Comparing PCS 7 OS Single Station Redundancy vs OS Server Redundancy Architecture

In SIMATIC PCS 7, the fundamental difference between OS Single Station Redundancy and OS Server Redundancy lies in their architectural role and client connectivity. OS Single Station Redundancy consists of two standalone operator stations that both run individual full projects, communicate directly with the controllers, and synchronize alarms and historical archives across a redundancy link; however, OS Clients CANNOT connect to an OS Single Station pair. In contrast, OS Server Redundancy consists of a dedicated pair of redundant data servers that maintain synchronized process databases and serve up to 64 dedicated OS Client operator stations over the Terminal Bus.

Executive Summary

When architecting supervisory control and data acquisition (SCADA) layers in SIMATIC PCS 7, control engineers must choose the appropriate high-availability topology based on plant size, physical control room layout, and growth projections. A frequent source of design errors is confusing 'Single Station Redundancy' with 'Software Server Redundancy'. System integrators sometimes select Single Station Redundancy intending to attach additional operator client terminals later, only to discover that Siemens architecture strictly prohibits client connections to Single Station pairs. This article provides a rigorous comparative analysis of OS Single Station Redundancy versus OS Server Redundancy, outlining their hardware requirements, synchronization mechanisms, operational constraints, and licensing models.

Architecture and Fundamental Mechanics

SCADA Topology Comparison, Data Synchronization, and Client Connectivity Rules:

Architecture of OS Single Station Redundancy:
Consists of exactly two physical computers acting as independent operator stations (OS Single Station A and B).
In SIMATIC Manager, two OS Single Station objects are configured in the station configuration, referencing the same project.
Both stations connect directly to the Plant Bus (AS controllers) and provide local operator visualization and control.
Synchronization: The WinCC Redundancy package synchronizes Alarm Logging, Tag Logging (process archives), and User Archives across an Ethernet redundancy link or serial connection.
Crucial Architectural Restriction: OS Clients (operator terminals) CANNOT connect to a redundant OS Single Station pair. The plant is permanently limited to exactly two physical operator interfaces.
Architecture of OS Server Redundancy (Client-Server Architecture):
Consists of a dedicated pair of redundant background servers (OS Server 1 and OS Server 2) and multiple independent OS Clients.
The servers acquire data from the AS controllers over the Plant Bus, execute archive logging, evaluate alarms, and manage batch records.
The servers synchronize all runtime data continuously via the internal redundancy mechanism.
Client Connectivity: Dedicated OS Clients (up to 64 per server pair) connect over the Terminal Bus. Clients download no local process databases; they dynamically view pictures and issue commands hosted by the servers.
Multi-Server expansion: A plant can host multiple server pairs (e.g. Area 1 Server Pair, Area 2 Server Pair) with Multi-Clients viewing tags from all pairs simultaneously.
Data Synchronization Mechanics Comparison:
Both architectures utilize the WinCC Redundancy software engine.
During normal operation, both partner stations log process values and alarms independently.
If Partner A fails, Partner B continues logging independently and flags the outage.
When Partner A restarts, the internal redundancy sync agent queries Partner B's SQL database and copies all missing message and tag archives across the redundancy link to achieve identical historical records.

Step-by-Step Implementation Procedure

Architectural Selection Criteria:
* Choose OS Single Station Redundancy if:
- The plant requires exactly two operator screens (e.g. small skid, water treatment facility, or package boiler).
- There is no requirement for remote operator clients, web clients, or multi-client consoles.
- Hardware budget and control cabinet space are constrained.
- Choose OS Server Redundancy if:
- The facility requires more than two operator stations (e.g. main control room with 4 to 12 operator desks).
- Future plant expansion is anticipated.
- Centralized archiving, SIMATIC BATCH, or SIMATIC Route Control integration is required.
Configuring OS Single Station Redundancy in SIMATIC Manager:
* In Component View, insert two SIMATIC PC Station objects: OS_Single_A and OS_Single_B.
* In HW Config, configure appropriate communication cards (CP 1623 or IE General).
* In SIMATIC Manager, right-click OS_Single_A -> Options -> OS Redundancy and assign OS_Single_B as redundant partner.
* Open WinCC Explorer on the master station, open Redundancy editor, define the redundancy network connection, and activate synchronization.
Configuring OS Server Redundancy in SIMATIC Manager:
* Insert two PC Stations: OS_Server_1 and OS_Server_2.
* In HW Config of Server 1, insert a WinCC Application (redundant);
* In Server 2, insert WinCC Application (stby);
* Insert multiple PC Stations for OS Clients (WinCC Application Client).
* Assign the server data package (.pck) generated by the server pair to each OS Client.
* Download server projects to both servers and client projects to client stations.

Verification

Start Runtime on both partner stations (either Single Station pair or Server pair).
Inspect the @RedundancyState internal tag; verify that one station is Master (1) and the partner is Standby (2).
Shut down Partner A for 5 minutes while generating process alarms on the active system.
Power on Partner A; observe the status tag transitioning to Synchronizing.
After synchronization completes, verify that the Alarm Logging archive on Partner A contains all alarms that occurred during its downtime.

Limitations and Operational Precautions

Under no circumstances can WinCC OS Clients connect to an OS Single Station Redundancy pair; attempting to assign client server data to a Single Station project is rejected by the engineering system. WebNavigator clients are supported on Single Stations with strict license limits, but full OS Clients require a Server architecture. Upgrading from Single Station Redundancy to Server Redundancy requires reconfiguring station types in SIMATIC Manager and procuring new server licenses.

Engineer FAQ

Can I connect an OS Client to a redundant OS Single Station?

No. OS Clients can only connect to dedicated OS Servers. Single Stations do not generate server data packages for client connections.

How many operator stations can an OS Server pair support?

A PCS 7 OS Server pair can support up to 64 OS Clients depending on communication cycle times and network topology.

What software synchronizes data between redundant stations?

WinCC Redundancy synchronizes Alarm Logging, Tag Logging, and User Archives between partner stations.

Do redundant single stations need separate physical CP cards?

Yes, each physical computer requires its own communication processor (e.g. CP 1623 or industrial Ethernet adapter) to interface with the AS.

What happens to recipes in SIMATIC BATCH if a server fails?

SIMATIC BATCH also supports server redundancy, running on redundant batch servers to ensure uninterrupted recipe execution.

Technical References

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Comparing PCS 7 OS Single Station Redundancy vs OS Server Redundancy Architecture