Preserving Tag Logging Process Value Archives During OS Compilation in SIMATIC PCS 7

In SIMATIC PCS 7 (including WinCC V7.4 and PCS 7 V8.2), process tags marked for archiving in CFC block I/O properties (S7_archive attribute) are automatically compiled by AS-OS Engineering into the default tag logging container SystemArchive. If an engineer manually creates a secondary process value archive (e.g., dedicated to binary tags) and moves tags into it inside WinCC Explorer, the next CFC and OS compilation will automatically reassign those tags back to SystemArchive. To preserve tags in a custom archive without compiler interference, engineers must uncheck the 'Archive' property on the block pin in CFC and manually configure the archive tag directly inside WinCC Tag Logging, or alternatively utilize WinCC archive filtering and trend controls within the unified SystemArchive architecture.

AS-OS Compilation Mechanics in Tag Logging

In SIMATIC PCS 7, the integration between the automation system and the operator station relies on the AS-OS Engineering compiler. When an engineer flags an I/O parameter on a CFC block for archiving (such as setting the archiving property to 'Long-term archiving' or 'Short-term archiving' in the block I/O dialog), the compiler registers this request in the PCS 7 engineering database.

During an OS compilation, the compiler accesses the WinCC SQL Server database, evaluates all block parameters bearing the S7_archive attribute, and synchronizes the Tag Logging editor. By design, the PCS 7 OS compiler manages a standardized, highly optimized circular archive structure named SystemArchive. Any tag flagged for archiving in CFC is strictly assigned to SystemArchive by automated system scripts.

The Conflict Between SystemArchive and Custom Process Archives

Engineers transitioning from standalone WinCC environments often prefer segregating analog process values and binary state events into distinct archive containers (e.g., AnalogArchive and BinaryArchive) to manage disk space or sampling rates independently.

Tag Logging Management ModeCreation MechanismBehavior During OS CompilationRecommended PCS 7 UseCFC-Driven Archiving (SystemArchive)Set via CFC Block Pin I/O PropertiesAutomatically regenerated and locked to SystemArchiveStandard DCS Best PracticeManual Relocation in WinCCMoved manually from SystemArchive to custom containerReverted to SystemArchive upon next OS compilationHighly Discouraged (Defeats DCS Automation)WinCC Decoupled ArchivingUnchecked in CFC; manually created in WinCC Tag LoggingPreserved indefinitely; ignored by OS compilationSupported for specialized OEM/Custom loggingCompressed Archive (Fast/Slow)Configured via WinCC Process Value Archive wizardGoverned by WinCC compression algorithmsSupported for long-term reduction

When an engineer manually moves compiled tags into a secondary archive in WinCC Explorer, the change operates only until the next OS compile. Because the source of truth in PCS 7 is the CFC engineering database, the compiler detects that the CFC pins specify archiving and overwrites the manual WinCC configuration, moving the tags back to SystemArchive.

Configuration Strategies: CFC Decoupling vs SystemArchive Best Practices

Depending on operational requirements, technical teams have two methods to manage process archives:

Approach 1: Embracing SystemArchive (Siemens Recommended DCS Standard)
Rather than fighting compiler automation, engineers should utilize SystemArchive for all CFC-driven tags. In WinCC V7.4 and PCS 7, SystemArchive uses advanced Microsoft SQL Server segmenting that efficiently handles mixed analog and binary data streams without performance penalties. Separation of data is achieved purely at the presentation layer using WinCC Online Trend Control filters, tag prefixes, or query criteria, rather than splitting physical archives.

Approach 2: Manual Decoupling for Custom Dedicated Archives
If project specifications strictly mandate an isolated custom archive for specific binary tags:
1. Deselect Archiving in CFC: In the CFC Editor, open the block properties for each target block pin. In the Archiving dropdown, select No Archiving.
2. Recompile the OS: Execute an OS compilation. This removes the tags from SystemArchive in WinCC Tag Logging.
3. Manually Create Archive in WinCC Explorer: Open Tag Logging in WinCC Explorer. Right-click Process Value Archives, select Archive Configuration, and create your custom archive (e.g., Binary_Event_Log).
4. Manually Insert Archive Tags: Under the custom archive, right-click, select New Process Tag, and select the runtime tag from the WinCC Tag Management hierarchy. Configure acquisition cycle, archiving cycle, and hysteresis.
5. Future Compiles: Because the CFC pin does not hold the S7_archive attribute, subsequent OS compilations will completely ignore these tags, leaving the custom archive undisturbed.

Verification

Verify archive integrity and compiler stability using the following checks:

CFC Property Audit: Check the target block pin properties in CFC and verify that Archiving is set to No Archiving.
OS Changes Compilation: Execute an OS Compile -> Changes Only from SIMATIC Manager.
Tag Logging Inspection: Open WinCC Tag Logging editor. Confirm that tags configured under the custom archive remain intact in their designated folder and have not migrated to SystemArchive.
Runtime Trend Validation: Open WinCC Runtime, launch the trend picture, and verify that the trend control reads historical values smoothly from CustomArchive\TagName.

Limitations and Operational Precautions

When decoupling archive configuration from CFC, observe these operational tradeoffs:

Increased Maintenance Overhead: Decoupled tags lose automatic lifecycle synchronization. If a block is renamed or deleted in CFC, the manual archive tag in WinCC becomes an invalid orphan and must be cleaned up manually.
Storage Segmentation Planning: Creating multiple process value archives fragments SQL Server disk I/O. Ensure the computer station has sufficient write throughput and storage space to manage multiple active database segments.
Redundancy Synchronization: Custom archives are fully synchronized by WinCC Redundancy, provided the archive is a standard Process Value Archive and not a third-party proprietary database.

Engineer FAQ

Why does PCS 7 force everything into SystemArchive?

PCS 7 is designed around automated mass engineering. Centralizing all process tags into SystemArchive allows the system to automate archive creation, optimize SQL segment allocation, manage backup paths via Process Historian, and eliminate manual tag linkage errors.

Can I rename SystemArchive in WinCC Tag Logging?

No. Renaming SystemArchive will cause the AS-OS compiler to regenerate a brand new SystemArchive on the next compile, leaving you with two separate archives and fragmented trend data.

Does storing binary tags in SystemArchive slow down analog trend retrieval?

No. WinCC SQL Server uses optimized indexing and compression for binary and analog values. Querying an analog trend filters by TagID and timestamp index, so binary records in the same segment do not impact retrieval latency.