Managing WinCC ServerPrefix Strings and Tag Name Length in OPC Architectures

In SIMATIC PCS 7 and WinCC client-server architectures, OPC tag strings often exceed third-party buffer limits due to prepended server qualifiers such as '@LOCALMACHINE::' or long symbolic server prefixes. Resolving excessive tag length is accomplished by assigning a short, concise symbolic computer name during server package creation in WinCC Explorer, updating OS client package references, or configuring OPC alias mapping. Changing the server prefix streamlines external OPC DA and OPC A&E tag paths while maintaining full client-server routing.

WinCC ServerPrefix Architecture and OPC Naming Conventions

In SIMATIC PCS 7 multi-client architectures, an OS Client accesses tags and alarms from distributed OS Servers by prepending a symbolic server prefix to the tag name:
ServerPrefix::Tag_Name (for example, OS_SRV_UNIT1::FIC101/PV_Out.Value)

When third-party supervisory systems or plant historians interface with WinCC via OPC DA (Data Access) or OPC A&E (Alarms & Events), the OPC server interface exposes tags using fully qualified hierarchical names. If an OPC client connects directly to a server where server data has not been parameterized symbolically, WinCC may prepend the default qualifier @LOCALMACHINE:: or the full network computer name (e.g., @LOCALMACHINE::Area::Unit::Tag).

In many legacy OPC client systems, SCADA platforms, and DCS gateway drivers, tag name buffer strings are restricted to 64, 128, or 256 characters. Long server prefixes combined with deep PCS 7 Plant Hierarchy paths can exceed these string buffers, causing tag discovery truncation, communication timeouts, or bad quality status (0x80004005).

Configuring Symbolic Names During Server Package Creation

To shorten or customize the server prefix generated by WinCC:

Open WinCC Explorer on the OS Server: Open the target OS project on the engineering station or server.
Access Server Data: In the navigation tree, right-click Server Data and select Open.
Configure Server Package Properties:
- Right-click the server package in the list and select Properties (or configure before generation via Options > Settings).
- Locate the Symbolic Computer Name field. By default, this is populated with the physical PC station name (e.g., PCS7_OS_SRV_01).
- Change the symbolic name to a short, standardized identifier (e.g., S1, SRV1, or A1).
Generate New Server Package: Right-click Server Data and select Create... (or execute an OS Compilation with package generation enabled).
Resulting Tag Strings: The generated prefix updates immediately from long formats to the shortened prefix:
S1::Area1/FIC101/PV_Out.Value.

Managing OPC A-E and OPC DA Namespace Formatting

For external OPC DA and OPC A&E communication gateways:

OPC Server Configuration Settings: In the WinCC installation directory, the OPC.ini configuration file governs specific exposure rules for the OPC server layer. Parameterizing options such as UseSymbolicPrefix or disabling redundant machine prefixes ensures the OPC tree presents simplified paths.
Local Client vs Remote Client Routing: When OPC clients execute locally on the WinCC Server, using OPC.SimaticNET or OPC.WinCC without specifying remote machine handles eliminates unnecessary @LOCALMACHINE prefixes.
Alias Mapping: For third-party historians with rigid tag length limits, deploy an OPC gateway (such as WinCC IndustrialDataBridge or Kepware KEPServerEX) configured with an alias dictionary that maps long PCS 7 hierarchy names to short numerical or alphanumeric register tags.

Impact on OS Clients and Distributed Systems

The following table details the operational impact and required engineering actions when modifying the ServerPrefix:

System ComponentImpact of Changing ServerPrefixRequired Engineering ActionOS ClientsPackage prefix mismatch; process screens show unlinked tagsRe-load updated Server Package via Server Data > Load... on all OS ClientsWinCC Picture WindowsTagPrefix assignments in scriptsVerify hardcoded script strings; use relative TagPrefix where possibleC-Scripts and VBScriptsScripts referencing explicit server names will failUpdate script references to match the new shortened symbolic prefixStandard APL FaceplatesAutomatic resolutionFully preserved; APL faceplates dynamically resolve server prefixesExternal OPC ClientsTag path changesRe-browse OPC namespace and update external client tag subscriptions

Important: Never rename the physical Windows computer name of an active PCS 7 server to shorten tag names. Only modify the Symbolic Computer Name in WinCC Server Data.

Verification

Verify that ServerPrefix changes resolve tag length issues:
1. Re-generate the server package on the OS Server and load it onto an OS Client.
2. In WinCC Explorer on the OS Client, verify under Tag Management that the server prefix reflects the new short name (e.g., S1::).
3. Launch an OPC test client (e.g., Siemens OPC Scout or Matrikon OPC Explorer) and connect to OPC.WinCC.
4. Browse the OPC namespace; verify that tag names now display the shortened prefix without @LOCALMACHINE:: and within length constraints.
5. Subscribe to a sample tag and confirm reliable data updates with quality Good (16#00C0).

Limitations and Operational Precautions

Essential precautions when modifying server packages:
- Modifying a server package on a live running plant disrupts tag communication on connected OS Clients until the new package is loaded and client runtime is refreshed.
- Changing symbolic server prefixes invalidates hardcoded tag paths in legacy C-scripts (GetTagDouble("OS_SRV::Tag")). Standardize on APL dynamic prefixing functions instead.
- If redundant OS servers are utilized, verify that both Server A and Server B generate identical symbolic server prefixes to maintain seamless client redundancy switchover.

Engineer FAQ

Where does the '@LOCALMACHINE::' prefix come from?

When an OPC client connects to WinCC on a standalone server or client without an imported server package, WinCC assigns @LOCALMACHINE as the default local namespace identifier.

Can I eliminate the ServerPrefix entirely on OS Clients?

No. OS Clients require a server prefix to determine which server hosts the requested tag or archive. However, on single-station systems (Single Station OS), tags do not require a prefix.

Do I have to recompile the OS to change the server package prefix?

You can change the symbolic computer name directly in the WinCC Server Data properties and re-create the package without executing a complete AS-OS compilation.