Troubleshooting Screen Scaling Distortion When Opening AOTC Trends in WinCC V7.4 and CEMAT

Screen scaling distortion and unexpected window shrinking when opening online trends (AOTC) in SIMATIC PCS 7 V9.0 SP2 / CEMAT V9.0 SP1 (WinCC V7.4 SP1 Upd10) is caused by multi-monitor DPI scaling mismatches and known coordinate evaluation defects in legacy trend calling scripts. When an operator right-clicks an analog measurement unit (C_MEASUR) to plot a curve, WinCC attempts to dynamically calculate popup window coordinates. If Windows display scaling is set to anything other than 100% (96 DPI), or if the screen layout contains mixed resolution monitors, the WinCC runtime viewport collapses to a smaller bounding box. To resolve this: set Windows display scaling strictly to 100% on all monitors, align screen resolutions in OS Project Editor, and apply WinCC V7.4 SP1 Update 14 or later.

Executive Summary

In cement and minerals processing facilities utilizing the SIMATIC PCS 7 Minerals (CEMAT) library alongside PCS 7 V9.0 SP2 (WinCC V7.4 SP1), control room operators frequently utilize the Advanced Operator Trend Control (AOTC) to analyze analog sensor trajectories. A critical operational failure occurs when operators right-click the engineering unit of an analog measured value (C_MEASUR): instead of smoothly opening the trend curve popup, the entire WinCC runtime interface collapses and shrinks to a fraction of the physical monitor size, forcing operators to deactivate and restart runtime to restore visibility. This article identifies the root cause within Windows DPI virtualization and WinCC coordinate scripts, and details the configuration and patch requirements to permanently stabilize trend displays.

Architecture and Fundamental Mechanics

DPI Virtualization, Screen Coordinate Mechanics, and AOTC Trend Architecture:

WinCC Runtime Coordinate Engine and Windows DPI Scaling:
SIMATIC WinCC V7.4 is designed around a fixed-coordinate graphics rendering pipeline based on native 96 DPI (100% Windows display scaling).
In modern widescreen control rooms featuring 4K or high-density Full HD monitors, operating system installations often default to display scaling factors of 125%, 150%, or 200%.
When Windows DPI scaling is active, Windows intercepts GDI draw calls and applies a coordinate transformation matrix (DPI virtualization). While static plant mimic screens scale reasonably well, dynamic script-driven popup windows that calculate absolute pixel coordinates (using SetLeft, SetTop, SetWidth, SetHeight) receive unscaled coordinates from Windows API functions but write them to scaled WinCC picture windows.
CEMAT AOTC Trend Calling Script Defect in C_MEASUR:
When an operator right-clicks the engineering unit of block icon C_MEASUR, an event script triggers the AOTC trend launcher (C_Trend.pdl or @PG_C_MEASUR_TREND.pdl).
In WinCC V7.4 SP1 Upd10, the trend opening script contains an unhandled coordinate recalculation loop that inadvertently modifies the root @Screen container dimensions when scaling factors are present.
The parent runtime frame collapses to match the smaller unscaled bounding box, shrinking the entire operating desk layout into the upper-left corner of the monitor.
Siemens Patch Resolution:
Siemens addressed this coordinate overflow defect in WinCC V7.4 SP1 Update 14 and consolidated the fix in subsequent PCS 7 V9.0 SP2 / SP3 update collections, ensuring trend popups preserve parent screen boundaries.

Step-by-Step Implementation Procedure

Standardize Windows Display Scaling on All Operator Stations:
* On every OS Client and OS Server station, right-click the Windows Desktop and select 'Display settings'.
* Under 'Scale and layout', select the dropdown for 'Change the size of text, apps, and other items'.
* Set the scaling factor strictly to 100% on all connected monitors.
* If multi-monitor setups are used, confirm that all physical monitors operate at identical native resolutions (e.g. 1920x1080) and 100% scaling.
* Log off and restart Windows to flush DPI virtualization caches.
Verify Screen Layout Settings in OS Project Editor:
* Open WinCC Explorer on the Engineering Station.
* Open OS Project Editor.
* On the 'Layout' tab, confirm that the selected resolution matches the physical display resolution (e.g. SIMATIC Standard 1920*1080).
* Ensure that option 'Size of the overview area' and screen partitions correspond to the active multi-monitor configuration.
* Click OK to apply layout configurations.
Install WinCC V7.4 SP1 Update 14 (or Higher):
* Download the latest Update Collection for WinCC V7.4 SP1 (minimum Update 14, recommended Update 18 or PCS 7 V9.0 SP3).
* Close all WinCC applications and execute Setup.exe with administrative privileges.
* Install the update on the Engineering Station, OS Servers, and all OS Clients.
* Restart each computer following update installation.
Update CEMAT Project PDLs (If Necessary):
* If customized trend PDLs are present in the project GraCS directory, back them up.
* Run the CEMAT project update script (D:/CEMAT_CS/BIN/Update_CS.bat) to ensure standard trend calling scripts are refreshed with patched versions.
Validate Runtime Trend Popups:
* Activate WinCC Runtime.
* Locate a C_MEASUR block icon on a plant graphic.
* Right-click the engineering unit (e.g. rpm or bar).
* Confirm that the AOTC trend window opens smoothly in the foreground while the background plant overview remains perfectly anchored at full screen resolution.

Verification

Verify Trend Popup Stability and Resolution Integrity:
1. Right-click analog measurement units across multiple plant graphic screens; verify that the trend window opens without altering parent screen resolution.
2. In Windows Display settings, confirm that display scaling remains locked at 100%.
3. In WinCC Explorer -> Help -> About, verify that the installed WinCC version reports V7.4 SP1 Upd14 or higher.
4. Verify that deactivating and reactivating runtime is no longer required after plotting multiple trends.

Limitations and Operational Precautions

Operational Precautions and Multi-Monitor Guidelines:
- Never mix monitors with different native aspect ratios (e.g. 16:9 mixed with 4:3) on the same multi-monitor operator console; WinCC picture navigation cannot seamlessly stretch across disparate aspect ratios.
- Do not allow Windows automatic scaling or remote desktop sessions (RDP) to alter DPI settings on active operator client consoles; use dedicated operator console logins.

Engineer FAQ

Why did right-clicking an analog unit shrink my entire WinCC runtime?

In WinCC V7.4 SP1 Upd10, Windows DPI scaling factors (>100%) cause the trend script to miscalculate screen coordinates, collapsing the parent @Screen window into an unscaled coordinate box.

What Windows display scaling is officially certified for PCS 7 WinCC?

Siemens officially certifies WinCC operation exclusively at 100% display scaling (96 DPI). Non-standard scaling factors cause graphical misalignment and coordinate errors.

What update resolves this AOTC trend scaling bug?

WinCC V7.4 SP1 Update 14 (or later) resolves the coordinate recalculation defect in AOTC trend popups.

Does this issue occur on Engineering Stations?

It often does not appear on Engineering Stations if the ES runs in a single-screen windowed mode or has different Windows DPI settings than multi-monitor operator stations.