Integrating Legacy S7-300 Ladder Logic into SIMATIC PCS 7 WinCC - Manual Tag Creation and AS-OS Compile Protection

In SIMATIC PCS 7, integrating legacy S7-300 controllers programmed in Ladder (LAD) and manually creating WinCC tags without them being overwritten by AS-OS compilations is achieved by segregating communication channels. In WinCC Explorer, create an independent connection under SIMATIC S7 PROTOCOL SUITE (e.g., Industrial Ethernet or TCP/IP) named S7_300_PLC. Tags created manually under this independent connection are completely ignored by the PCS 7 AS-OS compiler. For discrete alarms, configure standard bit-message blocks in WinCC Alarm Logging tied to S7-300 data block addresses.

The AS-OS Compiler Automation Paradigm vs Manual SCADA Engineering

In standard SIMATIC PCS 7:
* The AS-OS Compiler:
Automates 100% of the tag management pipeline. When an engineer clicks Compile OS, the compiler parses CFC charts, extracts all S7_m_c := 'true' variables, builds structure tags, configures alarms, and links faceplates under Named Connections.
* The Legacy Integration Challenge:
When expanding an existing plant, engineers often need to bring legacy standalone S7-300/400 PLCs programmed in standard STEP 7 Ladder Logic (LAD) with DB bit alarms into the PCS 7 WinCC runtime.
* The Danger:
If an engineer manually creates tags inside the PCS 7 compiled S7 connection, the next AS-OS compilation will purge them.

Why AS-OS Compilations Overwrite Manually Created WinCC Tags

During an AS-OS compile (especially with Memory reset):
* The compiler reconciles the WinCC tag database against the S7 program symbol table.
* Any tag located inside the compiled S7 Named Connection that does not correspond to an active CFC block instance is flagged as an orphan and deleted.
* To prevent data loss, manually created tags must reside in an independent channel container outside the compiler's authority.

Channel Segregation Strategy for Non-PCS 7 Controllers

To protect manual tags permanently:
* Create an Independent Connection:
In WinCC Explorer -> Tag Management -> SIMATIC S7 PROTOCOL SUITE, add a dedicated connection under Industrial Ethernet or TCP/IP (e.g., S7_300_Packaging).
* Connection Independence:
The AS-OS compiler only synchronizes Named Connections declared in NetPro. An independent TCP/IP connection created manually inside WinCC is completely ignored by the compiler, guaranteeing 100% permanence across all future compilations.

Comparison Table: AS-OS Automated Tags vs Manually Created WinCC Tags

The table below outlines operational differences between tag types in PCS 7 WinCC:

Parameter / FeatureAS-OS Compiled TagsManually Created S7-300 TagsCreation MethodAutomated via CFC / SFC compilationManual entry in WinCC Configuration StudioConnection ProtocolS7 Named ConnectionS7 TCP/IP / Industrial EthernetAlarm GenerationAlarm_8P event frames from controllerBit Alarms polled cyclically by WinCCImpact of OS Memory ResetCompletely rebuilt from CFCUntouched (100% Preserved)Faceplate CompatibilityStandard PCS 7 APL FaceplatesCustom WinCC Graphic Objects / PopupsProcess Object LicensingConsumes AS & OS Process Objects (PO)Consumes standard WinCC Tags / PowerTags

Step-by-Step Procedure to Integrate S7-300 Ladder Tags into PCS 7 WinCC

Configure Physical Network Connection:
Assign an IP address to the S7-300 Ethernet CP/PN port on the plant network subnet.
Add Independent S7 Connection in WinCC:
* Open WinCC Explorer on the Engineering Station.
* Open Tag Management.
* Under SIMATIC S7 PROTOCOL SUITE -> TCP/IP, right-click and select New Connection.
* Name the connection S7_300_PLC.
* In Connection Properties, enter the S7-300 IP address, Rack 0, and Slot 2.
Create Process Tags:
* Inside the S7_300_PLC connection, create tag groups (e.g., Analog_Inputs, Motors).
* Manually create tags mapping to S7-300 DB addresses (e.g., DB10.DBD0 Float, DB20.DBX0.0 Binary).
Configure Discrete Bit Alarms:
* Open Alarm Logging in WinCC.
* Create a new message class (e.g., S7-300 Alarms).
* Add discrete bit messages bound to your manual binary tags.
Develop Process Graphics in Graphic Designer:
In Graphic Designer, bind graphic symbols and I/O fields directly to the manual S7-300 tags.
Execute OS Compilation:
Perform an AS-OS compilation of your PCS 7 controllers. Confirm that the compile completes successfully and that the S7_300_PLC connection and tags remain completely intact.

Verification

To verify manual tag integration:
1. Activate WinCC OS Runtime.
2. Open WinCC Channel Diagnosis. Verify that both the PCS 7 Named Connections and the TCP/IP -> S7_300_PLC connection display status OK (green).
3. Toggle a bit in the S7-300 controller via a VAT table. Verify that the corresponding graphic indicator updates in WinCC Runtime.
4. Execute an Entire OS Compile with memory reset from SIMATIC Manager. Confirm that all S7-300 tags and alarms survive without modification.

Limitations and Operational Precautions

No APL Faceplate Magic: Manually created tags cannot automatically open APL faceplates because APL faceplates require structured status bytes (Value and ST). Use standard WinCC controls or custom popups for S7-300 equipment.
Polling Latency: Unlike event-driven Alarm_8P alarms, WinCC bit alarms rely on cyclic network polling (typically 500 ms to 1 s). Do not use manual bit alarms for millisecond sequence-of-events recording.
Licensing Disclaimer: S7-300 manual tags communicating via standard protocol channels consume WinCC runtime tag licenses (6AV6381-...) validated against the applicable Siemens catalog and authorized distributor quotation.