Why AS-OS Compilation Is Mandatory for Process Tags in PCS 7 and When Manual Tag Creation Is Permissible

In SIMATIC PCS 7 V8.1, the automated AS-OS Compilation ('Compile OS') is the mandatory, foundational method for generating process tags in WinCC. By flagging CFC block parameters with the operator control attribute (S7_m_c := 'true'), the AS-OS compiler automatically creates all required WinCC process tags, message configurations, alarm logs, archive linkages, and Named Connection handles with 100% relational integrity. While manual tag creation in WinCC Tag Management is technically possible—and permissible for internal UI script variables or third-party OPC tags—manually creating tags for S7-400 process blocks is strictly prohibited in PCS 7 because it bypasses alarm routing, doubles engineering effort, and corrupts faceplate integration.

The Foundation of PCS 7 Integrated Engineering: AS-OS Compilation

SIMATIC PCS 7 distinguishes itself from traditional standalone SCADA/PLC packages through Integrated Engineering:
* The Standalone SCADA Workflow (Classic STEP 7 + WinCC):
* An engineer programs a PLC in STEP 7, notes memory addresses (DB10.DBD20), opens WinCC Tag Management, manually types tag names, selects data types, manually creates alarms in Alarm Logging, and configures faceplates.
* The PCS 7 Automated Compilation Workflow:
* The engineer designs control logic visually in Continuous Function Charts (CFC).
* The AS-OS compiler (Compile OS) extracts all technological tags, unit strings, range limits, and alarm definitions in a single automated pass.
* Thousands of interconnected WinCC tags and alarm messages are generated in seconds without manual human entry errors.

Automated Tag Generation via the S7_m_c Attribute in CFC

The mechanism governing tag compilation is the system attribute S7_m_c (S7 Monitoring and Control):

How It Works:
When an APL block (such as PIDConL) is compiled, any input or output declared with S7_m_c := 'true' is earmarked for the OS.
In CFC, selecting Chart > Compile generates system communication descriptors.
Executing OS Compilation:
1. In SIMATIC Manager, right-click the OS Server object: select Compile.
2. Select compilation scope: Tags and messages (and optionally Picture Tree and SFC Visualization).
3. Choose compilation mode: Changes Only for routine edits or Entire OS for first-time builds.
4. The compiler populates WinCC Tag Management with structured tag groups named after the Plant Hierarchy (e.g., Plant1/Unit2/TIC101.PV_Out#Value).

Consequences of Manually Creating Process Tags in WinCC

Attempting to bypass AS-OS compilation by creating S7-400 process tags manually in WinCC Tag Management triggers severe operational failures:

Severed Alarm System: WinCC Alarm Logging relies on raw alarm messages generated by controller blocks (ALARM_8P). Manually created tags cannot bind to ALARM_8P message handles.
Broken APL Faceplates: APL faceplates expect structured tag groups containing .Value, .ST (Quality Code), and .Status1. Manually created tags lack structure metadata, leaving faceplates in a permanently faulted state.
Loss of Consistency: If a block address moves in controller memory during a delta download, the AS-OS compiler updates WinCC automatically. Manually created tags will point to stale memory words, reading corrupted data!

Permissible Scenarios for Manual Tag Creation in WinCC Tag Management

Manual tag creation in WinCC Explorer is permissible strictly for non-DCS peripheral variables:

Internal WinCC Tags (@ and User Variables):
* Screen navigation variables, temporary calculation tags, local operator workstation flags, and user interface script memory.
Third-Party OPC / Modbus Communication:
* Reading auxiliary parameters from a package chiller or building management system over the WinCC OPC Channel or Modbus TCP driver where no PCS 7 AS controller is involved.
External SQL Database Connectivity:
* Holding temporary string buffers for barcode scanners or laboratory information management system (LIMS) reports.

Comparison of Automated AS-OS Compilation vs Manual Tag Creation

The table below contrasts AS-OS Compilation with manual tag creation in SIMATIC PCS 7.

Engineering MetricAutomated AS-OS CompilationManual Tag Creation in WinCCData Consistency100% Guaranteed by CompilerProne to human memory offset errorsAlarm IntegrationAutomatic binding to ALARM_8PImpossible for PCS 7 block alarmsAPL Faceplate CompatibilityNative support (Struct, ST, Status1)Broken (Missing structured metadata)Engineering Time (1000 Tags)30 SecondsDays of manual data entryDelta Download SupportFully automated Changes-OnlyRequires manual coordinationRecommended ApplicationAll S7-400 / CPU 410 Process TagsInternal UI tags and third-party OPC

Verification

To confirm that tags generated via AS-OS compilation are healthy:
1. In SIMATIC Manager, perform an OS compilation: select Compile OS > Changes Only.
2. Confirm compilation completes with 0 Errors.
3. Open WinCC Explorer: open Tag Management.
4. Expand SIMATIC S7 Protocol Suite > Named Connections: confirm that structured tag groups named after your plant hierarchy exist.
5. In WinCC Runtime, open an operator picture: verify that block icons bind to the compiled tags and faceplates open with dynamic live values and valid signal status.

Limitations and Operational Precautions

Never delete or rename auto-compiled tags inside WinCC Tag Management; any subsequent OS compilation will recreate deleted tags and orphan manually edited references.
Ensure plant hierarchy folders in SIMATIC Manager are named concisely; excessively long hierarchy paths can exceed WinCC tag name character limits (max 128 characters).
When adding custom SCL blocks, always decorate exposed variables with { S7_m_c := 'true' } before compiling.
Always validate final software licensing, tag capacity tiers, and system documentation against the applicable Siemens catalog and authorized distributor quotation.

Engineer FAQ

Why does PCS 7 not recommend manual tag creation?

Because PCS 7 is an integrated Distributed Control System (DCS). AS-OS compilation binds tags, alarms, faceplates, archives, and security permissions in a single atomic pass, eliminating human configuration errors.

How do I make a custom variable compile to WinCC automatically?

In your SCL or CFC block definition, add the system attribute { S7_m_c := 'true' } to the variable declaration. During OS compilation, WinCC will automatically create the tag.