Fix PCS 7 Message Class Dropdown Not Expanding: Dual-Monitor

Problem Overview

When configuring alarm behavior on Advanced Process Library (APL) function blocks inside the SIMATIC PCS 7 CFC Editor, the engineer is expected to open the Object Properties dialog, switch to the General tab, and click the Messages... button. The resulting message configuration dialog exposes a Message Class drop-down for every message row that is not greyed-out. In affected installations the drop-down never opens: clicking the arrow does nothing, the control appears locked, and the engineer cannot change the class from, for example, Alarm - above to Warning - above or Fault - below.

The symptom is observed on blocks such as Alarm8P, Alarm8, MSG_LOCK, SMC_DIR, and any APL block that derives from the standard message-capable FB family. It is independent of the program logic, the CPU firmware (S7-300 / S7-400 / AS 410 / AS 410E), and the project hierarchy; only the engineering station's display topology matters.

Affected Versions and Software Components

How to Reproduce the Defect

1. Log in to a Windows engineering station on which PCS 7 ES has been installed identically to a working reference station (same DVD, same Setup, same Hotfix bundle).
2. Extend the desktop to a second monitor (DisplayPort, DVI, HDMI, or USB graphics adapter) so the primary device is monitor 1 and the secondary device is monitor 2.
3. Open the SIMATIC Manager → project → CFC Editor and load a chart that contains an APL block such as FB 1869 <ALARM_8P> or FB 49 <SMC_DIR>.
4. Right-click the block header and choose Object Properties... (or double-click the block body).
5. On the General tab, click Messages.... The Message Configuration dialog appears.
6. Click the Message Class drop-down arrow for any non-greyed row. Expected: the list of available classes appears. Actual: nothing happens, the arrow does not react, the control is unresponsive.
7. Drag the entire Message Configuration dialog from monitor 2 to monitor 1 without closing it, then click the same drop-down. The list opens normally.

The defect is therefore tied to the screen on which the parent dialog is rendered, not to the project, the chart, the user, or the block instance.

Root Cause Analysis

The Message Configuration dialog is a Win32 child window owned by the CFC Editor's MDI frame. The drop-down is a ComboBoxEx32 control that uses a drop-down list, not a simple list. When the parent dialog is positioned on a non-primary display, the OS computes the candidate popup rectangle relative to the monitor that owns the top-left corner of the parent. If that rectangle is clipped, fully obscured, or if the parent has the WS_EX_TOPMOST bit cleared in a way that conflicts with the secondary monitor's DWM composition policy, the popup is either:

• placed at coordinates that the OS interprets as off-screen, and therefore never drawn, or
• drawn behind the parent dialog and therefore not visible to the user.

Microsoft documents the underlying behavior in the Win32 ComboBox reference: the popup is created with WS_EX_TOPMOST derived from the parent, and its z-order is decided by SetWindowPos using the parent's monitor handle. On extended desktops the monitor handle of the parent is not always the primary monitor handle, and the secondary monitor's composition state can differ when the dialog is moved there after creation (a classic DWM timing issue when a window is dragged across a monitor boundary without a full WM_MOVE/WM_SIZE cycle).

Siemens has not published a defect entry that explicitly references this Win32 behavior in the public Knowledge Base at the time of writing. The behavior is, however, consistent with a known class of issues in SIMATIC PCS 7 engineering dialogs (ID 109812497), where dialog state depends on which monitor hosts the modal parent. The Configure message classes tab of the WinCC Explorer likewise uses a tree control whose expand arrow can fail on monitor 2 for the same reason.

Immediate Workarounds (No Code Change Required)

Workaround 1 - Keep the dialog on the primary monitor

1. Open the Message Configuration dialog.
2. Drag the title bar to the primary monitor (typically the one labeled 1 in Settings → Display → Identify).
3. Click the Message Class drop-down. The list opens.
4. Make all required class changes while the dialog remains on monitor 1.

Workaround 2 - Maximize the dialog

1. Double-click the title bar of the Message Configuration dialog to maximize it.
2. The drop-down will then open inside the maximized client area, which always fits on a single monitor.

Workaround 3 - Move the parent CFC Editor to monitor 1

1. Drag the CFC Editor's MDI frame from monitor 2 to monitor 1.
2. Re-open the Message Configuration dialog. The drop-down will work as long as the parent is hosted on the primary monitor.

Structural Workarounds (Configuration Change)

Option A - Set the secondary monitor as a duplicate of the primary

1. Press Win+P and choose Duplicate instead of Extend.
2. This defeats the multi-monitor productivity benefit but removes the dialog clipping condition entirely.

Option B - Pin the CFC Editor to the primary monitor

1. Right-click the CFC Editor shortcut → Properties → Shortcut tab → Run: select Normal window.
2. Use a third-party utility such as DisplayFusion or the Windows Shell.Application rule to force the process window to monitor 1.

Option C - Disable per-monitor DPI awareness for the ES shell

1. Open regedit and navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers.
2. Add a string value whose name is the full path of S7PCTE64.exe (or S7PCTE.exe on 32-bit installations) and whose data is ~ DPIUNAWARE.
3. Restart the engineering station.
4. The dialog will then render in system DPI rather than per-monitor DPI; the drop-down stops clipping because the popup rectangle is calculated in screen coordinates independent of the secondary monitor's scaling factor.

Configuration Surfaces That Can Mask the Issue

When the Message Class drop-down is greyed out entirely (rather than just unresponsive), the cause is different. The following table distinguishes the two failure modes.

Verifying the Fix

1. Re-open the same CFC chart and double-click the same APL block.
2. Confirm that Object Properties → General → Messages... opens the dialog on the intended monitor.
3. Click the Message Class drop-down. Verify that the list of classes (e.g. Alarm - high, Alarm - low, Warning - high, Warning - low, Tolerance - high, Tolerance - low, Fault, Process, Operator request, Status, System, Failure, PLC fault) is shown.
4. Select a class, click OK, recompile the chart (Ctrl+B), and download the program to the AS.
5. In WinCC Explorer, open the online test for the same message number and trigger the alarm. Verify the runtime shows the new class color and icon.

Programmatic Alternative: Editing Message Class from the Chart

If the dialog cannot be made to work in the field, the same change can be applied by editing the block instance directly in the CFC chart. Open the block's Inputs/Outputs tab and modify the MSG_CLASS attribute value at the instance level. The value is a 16-bit WORD in the standard PCS 7 class registry. The reference table is:

Editing at the instance level bypasses the Win32 dialog entirely and is therefore unaffected by the multi-monitor clipping bug. The change is committed on the next chart compile and download.

Status Output Diagnostics During Engineering

Although the dialog issue is in the engineering host, the underlying message class infrastructure is governed at runtime by the STATUS output of the S7-300/S7-400 message instructions. The PCS 7 documentation cross-references the standard SIMATIC error codes; an excerpt relevant to message generation is:

The complete S7-300/S7-400 message error reference is published by Siemens at Error messages - S7-300, S7-400 (TIA Portal documentation).

Long-Term Recommendations

• Standardize all PCS 7 ES installations on the same Windows build, same multi-monitor topology, and same primary-monitor designation. Document this in the project's Engineering Guidelines.
• Open a Siemens support request (Webform code 9F, product 6DL5xxx) referencing the CFC Editor drop-down clipping on extended desktops to obtain a definitive defect number and any future hotfix.
• Prefer editing the MSG_CLASS instance attribute over the dialog when scripting bulk changes (e.g. during plant-wide re-classification of warnings to alarms). This avoids the multi-monitor clipping condition and is reproducible in unattended builds.
• Train commissioning engineers to keep the CFC Editor and any of its modal children on the primary monitor until a corrective fix is published.

Frequently Asked Questions