Distributing OS Clients Across Redundant WinCC OS Servers via Preferred Server Configuration

In SIMATIC PCS 7 V8.1, balancing operator station loads across a redundant WinCC OS server pair (Server A and Server B)—so that half of the OS clients connect to Server A and the other half connect to Server B—is configured using the Preferred Server setting in WinCC Server Data. In SIMATIC Manager on the Engineering Station, open the WinCC project for each OS Client, navigate to Server Data, right-click the loaded server package, and select 'Configure...'. In the configuration dialog, set 'Preferred Server' to Server A for Clients 1 and 2, and set 'Preferred Server' to Server B for Clients 3 and 4. Download the updated projects to the OS clients. In runtime, each client connects directly to its designated preferred server, automatically failing over to the partner server only if its preferred server goes offline.

Executive Summary

In control room environments operating redundant WinCC OS servers (Server A and Server B) with multiple OS Clients, all clients by default connect to whichever server is designated the active Master (typically Server A). This leaves Server A handling 100% of graphic picture requests, script processing, and operator commands, while the redundant partner server (Server B) sits largely idle, mirroring archives. To optimize network bandwidth, balance server CPU loads, and improve operator screen refresh rates, system engineers distribute client connections evenly. This guide provides the complete procedure for configuring Preferred Servers in WinCC Server Data, explaining runtime switchover and failback behavior.

Architecture and Fundamental Mechanics

The client-server communication architecture of WinCC Redundancy in PCS 7 governs how operator clients bind to redundant server pairs:
1. Default Master Connection Behavior:
- When an OS Client starts up without a designated preferred server, it evaluates the Server Data package (<Project>.pck) and connects to the active Master server.
- If four clients exist, all four connect to Server A, consuming communication sockets and graphics rendering resources on a single machine.
2. The Preferred Server Mechanism:
- In WinCC Server Data, an engineer can assign a Preferred Server (Server A or Server B) independently for each OS Client project.
- When an OS Client launches WinCC Runtime:
a. It checks the availability of its assigned Preferred Server.
b. If the Preferred Server is running (even if operating in the 'Standby' redundancy role), the client establishes its primary visualization and tag connection to that server.
c. The client queries alarms and process variables directly from its Preferred Server.
3. Dynamic Failover and Automatic Failback Mechanics:
- Failover: If an OS Client's Preferred Server fails or undergoes planned maintenance, the client automatically switches its communication channel to the redundant Partner Server within 2 to 5 seconds, maintaining continuous operator visibility.
- Failback (Switchback): When the Preferred Server restores operation and completes redundancy archive synchronization, the OS Client detects partner recovery and automatically switches its operational connection back to its Preferred Server, re-establishing the balanced 50/50 load distribution.

Step-by-Step Implementation Procedure

Compile and Generate Server Data on the Engineering Station:
a. Open SIMATIC Manager on the ES.
b. Compile the redundant OS Server project (Server A).
c. Under the OS Server object, open the 'Server Data' folder, right-click, and select Generate Server Data.
d. Confirm that the updated package file (<ServerA_Project>.pck) is generated.
Assign Server Data to All OS Clients:
a. In SIMATIC Manager, select the OS Client projects (e.g., CLNT_01, CLNT_02, CLNT_03, CLNT_04).
b. Execute Options -> 'Assign OS server...' to ensure all clients reference the redundant server package.
Configure Preferred Server for Client Group 1 (Clients 1 and 2):
a. In SIMATIC Manager, open the WinCC Explorer project for CLNT_01.
b. In the WinCC Explorer tree, expand Server Data.
c. Right-click the loaded server package (e.g., OS_SRV_A) and select Configure....
d. In the 'Configure Server Data' dialog:
- Locate the field Preferred Server.
- Select Server A from the drop-down list.
  e. Click OK and close WinCC Explorer for CLNT_01.
  f. Repeat this configuration for CLNT_02 (Preferred Server = Server A).
Configure Preferred Server for Client Group 2 (Clients 3 and 4):
a. In SIMATIC Manager, open the WinCC Explorer project for CLNT_03.
b. Expand Server Data, right-click the server package, and select Configure....
c. In the 'Configure Server Data' dialog, set Preferred Server to Server B.
d. Click OK and close WinCC Explorer for CLNT_03.
e. Repeat this configuration for CLNT_04 (Preferred Server = Server B).
Download Updated Projects to OS Clients:
a. In SIMATIC Manager, right-click each OS Client and select PLC -> Download.
b. Select the destination network paths and transfer the updated client projects.
Activate Runtime and Verify Client Connections:
a. Start WinCC Runtime on all four OS Clients.
b. Verify that Clients 1 and 2 connect to Server A, while Clients 3 and 4 connect to Server B.

Verification

On OS Server A, open Windows Task Manager or WinCC diagnostics: verify that only Clients 1 and 2 maintain active connection handles.
On OS Server B, verify that Clients 3 and 4 maintain active connection handles, confirming 50/50 balanced load.
On Client 3, open the WinCC Status banner: verify the connected server indicator shows Server B.
Gracefully deactivate Server B: verify Client 3 seamlessly fails over to Server A without process interruptions. Reactivate Server B: confirm Client 3 switches back to Server B once redundancy synchronizes.

Limitations and Operational Precautions

Ensure both redundant OS servers have identical hardware specifications and memory capacity. If one server is older or slower, allocate a higher proportion of clients to the higher-performing server rather than a strict 50/50 split.

Engineer FAQ

Can an OS Client connect to a server operating in the 'Standby' redundancy role?

Yes. If configured as the Preferred Server, an OS Client connects directly to the Standby server for process visualization and operator control.

Where is the Preferred Server configured in PCS 7?

In the OS Client's WinCC project under Server Data -> Right-click package -> Configure -> Preferred Server.

What happens if a client's Preferred Server fails?

The OS Client automatically switches its communication channel to the redundant partner server within seconds.

Do OS clients switch back automatically when their Preferred Server recovers?

Yes. Once the Preferred Server re-synchronizes with its partner, the client automatically switches back to its designated preferred server.

Does balancing clients across servers improve system performance?

Yes. It distributes network bandwidth and server CPU loads evenly, preventing a single server from becoming a bottleneck during alarm storms.

Technical References

Related Tags

Preferred Server
WinCC Redundancy
OS Clients
Load Balancing
Server Data
PCS 7 V8.1

20 respuestas

Distributing OS Clients Across Redundant WinCC OS Servers via Preferred Server Configuration