Restoring WinCC OS Client Trend Views Following Server Power Outages in PCS 7

When a SIMATIC PCS 7 OS Client that remained powered on during an OS Server power outage fails to display trend curves after server restoration—while other clients that power-cycled recover normally—the issue is caused by a stale Split Screen Manager (SSM) DCOM binding or unassigned Standard Server in WinCC Explorer. When an OS Server drops communication abruptly, an un-restarted OS Client locks its trend request threads into a local fallback state, attempting to query a local non-existent TRG folder rather than reconnecting to the server archive. Resolving this fault requires verifying the Standard Server assignment for "SSM" and "Alarms" in WinCC Explorer, clearing local temporary cache files (.dcf), and re-initializing the client WinCC runtime session.

WinCC Client-Server Trend Architecture and TRG Folders

In a SIMATIC PCS 7 Multi-Client architecture, OS Clients do not host local process archives or trend configuration databases. Instead, historical process values, archive segments, and runtime trend group templates (.trg files) reside centrally on the OS Server within the project's TRG directory.

To access these resources across the network, the OS Client relies on two architectural mechanisms:
1. Server Data Packages (.pck): Exported from the OS Server and loaded on the client to define available archive tags and server network addresses.
2. Standard Server Assignment: Designates which specific OS Server acts as the default provider for global system services, specifically Alarms and Split Screen Manager (SSM).

The Split Screen Manager is the WinCC component responsible for coordinating multi-monitor workspaces, picture selection, and trend group synchronization across the client network.

Mechanism of Client Thread Lockout During Power Loss

When an OS Server experiences a sudden loss of electrical power while an OS Client remains energized, the client's WinCC runtime engine undergoes an abnormal communication disconnection:

Client ConditionNetwork ReactionTrend Query MechanismOperational StatusPower-Cycled ClientReboots into clean Windows stateRe-establishes clean DCOM link to restored OS ServerTrends display immediately upon startupContinuously Powered ClientDCOM socket breaks abruptly; server disappearsSSM thread enters blocking wait; falls back to local pathTrends fail; client queries local non-existent TRG folderClient Missing Standard ServerConnects to server for tags, but SSM unassignedClient cannot resolve which server hosts the TRG repositoryTrends blank; curves refuse to loadRedundant Pair ResynchronizingStandby server recovering archivesClient queries server currently undergoing archive syncTemporary trend latency during archive rebuild

Because the third client was never restarted, its internal WinCC DCOM proxies remained tied to stale TCP socket descriptors. Even after the physical server powered back on, the client's Split Screen Manager service failed to re-bind to the server's remote trend directory, mistakenly querying a local path on the client's hard drive.

Configuring Standard Server for Split Screen Manager (SSM)

To ensure that an OS Client always knows where to query trend groups and archive curves:

Open WinCC Explorer on the problematic OS Client project.
In the navigation tree, locate Server Data.
Right-click Server Data and select Standard Server....
In the dialog box, inspect the components:
- Alarms: Must be assigned to the primary OS Server symbolic name (e.g., OS_SRV_PRJ).
- SSM (Split Screen Manager): Must be assigned to the identical OS Server symbolic name.
- Archives (if applicable): Assigned to the OS Server or Process Historian.
If SSM is set to <No Standard Server>, the client cannot locate global trend groups and will fail to display shared curves. Select the appropriate server and click OK.

Step-by-Step Recovery and Cache Purge Procedure

To completely clear stale DCOM handles and restore trend visualization on the affected client:

Deactivate WinCC Runtime: On the affected OS Client, close WinCC Runtime completely.
Close WinCC Explorer: Exit WinCC Explorer. Open Windows Task Manager and confirm that background processes (CCApp.exe, CCClient.exe, pdlrt.exe) have completely stopped.
Purge Client Project Database Cache:
- Open Windows Explorer and navigate to the OS Client project directory on disk (e.g., D:\PCS7_Projects\Client_03\).
- Locate the temporary database cache file: <ProjectName>.dcf.
- Delete the <ProjectName>.dcf file. (WinCC will automatically regenerate a fresh, uncorrupted .dcf file upon next launch).
Restart Simatic Shell / Network Binding:
- In Windows Explorer, right-click Simatic Shell -> Settings. Confirm the correct Terminal Bus network card is selected.
Reboot the OS Client: Restart Windows to flush all lingering RPC/DCOM communication handles.
Activate Runtime: Start WinCC Runtime; open the trend display; the client now re-reads the server TRG directory cleanly.

Verification

To verify that trend functionality is fully restored:
1. Open the WinCC Runtime trend window on the recovered OS Client.
2. Select an existing trend group (e.g., Boiler_Steam_Trends); verify that archive curves render immediately without error dialogs.
3. Use the ruler tool to inspect historical data points recorded before and after the server power outage; confirm continuous curve progression.
4. Create and save a new test trend group from the client; verify that the .trg file is successfully created in the OS Server's TRG folder.

Limitations and Operational Precautions

Never delete .mdf or .ldf database files from an OS Client or Server project directory; only the temporary configuration file <ProjectName>.dcf should be purged during troubleshooting. When an OS Server recovers from an unplanned power outage, allow redundant servers several minutes to complete Tag Logging archive synchronization before diagnosing client display faults.

Engineer FAQ

Why did the client that never lost power fail to see trends while restarted clients worked?

Restarted clients created fresh network connections upon booting. The continuously powered client retained stale DCOM handles and locked its Split Screen Manager into an offline local fallback state.

What does the .dcf file do in WinCC?

The .dcf file is a local configuration cache that stores runtime connection parameters and server package references. If corrupted during a network drop, it prevents proper server resource resolution.

Is the Standard Server configuration required on all OS Clients?

Yes. In PCS 7 Multi-Client architectures, every OS Client must have its Standard Server configured for both Alarms and SSM to ensure coordinated alarm messaging and trend group access.