Expanding Redundant Single-Station OS to Multi-User Client-Server in PCS 7

Expanding an existing SIMATIC PCS 7 redundant single-station architecture (two independent OS single stations with redundancy) to add local monitoring for a new plant section can be accomplished by either converting the system to a true OS Server-Client architecture or adding a Reference OS single station. Converting to a redundant OS Server with OS Clients is the technically superior, guardrail-compliant solution: it provides centralized archive synchronization, operator audit trails, and unified alarm tracking across all stations, but requires Windows Server on both OS Servers. Alternatively, adding a Reference OS single station minimizes engineering by sharing the main OS project, but leaves historical data and operator messages isolated without alarm synchronization.

Architectural Evaluation: Single Station vs Client-Server

In SIMATIC PCS 7, a Redundant Single-Station (Redundante Einzelplatzstation) consists of two standalone OS stations communicating independently with the AS controllers while cross-communicating via a serial or Ethernet redundancy link for tag logging and alarm synchronization. When a new facility area (such as a tank farm or utility room) requires local visualization and control, engineering teams face the choice between expanding the standalone topology or converting to a distributed OS Client/Server model.

A third single station cannot participate in the redundancy synchronization pair of the existing two stations. In PCS 7 WinCC Redundancy, synchronization is strictly limited to exactly two partner servers. Consequently, any third standalone station operates as an independent node with separate database archives and uncoordinated alarm acknowledgment.

Option Analysis: Upgrade to Server-Client, Reference OS, or Panel

Three architectural options exist for adding visualization to the new plant section:

Conversion to Redundant OS Server + OS Client (Recommended):
- Upgrade the two single stations into a redundant OS Server pair using server upgrade licenses.
- Add an OS Client at the tank farm. The OS Client loads the server data package, requiring zero local database maintenance.
- Operating System Requirement: Both OS Servers must run Microsoft Windows Server (e.g., Windows Server 2012 R2 or 2016). While legacy WinCC allowed up to 3 clients on desktop Windows for non-redundant servers, redundant OS servers in PCS 7 strictly require Windows Server operating systems. The OS Client can run standard Windows 7 or Windows 10.
Adding a 3rd Single Station as a Reference OS (Referenz-OS):
- A Reference OS is an OS object in SIMATIC Manager that points directly to the configuration of a base OS single station.
- Only the base OS is compiled and edited; the Reference OS automatically inherits pictures, tags, and scripts, dramatically reducing configuration effort.
- Disadvantage: Requires a full Single Station Runtime license. Most critically, alarms acknowledged on the third station do NOT clear on the main redundant pair, and operator control actions (Bedienmeldungen) are logged only locally.
Local Operator Panel (SIMATIC Comfort Panel / Web Client):
- For monitoring-only or simple pump start/stop, a SIMATIC Comfort Panel or WinCC WebUX / Web Navigator client can connect directly to the AS or OS Server.

Technical and Licensing Comparison Matrix

The following table compares the architectural alternatives for expanding the PCS 7 visualization system:

Expansion OptionOperating System RequirementLicensing RequirementsAlarm & Tag SynchronizationEngineering Maintenance EffortRedundant Server + OS ClientWindows Server on Servers; Win 10 on ClientSingle Station to Server Upgrade + OS Client RT100% Full Synchronization across all clientsSingle project maintenance on server3rd Single Station (Reference OS)Windows 10 or Windows ServerFull OS Single Station RT License + AS/OS RT POsIsolated; No synchronization with redundant pairLow (Inherits configuration from Base OS)3rd Independent Single StationWindows 10 or Windows ServerFull OS Single Station RT License + AS/OS RT POsIsolated; Independent archives and alarmsHigh (Separate compile, graphics, and tag management)WinCC Web Navigator / WebUXHost OS Server OS; Browser on Web ClientWeb Navigator Server License (e.g., 1-client or 3-client)Server-synchronized via web sessionMinimal (Uses existing WinCC pictures published to web)

[!NOTE]
Commercial Accuracy Guardrail: Exact software MLFBs (such as 6ES7658-2AB58-0YD5 for Server upgrades or 6ES7658-2CX58-0YB5 for OS Clients) depend on the exact PCS 7 release version (V8.0, V8.2, or V9.0) and runtime Process Object quantities. Licensing requirements must be validated using the Siemens Catalog ST PCS 7.

Engineering Procedure for Adding a Reference OS Single Station

If selecting the Reference OS single station approach to avoid immediate operating system upgrades:

In SIMATIC Manager Component View, right-click the Multiproject and insert a new SIMATIC PC Station.
Open the PC Station Hardware Configuration and assign the local network card and WinCC Application.
In the tree, right-click the PC Station -> Insert New Object -> Reference OS.
Open the Properties of the Reference OS. Under the Target OS tab, browse and select the existing Base OS Single Station.
In the Path to Target OS Computer, enter the network share path to the local project folder on the third station.
Compile the Base OS. When compilation completes, right-click the Reference OS and select PLC -> Download.
Start WinCC Runtime on the third station. The station runs an identical project instance using its own local SQL database.

Verification

To verify the multi-station operational behavior:
1. If configured as an OS Client, verify that opening a picture on the client displays server tags with instantaneous updates.
2. Acknowledge an active alarm on the client or third station. Confirm whether the alarm horn and alarm banner acknowledge simultaneously on the main control room stations.
3. Trigger a manual motor start from the new station and verify that the Operator Input Message is recorded in the central Alarm Logging journal.
4. Deactivate the new station for 30 minutes, reactivate it, and observe whether tag logging trends exhibit missing gaps or backfill historical data.

Limitations and Operational Precautions

No Alarm Synchronization on Reference OS: Standalone and Reference OS single stations do not synchronize alarms. If an operator acknowledges an alarm at the tank farm, the central control room alarms will continue to flash and sound audible horns.
Audit Trail Integrity: Regulated pharmaceutical or chemical processes requiring FDA 21 CFR Part 11 compliance cannot use isolated single stations because operator actions cannot be consolidated into a single tamper-proof audit trail.
Network Bandwidth: Each single station opens direct S7 connections to the S7-400 CPU, consuming controller communication resources. OS Clients connect to the OS Server, preserving CPU communication channels.

Engineer FAQ

Can an OS Server run on Windows 7 or Windows 10 in PCS 7?

In PCS 7 V8.x and V9.x, redundant OS Servers strictly require Microsoft Windows Server operating systems. A non-redundant OS Single Station acting as a server with up to 3 clients was historically permitted on workstation OS, but redundancy requires Windows Server.

What happens if the Base OS is compiled while the Reference OS is running?

Compiling the Base OS updates the base project database. To apply changes to the Reference OS, the Reference OS must be closed and re-downloaded from SIMATIC Manager.

Does an OS Client consume AS Process Object (PO) licenses?

No. AS RT PO licenses are consumed exclusively by the controller, and OS RT PO licenses are booked on the OS Server. OS Clients only require an OS Client Runtime software license.

Can I use a SIMATIC Comfort Panel instead of a PC-based OS Client?

Yes, Comfort Panels can communicate with S7-400 controllers via Industrial Ethernet, but they require independent TIA Portal engineering, lack APL faceplate integration, and do not synchronize alarms with PCS 7 WinCC.