Troubleshooting OS Download Discrepancies in SIMATIC PCS 7 - Why Tags Are Missing on Reference OS Stations

In SIMATIC PCS 7, downloading an OS project from an Engineering Station (ES) to a target computer configured as a 'Reference OS' station copies only graphic pictures (.pdl) and local computer properties, leaving the target station with an empty or non-updating tag management database. By design, a Reference OS (OS Ref) does not host an independent Tag Management database; it is an architectural pointer designed to reference and read process tags directly from a master base OS Server. If the target PC station is intended to operate as an independent runtime station or redundant partner, it must be configured as a standard 'OS Application' (or 'OS Single Station') rather than an 'OS Reference'.

Architecture of OS Application vs OS Reference (OS Ref)

In SIMATIC Manager Component View, inserting a SIMATIC PC Station allows engineers to add different types of WinCC runtime applications:
1. OS Application (WinCC Appl.):
- Represents a complete, autonomous Operator Station database.
- Possesses its own S7 connection configuration, Tag Management catalog, Alarm Logging database, and Tag Logging archives.
- When compiled in SIMATIC Manager, the compiler transfers all S7-400 structure tags, raw variables, and message texts into that specific station database.
- When downloaded, the ES transfers both the graphics (GraCS) and the complete relational database files (.mdf and .ldf).
2. Reference OS (WinCC Appl. (Ref)):
- Designed for multi-operator configurations where multiple client monitors share identical graphics and access the same base server.
- Crucial Architectural Principle: A Reference OS does not compile tags from the AS. It simply copies the graphic screens from its assigned Base OS and configures its local communication channel to point directly to the Base OS database.
- When an engineer downloads an OS Ref, SIMATIC Manager deliberately skips transferring Tag Management structure records.

The Operational Role of Reference OS in Minimal Architectures

In the Siemens PCS 7 manual 'Minimal Configurations', an OS Reference is used to reduce engineering maintenance:
- An engineer configures a single master OS project (e.g. OS_Base).
- A second station is inserted as OS_Ref, pointing to OS_Base.
- When changes are compiled on OS_Base, the engineer only needs to compile once.
- However, if the target PC is physically an independent OS Single Station connected directly to the S7-400 controller via Industrial Ethernet, deploying an OS Ref causes total failure: the screen opens with block icons, but every block icon displays yellow exclamation marks or invalid data flags because the local WinCC Tag Management contains zero S7 tags.

Converting an OS Reference to an Independent OS Application

To ensure both target stations receive full Tag Management databases:

Check Station Configuration in SIMATIC Manager:
- Open SIMATIC Manager Component View.
- Expand the target PC Station.
- Check the software object under the Configuration container:
- If the object icon displays an arrow or reads WinCC Appl. (Ref), it is a Reference OS.
Replace with Standard OS Application:
- Right-click the PC Station and open HW Config.
- In HW Config, delete the WinCC Appl. (Ref) component from the virtual rack.
- In the hardware catalog, expand:
SIMATIC PC Station > HMI > WinCC Appl. (or WinCC Appl. Stby for redundant pairs).
- Insert WinCC Appl. into the rack slot.
- Save and compile HW Config.
Reassign OS Project:
- In SIMATIC Manager, right-click the newly inserted OS object and select Object Properties.
- Ensure the OS path and name are correctly specified.
Compile Entire OS:
- Right-click the OS project, select Compile, and execute Entire OS.
- Verify that the compiler generates structure tags and communication connections for both stations.
Download to Target Station:
- Ensure WinCC Runtime and WinCC Explorer on the target PC are closed.
- Right-click the OS project in SIMATIC Manager and select PLC > Download.
- Verify that both the project database files and graphics folders are transferred completely.

OS Station Object Type Comparison and Download Scope Table

The following table compares the functional capabilities and download scope of PCS 7 OS station types:

OS Object Type in HW ConfigTag Database CompiledArchive Logging HostedDownload Scope from ESIntended Operational ArchitectureWinCC Appl.Yes (Autonomous DB)Yes (Local Tag/Alarm Logging)Full Project (.mdf + GraCS)Primary OS Server or OS Single StationWinCC Appl. StbyYes (Mirrored DB)Yes (Redundancy Sync)Full Project (.mdf + GraCS)Redundant Standby OS PartnerWinCC Appl. (Ref)No (Shares Base DB)No (References Base Server)Graphics Only (GraCS + Config)Secondary visual terminals sharing one OS ServerWinCC Appl. ClientNo (Loads Server Package)No (Queries Server via RPC)Client Project (.mcp + Local GraCS)Multi-Client operator console

Verification

To confirm that the target OS station possesses a fully functional Tag Management database:
1. On the target PC, launch WinCC Explorer and open the downloaded project.
2. In the navigation tree, open Tag Management > SIMATIC S7 PROTOCOL SUITE > Named Connections.
3. Verify that the AS connection exists and displays all expected structure tags (e.g. TIC_101, PUMP_01).
4. Activate WinCC Runtime on the target station:
- Verify that process graphics display live analog values and active motor states.
- Confirm that block icons do not display gray disconnected states or yellow question marks.
5. Issue a setpoint change from the target station and verify immediate execution on the controller.

Limitations and Operational Precautions

When managing OS downloads:
1. Never Download to an Active Runtime Station: Downloading an OS project while WinCC Runtime is running on the target PC causes file transfer sharing violations and corrupts the SQL Server database. Always deactivate runtime before downloading.
2. Matching Path Structures: The project storage path configured in SIMATIC Manager (e.g. D:\WinCC_Projects\OS1\) must physically exist and match the shared network directory permissions on the target computer.
3. Redundant Server Download Sequence: In redundant OS pairs, never download to both servers simultaneously. Always download to the Standby Server first, verify operational integrity, and then download to the Partner Server.

Engineer FAQ

Why did the Reference OS download succeed if no tags were copied?

Because an OS Reference download is functioning as designed by transferring only screen graphics and pointing to the master database. The download tool did not encounter an error because it followed the Reference OS rules.

Can an OS Reference station communicate with the PLC if the Base OS is powered off?

No. An OS Reference station relies entirely on the active runtime database and communication channels of the Base OS Server. If the Base OS is offline, the Reference OS loses all process data.

What is the difference between an OS Client and an OS Reference?

An OS Client is a distinct WinCC architecture that imports 'Server Packages' across the terminal bus. An OS Reference is a physical clone of a single station project sharing a common database.

Can I convert an OS Ref to a standard OS Appl without deleting it?

No. In HW Config, the module type must be physically deleted and replaced with a standard WinCC Appl. to allow the OS compiler to generate local tag records.