Resolving WinCC Unified OpenFaceplateInPopup Parse JSON Failed

Problem Summary

The WinCC Unified Runtime Trace Viewer records the following diagnostic line every time a pop-up faceplate is opened from inside another faceplate:

session: 0010, trace: Parse JSON failed
90570 | ... | SIM_24220041_NEO_V106 | GfxRTS(22) | UAWebRHBase | GfxJSONFilter | Error | 2025.02.26 09:59:13.911970 | 50836 (0x9cf4) | session: 0010, trace: Parse JSON failed

Despite the trace entry, the HMI behaviour appears to be correct: the pop-up window opens with the expected faceplate instance, the user can interact with it, and closing the pop-up returns control to the parent screen. The error is a diagnostic message produced by the runtime JSON-serialization filter (component GfxJSONFilter) when it fails to build the payload that is forwarded to the faceplate proxy on the Unified Collaboration / Runtime back end.

The typical customer report pattern looks like this:

• Engineer drags a Popup Screen window element into a screen and wires a button's Click event to a script that calls Faceplate.OpenFaceplateInPopup(...).
• Trace Viewer shows red Error entries on every pop-up invocation.
• Functional test passes; the engineer's first instinct is to ignore the trace.
• The same code path silently breaks inheritance of interface data in nested faceplate types, so the underlying root cause must be understood and fixed.

Affected Versions and Environment

Root Cause Analysis

The OpenFaceplateInPopup method exists in two scopes with two different signatures. The runtime serializer (GfxJSONFilter) builds a JSON payload that carries the pop-up parameters from the script context to the faceplate container. If the calling code does not supply all of the parameters that the serializer is hard-wired to emit, the resulting JSON object becomes malformed and parsing fails when the proxy on the receiving side attempts to deserialize the message.

Scope 1 — UI scope (document level)

Available globally on the screen, this is the method documented as UI.OpenFaceplateInPopup() in the WinCC Unified JavaScript reference. The signature in V20 is:

UI.OpenFaceplateInPopup(
  screenName     : String,
  parameters?    : Array<Variant>,
  modal?         : Boolean,
  position?      : Point,
  closeOnLostFocus? : Boolean
) : Screen

All parameters are documented as optional. When the call site is the UI scope, the JSON filter constructs a default parameter object from the screen configuration and the trace entry is not generated. The official reference is in the TIA Portal V20 cloud docs: UI.OpenFaceplateInPopup() (RT Unified) — TIA Portal V20.

Scope 2 — FaceplateType scope (the affected call)

When a script is written inside a faceplate type — for example in a button on the faceplate's root container — the Faceplate object exposes its own OpenFaceplateInPopup helper. The effective signature is:

Faceplate.OpenFaceplateInPopup(
  faceplateType : String,   // type name of the pop-up faceplate
  title?        : String,   // window title
  modal?        : Boolean,  // true = blocks input on the parent screen
  center?       : Boolean   // true = centres the pop-up on the screen
) : Screen

Although the documentation labels the last three arguments as optional, omitting them yields a parameter list that the JSON filter does not know how to serialize. The filter is hard-coded to emit four well-typed fields; it receives only one or two and writes a partial object into the wire payload. The receiver tries to parse the truncated message, the parser throws, and the runtime logs Parse JSON failed. The pop-up appears to open because the visual component is constructed before the JSON filter is invoked; the trace entry is logged on the property-inheritance roundtrip that follows.

Why interface data disappears

The FaceplateType scope also differs in how interface data is propagated. The parent faceplate's Properties (UDT, UDT_HMI, Index, SecurityNo, etc.) are inherited by the opened faceplate. If the JSON filter fails, the inheritance is broken for the missing keys. Code that relies on those keys reading the correct value will silently fall back to defaults. The same applies to Faceplate.Properties.UDT, Faceplate.Properties.UDT_HMI, and Faceplate.Properties.Index that are typically passed in via a let data = {...} block when the call site is the UI scope.

Solution

Always pass the full argument list when calling Faceplate.OpenFaceplateInPopup from within a faceplate type. The minimal corrected call is:

// INCORRECT — throws "Parse JSON failed" in Trace Viewer
Faceplate.OpenFaceplateInPopup("FP_pop_up_empty_V_0_0_2", title);

// CORRECT — all four arguments supplied, trace stays clean
Faceplate.OpenFaceplateInPopup("FP_pop_up_empty_V_0_0_2", title, true, false);

Argument semantics for the fix:

Pattern for passing interface data to the child faceplate

Use the inherited Faceplate.Properties object so the child faceplate receives the same data the parent has — without re-typing it. The recommended pattern is:

// In the parent faceplate, e.g. on a button's Click event
let sTitle = "Detail — Item " + Faceplate.Properties.Index;
Faceplate.OpenFaceplateInPopup(
  "FP_PopUp_Empty_V_0_0_2",
  sTitle,
  true,   // modal
  false   // centered manually via screen coordinates
);

The child faceplate sees the UDT, UDT_HMI, Index, and SecurityNo inherited from the parent automatically. Do not attempt to pass an additional data object in the call — the FaceplateType signature does not support one, and doing so will trigger the same Parse JSON failed error.

Pattern when you need to forward a custom data set

If the application needs to forward non-interface data (for example, a temporary value computed in script), use a tag on the HMI as a side channel rather than a fifth argument:

Tags("PopupContext") = Faceplate.Properties.Index;
Faceplate.OpenFaceplateInPopup("FP_PopUp_Empty_V_0_0_2", title, true, false);

The child faceplate reads Tags("PopupContext") on its OnLoaded event. This pattern keeps the JSON payload within the four documented fields and avoids triggering the filter error.

Verification

1. Compile and download the project to the Unified Runtime (Panel or PC).
2. Open WinCC Unified Trace Viewer (Start > Siemens Automation > WinCC Unified > Trace Viewer, or Start > Run > ...<RT path>\Bin\RTILtraceViewer.exe).
3. Filter on the component GfxJSONFilter and severity Error.
4. Trigger the pop-up by clicking the configured button. The viewer must not produce a Parse JSON failed entry on this component.
5. Open the pop-up child faceplate and confirm on the OnLoaded event that this.Properties.UDT, this.Properties.Index, and this.Properties.SecurityNo are populated as expected.
6. Exit the pop-up, return to the parent, repeat the action 10 times to make sure the trace is clean on every invocation.

Method Signatures Compared

Underlying JSON Filter Mechanics

WinCC Unified uses an OPC UA Pub/Sub-inspired message format between the rendering engine and the property container. Each faceplate container publishes a typed schema; the GfxJSONFilter component is responsible for translating the JavaScript call into a message that matches that schema. When arguments are undefined, the serializer has two options:

• Inject a schema-valid default — the option the UI scope uses, because the screen object is always available to provide defaults.
• Emit undefined literally — the option the FaceplateType scope currently takes, because the faceplate container is not a child of the script's this.

Option two produces JSON of the form {"a":"FP_pop_up_empty_V_0_0_2","b":"title","c":,"d":}, which the receiving parser rejects. This is the same kind of error surfaced by JSON.parse in browser JavaScript when it receives malformed text. The Mozilla Developer Network documents the equivalent error in SyntaxError: JSON.parse: bad parsing — MDN; the WinCC runtime uses the same engine to deserialize property updates, so the symptom is the same.

Other Scenarios That Produce "Parse JSON failed"

The error is not unique to the FaceplateType scope. The Trace Viewer filter GfxJSONFilter reports the same string for several distinct issues, all of which should be ruled out before assuming the faceplate-popup cause:

Best Practices for Pop-up Faceplates

1. Always supply all four arguments when calling from a FaceplateType scope. Build a thin wrapper if the call site would otherwise become noisy.
2. Centralise the wrapper in a script module so the contract is enforced across the project:

// MyFaceplateUtils.js
function openDetailPopup(sType, sTitle, bModal, bCenter) {
  Faceplate.OpenFaceplateInPopup(sType, sTitle, bModal, bCenter);
}

3. Prefer the inherited Properties over a side-channel tag whenever the data is already on the parent faceplate's interface.
4. Pre-compute the title before the call so the JSON payload contains a string literal, not a function reference that the filter cannot serialize.
5. Avoid recursive pop-ups: a faceplate inside a pop-up that calls OpenFaceplateInPopup again on its own type produces a stack of modal windows and complicates the JSON filter because the inheritance chain grows. Use a confirmation pattern via a tag instead.
6. Keep modal pop-ups short-lived: a modal window blocks the parent screen from refreshing tags, which can cause connection loss alarms if the user walks away.
7. Test on the target panel before shipping: PC RT masks some errors because the host browser engine is more forgiving of malformed JSON. The Panel runtime is stricter, which is usually where the trace entry shows up first.

Commissioning Checklist

Related Methods to Audit

Several adjacent methods share the same four-argument design and trigger the same filter when arguments are dropped. Audit them at the same time:

• Faceplate.OpenFaceplateInPopup — pop-up window with the four-argument signature described above
• UI.OpenScreenInPopup — pop-up hosting a full screen, accepts the same optional fields
• Screen.OpenScreenInPopup — instance-level variant; defaults are inferred from the parent screen object
• Faceplate.OpenFaceplateInMainWindow — replaces the main window content with a faceplate; default arguments are required for the same reason

All of them route through GfxJSONFilter. Treat the trace entry as a contract violation: the contract is "four typed fields, no undefined values".

FAQ