WinCC Log In Dialog Hidden After Runtime Activation: Multi-Monitor Fix

Problem Details

On SIMATIC WinCC V7.x Runtime stations running a dual-monitor configuration, the Log In dialog from the WinCC User Administrator (or any screen that calls the @UserName login form) appears correctly the very first time the project is activated. After the user logs out, or after the runtime process is brought back to the foreground from another application, the Log In dialog no longer displays on screen. The dialog still exists in the Windows task list — Alt+Tab cycles through it, and it is visible in Task Manager as a running PDLRT.exe child window — but it never receives a visible rectangle on the active desktop.

Symptoms reported in the field:

• First activation: Log In dialog displays normally on Monitor 1.
• Subsequent login attempts: dialog is created but rendered off-screen, on a monitor that is no longer active, or at coordinates outside the visible desktop.
• Alt+Tab shows the dialog title (e.g. System Login) but selecting it does not bring it to the front.
• Clicking the WinCC taskbar icon hides the icon highlight instead of restoring the dialog.
• Stopping and re-activating the runtime temporarily restores the dialog — until the next login cycle.

This behavior has been observed most often on:

• WinCC V7.0 SP2 + Update 4 through Update 14
• WinCC V7.3, V7.4 SP1
• Windows 7 Professional SP1 64-bit and Windows 10 LTSC 2019 with multi-monitor (extended desktop) configurations

Root Cause

Two independent mechanisms combine to hide the dialog:

1. OS Project Editor monitor assignment

The OS Project Editor writes the monitor layout into the project registry under HKEY_LOCAL_MACHINE\SOFTWARE\Siemens\WinCC\<ProjectName>\RT\Monitors. Each configured monitor entry stores the resolution, position (X, Y origin), and a flag that pins Runtime windows to that monitor. When the editor is run on a workstation with two displays and then the same project is deployed to a station with only one display (or with the secondary monitor rotated, disabled, or replaced with a different panel), Runtime still attempts to open child dialogs — including the Log In window — at the saved coordinates of the now-missing monitor. The dialog is created successfully but rendered outside the visible work area.

2. Window position caching in the WinCC dialog layer

WinCC Runtime uses the Microsoft GetWindowPlacement / SetWindowPlacement calls to persist the last position of every PDL-window-level dialog. The placement data is written into HKEY_CURRENT_USER\Software\Siemens\WinCC\<ProjectName>\<ServerName>\Dialogs on graceful exit. If the user dismissed the Log In dialog on Monitor 2 before it was last visible, the cached rcNormalPosition rectangle points to Monitor 2. After the second monitor is disconnected, Windows will still honour that placement, opening the dialog where the missing monitor used to be.

3. Aero Snap / Taskbar pin interaction

On Windows 7, Aero Snap will restore a window to the last monitor it was snapped to. If the WinCC Runtime taskbar icon was last clicked while the dialog was on Monitor 2, every subsequent Activate on the WinCC taskbar routes the dialog to that monitor only.

The combined effect is identical to what is observed: the dialog exists (Alt+Tab confirms it), but it has no visible rectangle on any connected monitor.

Solution

Apply the steps below in order. Each step addresses one of the three root causes and can be verified independently.

Step 1 — Realign the OS Project Editor to the active workstation

1. Close the WinCC Runtime on the affected station.
2. Open WinCC Explorer and select the project.
3. Right-click the project name and choose OS Project Editor.
4. On the Monitor Configuration tab, verify the number of monitors matches the physical displays connected to the Runtime station. If only one monitor is connected, set Number of monitors = 1 and clear all secondary monitor entries.
5. Confirm the primary monitor resolution (e.g. 1920 × 1080) matches the connected display, not a previously configured virtual monitor.
6. Click OK and rebuild the Runtime database by selecting Rebuild All in WinCC Explorer. This regenerates the Monitors registry key.

Step 2 — Clear the cached dialog placement

1. Stop Runtime and exit WinCC Explorer.
2. Open regedit with administrator rights.
3. Navigate to: HKEY_CURRENT_USER\Software\Siemens\WinCC\<ProjectName>\<ServerName>\Dialogs
4. Delete the subkey System Login (or the localized name such as System-Anmeldung). This forces the next activation to re-create the dialog at its default coordinates on Monitor 1.
5. Optional: delete all subkeys under ...\Dialogs to reset every Runtime dialog to default positions.
6. Close regedit and restart Runtime.

Step 3 — Recover the off-screen dialog manually

If the project cannot be restarted immediately, the hidden dialog can be dragged back into the visible work area without losing the running session:

1. Click the WinCC Runtime taskbar icon once to give the application focus (even if no window appears).
2. Press Alt+Tab until System Login is highlighted.
3. Release the keys and immediately press Alt+Space to open the window's system menu.
4. Press M to select Move. The cursor will jump to the (invisible) dialog.
5. Press the left arrow or up arrow once — Windows snaps the dialog back into the visible desktop.
6. Move the mouse to reposition the dialog and click to release.

Step 4 — Disable Windows Aero Snap on the Runtime taskbar

1. Right-click the Windows desktop → Personalize.
2. Select Window Color and Appearance → Open classic appearance properties.
3. Click Effects and uncheck Prevent windows from being snapped to the side of the screen is incorrect — instead, run regedit and set HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics\MinAnimate = 0 only if the snap animation is misbehaving.
4. For Windows 10 LTSC, open Settings → System → Multitasking and toggle off Snap windows.

Step 5 — Pin the Log In dialog to a specific monitor

For multi-monitor stations where the dialog must remain on Monitor 1 regardless of caching, set the dialog start position in the Graphics Designer:

1. Open the screen that triggers the Log In (usually a header or login screen PDL).
2. Insert a User Administrator control or call SSMOpenDialog("System Login") via a C-script.
3. In the dialog properties, set Position to Center, parent screen and uncheck Remember position if available.

Step 6 — Update graphics driver and Windows

Some older Intel HD and NVIDIA NVS drivers incorrectly report EDID data when a secondary monitor is hot-unplugged. Install the latest WHQL driver from the OEM (Dell, Lenovo, HP, Siemens) and apply the latest Windows cumulative update. Reference: Siemens Industry Online Support for the compatibility matrix of WinCC V7 SP2 with Windows 7 SP1 graphics drivers.

Verification

After applying the steps above, validate the fix with the following checklist:

Manual placement checks that indicate success:

• Alt+Tab cycles through WinCC Runtime and shows only one icon, with the dialog attached.
• Task Manager → PDLRT.exe → right-click → Bring to front brings the visible window, not a hidden one.

Diagnostic Decision Matrix

Preventive Configuration for New Projects

When commissioning a new dual-monitor WinCC Runtime station:

1. Always run the OS Project Editor on the destination hardware, not on the engineering station. Monitor EDID data differs between displays and the editor writes exact coordinates.
2. In WinCC Explorer, set the Runtime start delay to 0 s so the Log In dialog opens after graphics are fully initialized.
3. Set Start Picture in Computer properties to the login screen PDL, not to the process overview. This guarantees the user is authenticated before any monitor-dependent picture is loaded.
4. Disable Windows screen saver and monitor power-off for the Runtime account via Group Policy. A sleeping monitor is sometimes reported as disconnected and triggers the same off-screen dialog placement.
5. For Windows 10 LTSC, disable the Show windows side by side feature and the Taskbar → Show windows from monitor X only option.

Related Issues

• Windows log-on screen not displaying after sleep. When a WinCC Runtime machine wakes from sleep, Windows can also route the OS-level log-on prompt to a disconnected monitor. The Microsoft Q&A thread PC does not display log in screen after waking up from sleep mode documents the same root cause at the OS layer and is sometimes misdiagnosed as a WinCC issue.
• Alarm control log-on hidden. The Alarm Logging log-on dialog uses the same WinCC dialog cache and can be hidden by identical mechanism.
• Picture Tree log-on hidden. Picture Tree Manager's log-on window is affected by the same registry key.

Reference Documentation

• SIMATIC WinCC V7.0 SP2 — System Manual, chapter Runtime configuration → OS Project Editor. Available at Siemens Industry Online Support, entry ID 109736590.
• SIMATIC WinCC V7.3 — System Manual, entry ID 109748952. Includes the V7.3 unified OS Project Editor.
• SIMATIC WinCC V7.4 SP1 — System Manual, entry ID 109755862.
• WinCC V7 compatibility matrix for Windows 7 SP1 and Windows 10 LTSC: see entry ID 109758561.