Resolving WinCC OS Server S7 Connection Failures After Adding New Plant Areas

When a newly added automation station or plant area in a SIMATIC PCS 7 multiproject works properly on the Engineering Station but fails to communicate on the WinCC OS Server (tags remain invalid or display disconnected status), the failure is caused by failing to download the updated connection configuration from NetPro to the PC Stations. Compiling the OS creates tag records in the WinCC database, but the underlying S7 Named Connection must be physically downloaded to the Station Configuration Editor of both the OS Server and Standby Server via NetPro (PLC -> Download to Current Project -> Selected Station). Once connection tables are downloaded, the server connects immediately.

Architecture of S7 Named Connections in Multiproject Systems

In SIMATIC PCS 7, communication between Operator Stations (WinCC OS Servers) and Automation Systems (S7-400 / CPU 410-5H) relies on S7 Named Connections established over Industrial Ethernet. Unlike classic tag polling using raw IP addresses, Named Connections utilize symbolic endpoints managed by the SIMATIC NET communication subsystem and the Station Configuration Editor on the PC Station.

When a project expands to include a new plant area or an additional controller, engineers insert new AS hardware and link the project in the multiproject. However, establishing active runtime communication requires a two-step engineering workflow: first, compiling the technological data from AS to OS; and second, downloading the updated connection database from NetPro to the local communication stack of every participating PC station.

Why OS Compilation Alone Does Not Establish Communication

A common misconception among automation engineers is that compiling the OS automatically updates all communication paths. Here is why communication fails if NetPro is overlooked:

Role of OS Compilation: OS Compilation reads technological block pins and generates tag management variables inside the WinCC SQL Server database. It binds these tags to a named connection name (e.g., AS_Area3@CPU).
Role of NetPro and Station Configuration Editor: The SIMATIC NET communication daemon (S7jcomx.exe / CCDataServer.exe) does not read the WinCC tag database to establish network sockets. Instead, it reads the local communication component configuration loaded into the Windows kernel via the Station Configuration Editor. If NetPro connections were never downloaded to the OS Server, SIMATIC NET has no routing table entry for AS_Area3@CPU, and the connection status remains permanently dead.

Comparison of Engineering Station vs OS Server Connection Paths

The table below contrasts why communication can function successfully on the Engineering Station while failing on the OS Server.

Diagnostic DimensionEngineering Station (ES)Target OS Server (Primary & Standby)Communication MechanismLocal STEP 7 internal connection (PC Internal / Named)Dedicated SIMATIC NET S7 Protocol Stack (CP 1623 / Softnet)Connection Table SourceDirect project database access in STEP 7Station Configuration Editor binary database (*.xdb / NetPro)Impact of Adding New ASES instantly accesses newly created AS in projectFails until NetPro connection table is explicitly downloadedRequired Download ActionNone (ES updates internal routing dynamically)Must execute PLC -> Download -> Connections and GatewaysRedundancy RequirementNon-redundant engineering stationMust download connections to both Server A and Server B

Step-by-Step Procedure to Download NetPro Connections to OS Servers

Follow this exact procedure to download S7 connection tables to the OS Servers after adding a new automation station:

Merge Multiproject Connections in NetPro:
- In SIMATIC Manager, open NetPro from the multiproject root.
- Verify that cross-project subnets (Industrial Ethernet / Plant Bus) are merged correctly via Edit -> Merge Subnets.
- Confirm that the S7 Named Connection from the OS Server to the new AS is displayed in the connection table.
Save and Compile NetPro:
- In NetPro, select Network -> Save and Compile.
- Choose Compile and check everything. Ensure compilation finishes with zero errors.
Download Connections to Target PC Stations:
- Highlight the Primary OS Server PC Station object in NetPro.
- Select PLC -> Download to Current Project -> Selected Station (or choose Connections and Gateways).
- In the target selection dialog, confirm the station name and click OK.
- Repeat the exact download step for the Standby OS Server.
Compile and Download OS Project:
- Recompile the OS in SIMATIC Manager (Changes Only or Entire OS).
- Download the OS project to the target server pair.

Verification

Verify live server communication through the following systematic checks:
1. Open the Station Configuration Editor on the OS Server: verify that the application and network adapter entries show run mode (green icons) without diagnostic flags.
2. Open WinCC Explorer on the OS Server, expand Tag Management -> SIMATIC S7 Protocol Suite -> Named Connections.
3. Right-click the newly added connection and select Connection Status: verify that status shows 'Connected' with a green checkmark.
4. Open a process graphic containing tags from the new area: confirm that process values update dynamically without gray invalid markers.

Limitations and Operational Precautions

Downloading connections from NetPro to a running PC Station updates communication tables online without requiring a full Windows reboot. However, in redundant server configurations, always verify that connection tables are downloaded to both partners. If Standby Server connections are omitted, server failover will result in an immediate plant-wide blackout for the new area because the standby partner cannot connect to the new PLC.

Engineer FAQ

Why does WinCC show '@... disconnected' on tags from a new PLC?

The prefix '@' in WinCC internal diagnostic tags indicates that the underlying Named Connection defined in NetPro has not been established or was not downloaded to the OS Server.

Can I download NetPro connections while WinCC Runtime is active?

Yes. Downloading 'Connections and Gateways' from NetPro updates the active SIMATIC NET connection table online without interrupting running WinCC graphics.

Do I need to download NetPro to OS Clients?

No. OS Clients communicate with OS Servers via WinCC Server-Client packages over the Terminal Bus and do not possess direct S7 connections to automation controllers.