case "Alarms"          : ActivateScreen "Screen_Alarms", 0
    Case "Trends_Temp"     : ActivateScreen "Screen_Trends_Temp", 0
    ' … 64 more cases …
    Case Else              : ShowSystemAlarm "Unknown screen: " & SmartTags("ScreenTag")
End Select

Scale-Out Variant: Textlist Selection

For 68+ screens, replace the IO field with a Textlist. Textlists in WinCC flexible 2008 support tag-driven entry selection at runtime, removing free-text input entirely and providing automatic localization through the runtime language.

Bind the Textlist output to ScreenTag. The user selects a row from a drop-down; pressing the button activates the corresponding screen. The displayed text changes with the HMI runtime language, while the underlying string tag stays constant.

Method 3 — TIA Portal V20 Unified Runtime

The Unified Runtime replaces WinCC flexible's VBScript with a JavaScript-based scripting API. Screen dynamization via tag triggers is configured per-screen in the screen editor. This is the recommended path for all new projects.

Step-by-Step Procedure

1. Open the project in TIA Portal V20.
2. Create a PLC tag PLC_State_Tag (String[30]) or an internal HMI tag with equivalent definition.
3. Open the target screen in the screen editor.
4. Select a UI element (e.g., the screen background or a hidden rectangle).
5. Open the Dynamization column in the properties pane.
6. Add a new dynamization → type Script.
7. Configure the trigger tag: select PLC_State_Tag (or the equivalent HMI tag).
8. Insert the script body:

   // TIA Portal V20 Unified — screen dynamization script
   // Trigger: PLC_State_Tag value change
   // Reference: docs.tia.siemens.cloud/r/en-us/v20/runtime-scripting-rt-unified
   
   let sScreenName = Tags("PLC_State_Tag").Read();
   
   if (sScreenName === undefined || sScreenName.length === 0) {
       HMIRuntime.Trace("Screen activation aborted: empty tag value");
       return;
   }
   
   let targetScreen = sScreenName.trim();
   
   // Guard against recursion
   if (HMIRuntime.Screens.BaseScreen === targetScreen) {
       return;
   }
   
   try {
       Screen.Items(targetScreen);  // existence check
       HMIRuntime.Screens.BaseScreen = targetScreen;
       HMIRuntime.Trace("Activated screen: " + targetScreen);
   } catch (e) {
       HMIRuntime.Trace("Failed to activate screen '" + targetScreen + "': " + e.message);
   }
   

9. Compile the project.
10. Download to the Unified Comfort Panel or start the PC Runtime simulation.

Verification

• Change PLC_State_Tag from the PLC (e.g., via a watch table) and observe the panel.
• Inspect the runtime trace: Start → Runtime → Trace Viewer or via the HMIRuntime.Trace API.
• Confirm no "Screen not found" errors in the diagnostic buffer.
• Verify the Screen.Items(name) call raises an exception (caught by try/catch) when the screen does not exist.

Feature Comparison

Configuration Reference

System Functions — WinCC Flexible 2008

Unified Runtime API — TIA Portal V20

Tag-Event Triggered Scripts

In addition to button triggers, both WinCC flexible and TIA Portal Unified support scripts that fire automatically when a tag value changes. This eliminates the "Go" button entirely — any PLC write to ScreenTag triggers the navigation.

WinCC Flexible 2008 Configuration

1. Right-click the tag ScreenTag → Properties → Events.
2. Select OnChange.
3. Attach a script that performs the same HmiRuntime.BaseScreenName = SmartTags("ScreenTag") assignment.
4. Set the tag acquisition cycle to "On demand" or 100 ms if the change originates from the PLC.
5. Add a debounce check using an internal LastValueTag to skip repeated equal writes.

TIA Portal V20 Unified Configuration

1. Open the screen editor → select an item → Properties → Dynamization.
2. Add a script trigger and select the tag.
3. Reference the official TIA Portal V20 example for syntax.
4. Set the trigger quality filter to "Good" only — ignore stale or uncertain values.

Commissioning Checklist

1. Compile the project with zero errors and zero warnings.
2. Transfer the runtime to the panel and confirm version timestamp.
3. Verify all tags are present: Start → Settings → OP → Tags (WinCC flexible) or Online → Tags (Unified).
4. Manually navigate to each screen via the IO field / Textlist and confirm.
5. Force the PLC tag to invalid values via the watch table and confirm graceful failure.
6. Toggle operator authorization down and confirm the navigation script is blocked.
7. Capture diagnostic buffer or trace output for at least one full navigation cycle.
8. Verify alarm/event logging captures every navigation if 21 CFR Part 11 / GMP audit trail applies.
9. Stress-test: write the tag 10 times per second for 60 seconds and confirm no screen flicker or runtime crash.
10. Sign off the FAT/SAT with the diagnostic buffer snapshot attached.

Troubleshooting Matrix

Performance & Safety Considerations

• Debounce rapid writes. A PLC writing ScreenTag in a 100 ms loop floods the script trigger. Use a 1-second debounce timer or only fire on value change (not every scan).
• Validate input. Free-text IO fields accept any string; a corrupted PLC value can navigate to unintended screens. Always validate against a known screen list before assignment.
• Authorization. Tie the navigation script to an operator authorization level to prevent unauthorized screen access. In WinCC flexible, set the event's "Authorization" property; in Unified, wrap the script in HMIRuntime.Authorization.Check().
• Logging. Use ShowSystemAlarm or HMIRuntime.Trace to log every screen change with timestamp and operator ID for audit trail compliance (FDA 21 CFR Part 11, GMP, ISA-95).
• License boundaries. Each tag used in a script counts toward the PowerTag license. WinCC flexible 2008 panels ship with 128 / 256 / 512 / 2048 PowerTags depending on the license. A project with 68 screens and 3 navigation tags stays well under 128 PowerTags, but adding 200 data tags will push toward the next license tier.
• Migration. Projects migrated from WinCC flexible 2008 to TIA Portal V20 require the VBScript to be re-written in JavaScript. The HmiRuntime.BaseScreenName property does not exist in Unified; use HMIRuntime.Screens.BaseScreen instead.
• Tag quality code handling. In Unified, always check Tags("PLC_State_Tag").Quality before activating — a "Bad" or "Uncertain" quality should not trigger navigation.

Related Documentation

• Siemens FAQ 25011172 — Dynamic Screen Selection in WinCC flexible
• TIA Portal V20 — Triggering a Screen Change with a Tag (Unified)
• Inductive University — Tag Event Scripts Video