1. Overview
SIMATIC PCS 7 V8.1 supports a redundant OS server pair (Master and Standby) that shares the runtime data of a process via server packages. In a typical PCS 7 plant there are four or more OS clients that connect to the OS server pair. By default WinCC distributes client connections across both servers evenly, but in many plants the load must be assigned explicitly: half the clients prefer Server A (the master at startup), the other half prefer Server B (the standby at startup). The configuration point is the WinCC Explorer Server Data editor of the client project on the Engineering Station (ES), where the Preferred Server column is set for each server package. The change is then compiled and downloaded from the ES to the affected OS client stations.
This article documents the full procedure for PCS 7 V8.1 (on Windows 7 for the OS clients and Windows Server 2008 R2 for the OS servers / ES), the prerequisites, the runtime implications of the Preferred Server setting, and the common pitfalls encountered when technicians edit the client locally instead of pushing the configuration from the ES.
2. Prerequisites
Before assigning Preferred Servers, verify the following items are in place. The configuration procedure assumes the redundant OS server pair is already commissioned and the OS client stations are integrated into the PCS 7 project.
| Item | Required value / state | How to verify |
|---|---|---|
| PCS 7 version | PCS 7 V8.1 (V8.1 SP1 or later recommended) | SIMATIC Manager → Help → About → "Installed PCS 7 Version" |
| WinCC version | WinCC V7.3 (PCS 7 V8.1 ships with this WinCC base) | Same dialog as above, "Installed WinCC Version" |
| ES operating system | Windows 7 SP1 (x64) or Windows Server 2008 R2 | winver from command prompt |
| OS client OS | Windows 7 SP1 (x64) - PCS 7 V8.1 does not support Windows 10 clients | winver |
| Redundant server pair | Server A (Master) + Server B (Standby) configured and synchronized | WinCC Explorer on the master → Redundancy → Status → "Master" |
| Server packages | At least one server package generated on the server project | Server project in WinCC Explorer → Server Data → packages listed |
| OS client projects | Each client exists as a project in the PCS 7 multiproject on the ES | SIMATIC Manager → component view → OS client object |
| User rights | Local Administrator on the ES, on every OS client, and on both OS servers | Computer Management → Local Users and Groups |
| Network | ES, server pair, and clients in same TCP/IP broadcast domain (or DNS resolvable) | ping serverA, ping serverB from ES and from each client |
| Time sync | All stations synchronized to a common NTP or domain controller | w32tm /query /status |
Reference: SIMATIC PCS 7 V8.1 Manual Collection and the WinCC V7.3 documentation available on the Siemens Industry Online Support portal.
3. Redundant Server Architecture in PCS 7 V8.1
PCS 7 V8.1 implements WinCC redundancy in two layers. The OS server pair runs an event-synchronized process image: when the master archives a value, a redundant write is propagated to the standby. On a controlled failover, the standby becomes master without operator-visible loss of the last acknowledged alarm.
The OS clients do not connect to both servers simultaneously. They connect to one server at a time, and they re-establish the connection to the surviving server on a failure event. The Preferred Server setting on the OS client is a startup preference, not a hard binding - the client still follows the redundancy partner on a failure.
| Concept | Behavior | Configuration location |
|---|---|---|
| Master / Standby | One server holds write authorization for archive and tags; the other receives event-synchronized copies | Server project → Computer properties → Redundancy |
| Server package | Logical container that defines which process data, alarms, and archives are served to which client group | Server project → Server Data → package list |
| Preferred Server | Which server the client tries to connect to first at startup | Client project → Server Data → "Preferred Server" column |
| Standard Server (legacy) | Older WinCC concept; replaced by Preferred Server for redundancy | Same dialog, retained for compatibility |
| Failover | Automatic: client loses connection → reconnects to surviving partner within 3-10 s depending on keep-alive settings | Server side: Redundancy → Watchdog parameters |
Splitting four clients into two groups of two is a standard topology: clients 1 and 2 are configured to prefer Server A, clients 3 and 4 to prefer Server B. At runtime, each client connects only to its preferred server; on a server failure, all surviving clients reattach to the live server.
4. Server Packages and the Preferred Server Concept
Server packages are defined on the server project in WinCC Explorer under Server Data. A package groups tags, alarms, and archive data for distribution. A typical PCS 7 plant will have 4 to 12 server packages such as:
- PACKAGE_AREAA_PROCESS
- PACKAGE_AREAA_ALARMS
- PACKAGE_AREAB_PROCESS
- PACKAGE_AREAB_ALARMS
- PACKAGE_UNIT1_ARCHIVE
- PACKAGE_UNIT2_ARCHIVE
On the client project, the same dialog lists the packages that the client is allowed to receive. The "Preferred Server" column accepts the symbolic computer name of the OS server (for example \OSSRV01 or \OSSRV02). The setting is per package, so a single client can prefer Server A for alarm packages and Server B for archive packages - this is unusual and not recommended.
5. Step-by-Step Configuration Procedure
The procedure below is the canonical way to assign two clients to Server A and two clients to Server B in a four-client PCS 7 V8.1 system.
5.1. Open the client project on the Engineering Station
- Start SIMATIC Manager on the ES.
- Open the PCS 7 multiproject.
- In the component view, expand the OS object that represents Client_1.
- Right-click the OS object and choose Open Object → WinCC Explorer (or start WinCC Explorer manually and load the client project: File → Open → select client project → Open).
- WinCC Explorer opens with the client project loaded. The computer icon now represents the OS client station, not the server.
5.2. Open the Server Data editor
- In the navigation tree of WinCC Explorer, expand the Server Data node.
- Right-click Server Data and choose Configure.
- The Server Data configuration table opens. Each row represents one server package that is available to this client; each column is a property of the assignment.
5.3. Set the Preferred Server column
- Locate the column labelled Preferred Server.
- Click the cell in the column for the first server package. A drop-down list appears with the symbolic computer names of the OS servers in the project (\OSSRV01, \OSSRV02 in this example).
- Select the server that this client should connect to first. For Client_1 and Client_2 choose
\OSSRV01; for Client_3 and Client_4 choose\OSSRV02. - Repeat for every server package listed for this client. A bulk fill is possible by selecting all rows, right-clicking the column header, and choosing Fill Down (or by using the keyboard combination after selecting multiple cells).
- Click OK to apply.
5.4. Example assignment table
| OS Client | Preferred Server (all packages) | Standby fallback |
|---|---|---|
| Client_1 (OS01) | \OSSRV01 (Master at startup) | \OSSRV02 |
| Client_2 (OS02) | \OSSRV01 (Master at startup) | \OSSRV02 |
| Client_3 (OS03) | \OSSRV02 (Standby at startup) | \OSSRV01 |
| Client_4 (OS04) | \OSSRV02 (Standby at startup) | \OSSRV01 |
5.5. Save and close WinCC Explorer
- File → Save in WinCC Explorer (the project change is committed to the local project database on the ES).
- Close WinCC Explorer.
6. Compile and Download to the OS Client
Changes made on the ES must be transferred to the runtime OS client by compile/download. The download overwrites any local configuration on the client.
- Return to SIMATIC Manager.
- Right-click the OS client object (for example OS01).
- Select PLC → Compile and Download Objects, or for a single client OS → Compile → Compile and Download Object.
- In the dialog, enable Check consistency before download to catch missing server packages or unresolved tags.
- Confirm the target computer. SIMATIC Manager uses the computer name configured in the OS object; verify that it matches the actual Windows computer name of the OS client (case-insensitive).
- Click Download. The download pushes the updated client project, including the new
ServerData.iniwith the Preferred Server column, to the OS client. - Wait for the success message. The download typically takes 30 s to 3 min depending on the project size and network.
- Repeat for OS02, OS03, OS04.
7. Verification
After the download, verify on the OS client that the Preferred Server change is effective.
- On the OS client, start WinCC Explorer in service mode (or open the project with WinCC Runtime stopped).
- Open Server Data → Configure. Confirm that the Preferred Server column matches the assignment table in section 5.4.
- Start WinCC Runtime on the client. After the splash screen, the main process picture should appear within 20-40 s.
- Open the Diagnosis window of WinCC Runtime (Ctrl + D, or menu Tools → Diagnosis) and check the Connections tab. The connected server should be the Preferred Server.
- For a deeper check, evaluate the internal tag
@ServerNamein a graphics object or via WinCC Tag Simulator.@ServerNamereturns the symbolic name of the server that is currently serving the client.
7.1. Optional: simulate a failover
- On the master server, stop the WinCC service:
net stop "CCAlgService"followed bynet stop "WinCC"(do this only in a maintenance window). - Watch the OS clients. Within 3-10 s the diagnosis window should show the surviving server.
- When the original server is brought back up, the redundancy re-syncs and the master role may be re-assigned to the original server; clients remain connected to whichever server is master.
8. Troubleshooting Matrix
| Symptom | Likely root cause | Resolution |
|---|---|---|
| "Preferred Server" column is greyed out | Server packages not yet generated on the server project | Open the server project, right-click Server Data → Generate Server Data, return to the client project |
| Client connects to the non-preferred server at startup | Preferred Server symbolic name does not match the Windows name; DNS / WINS resolution issue | Open the Server Data editor on the client and confirm the symbolic name (for example \OSSRV01) resolves in ping from the OS client |
| Local edit on the OS client disappears after a few days | Someone re-downloaded the client from the ES and overwrote the local change | Move all client configuration to the ES and download from there; document the procedure |
| Download fails with "Computer not reachable" | Firewall on the OS client blocks DCOM / SMB; service account mismatch | Open ports 135, 139, 445 and the DCOM range; verify that the SIMATIC Manager user is local admin on the client |
| Client shows "No connection to server" with both servers up | Server packages have not been activated on the client (the column "Activate" is unchecked) | Open the Server Data editor on the client, enable the Activate column for every package, save, download again |
| Runtime starts but graphics show "No values" | The client receives a package from a server whose project differs from the one the graphics object references | Re-compile the OS, then re-download to all clients in a single batch to ensure version consistency |
| Failover does not happen after stopping the master | Watchdog time too long, or the redundancy partner is not licensed for the same packages | Verify the license of Server B (WinCC RT / Archiving / Redundancy); lower the redundancy timeout in the server project |
9. Best Practices
- Edit only on the ES. All OS client configuration, including the Preferred Server column, must be performed on the ES and downloaded. Local edits on the client are temporary and will be overwritten.
- Use symbolic computer names, not IP addresses. The Preferred Server column accepts the symbolic name (\OSSERVER01) which is resolved at runtime. Hard-coded IP addresses break when the server hardware is replaced.
- Distribute clients evenly. In a four-client topology, prefer two clients per server. Unbalanced assignments defeat the purpose of load distribution.
- Test failover at least once per year. Schedule a maintenance window to stop the master and confirm that all four clients reconnect to the standby.
-
Monitor with @ServerName. Add a small text field in the standard project header that displays
@ServerName, so operators immediately see which server is serving their client. - Document the assignment. Maintain a project-specific matrix (see section 5.4) in the project documentation. Future maintenance windows often rotate personnel, and the matrix prevents guesswork.
- Re-compile after any server-side change. If server packages are added or removed, re-generate Server Data on the server, then re-download every client. A stale client project will silently ignore packages it does not recognize.
10. Related Parameters and Internal Tags
WinCC exposes several internal tags that help verify the Preferred Server setting at runtime. These tags can be added to graphics objects for diagnostic purposes.
| Internal tag | Meaning | Typical use |
|---|---|---|
| @ServerName | Symbolic name of the server currently serving the client | Header text, e.g. "Served by: @ServerName" |
| @RM_SERVER_NAME | Name of the redundancy master (visible on both master and standby) | Confirm master/standby role from any client |
| @RM_MASTER_NAME / @RM_STANDBY_NAME | Configured master / standby computer names | Distinguish configured roles from actual runtime role |
| @DBSERVER | Database server name (relevant for archive redundancy) | Verify archive failover |
| @RedundancyState | 0 = no redundancy, 1 = master, 2 = standby | Drive a status icon in the picture |
The actual symbol names vary slightly between PCS 7 versions. Always confirm the exact name with the WinCC Tag Manager on the running project (right-click Internal Tags → Filter → type @).
11. Comparison: Preferred Server vs Standard Server
Older WinCC projects used the Standard Server concept. The Preferred Server column was introduced to allow a per-package assignment and to clarify the redundancy semantics. New PCS 7 V8.1 projects should always use Preferred Server.
| Feature | Standard Server (legacy) | Preferred Server (PCS 7 V7.1+) |
|---|---|---|
| Configuration scope | One setting per client | One setting per server package per client |
| Redundancy awareness | Implicit | Explicit; failover is well-defined |
| Recommended for new PCS 7 V8.1 projects | No | Yes |
| Backward compatible | Yes (read-only on new clients) | Yes (new clients read the legacy setting on first load) |
Reference: SIMATIC PCS 7 V8.1 Process Control System manual and the WinCC V7.3 Information System under Redundant Systems.
12. Operational Notes
- The Preferred Server column is interpreted at the moment the OS client runtime starts. Mid-session changes to the Server Data on the server side do not retroactively change a running client; the client keeps its current connection until the next runtime restart or failover.
- If a project migrates from a single-server PCS 7 to a redundant pair, the Preferred Server column on every client is automatically set to the original (now master) server on the first compile after migration. The procedure above is still required to re-distribute the clients to the new standby.
- When PCS 7 V8.1 is upgraded in-place to V8.2 or V9.0, the Preferred Server setting is preserved. No re-configuration is required unless the symbolic computer names of the servers are changed.
- The number of OS clients per OS server is licensed. PCS 7 V8.1 OS server licenses allow up to 32 clients per server, but the practical limit is the network and process tag throughput (typically 8-12 clients per server for a busy plant).
FAQ
Where is the Preferred Server column configured in PCS 7 V8.1?
Open the OS client project in WinCC Explorer on the Engineering Station, right-click Server Data, choose Configure, and set the Preferred Server column for every server package. Save the project, then compile and download the OS client from SIMATIC Manager.
Can the Preferred Server be edited directly on the OS client?
Technically yes - WinCC Explorer on the client can open the same dialog. However, the next compile/download from the ES will overwrite the local change. Always edit on the ES and download to the client to keep the configuration under version control.
Why is the Preferred Server column greyed out on my client project?
The server packages have not been generated on the server project. Open the server project in WinCC Explorer, right-click Server Data, and select Generate Server Data. After regeneration, restart the client project in WinCC Explorer and the column will be editable.
How do I verify which server an OS client is currently connected to?
Open the Diagnosis window in WinCC Runtime (Ctrl+D) on the client and check the Connections tab. Alternatively, evaluate the internal tag @ServerName in a graphics object - it returns the symbolic name of the active server.
What happens to the client connections when the master server fails?
The OS clients detect the loss of connection within the configured watchdog time (typically 3-10 s) and automatically reconnect to the surviving standby server. The Preferred Server setting is only the startup preference; failover is automatic and the Preferred Server column does not need to be changed for failover to work.