Resolving PCS 7 OS Client Download Failure on Terminal Bus

A single Operator Station (OS) client in a SIMATIC PCS 7 plant refuses to download the WinCC Runtime project, or downloads successfully but the top-level navigation buttons stay blank in Runtime. All other OS clients in the same redundant server pair work normally, the project has been re-inserted, and the same client project has been tested on a different hardware image with identical results. This class of fault is almost always rooted in one of three layers: terminal-bus communication, the OS client editor configuration in the PCS 7 multiproject, or the WinCC Runtime/SQL database state on the target station.

This reference walks through the diagnostic sequence that a commissioning or service engineer should run before opening a Siemens support ticket, with the specific checks, service names, registry paths, and verification steps required to isolate the failing layer.

1. Problem Description and Symptom Matrix

Define the failure precisely before changing configuration. The matrix below maps common symptom patterns to the most likely subsystem.

Confirm which row applies to the failing client. The rest of this article assumes the most common case observed in the field: download completes (with or without a transient error), Runtime starts, and the navigation button bar is empty.

2. PCS 7 OS Client Architecture Context

A standard PCS 7 OS landscape with section-divided clients and one redundant server pair looks like this:

+-------------------+        Terminal Bus (TCP/IP)        +-------------------+
|  OS Server (Mas.) |<--------------------------------->|  OS Server (Stan.)|
+-------------------+                                    +-------------------+
        ^                                                      ^
        |   Plant Bus (S7 / Industrial Ethernet)               |
        |                                                      |
        v                                                      v
   AS 410 / AS 400 stations                              Redundancy link
        ^                                                      ^
        |                                                      |
   +----+----+----+----+                                      |
   |  10 OS Clients     |<------------------------------------+
   |  - 3 Section A     |   Each client is its own WinCC project
   |  - 3 Section B     |   + Reference clients for the others
   |  - 3 Section C     |
   |  - 1 Utilities     |
   +-------------------+

Each client in PCS 7 is a fully independent WinCC project that contains only the picture tree, area hierarchy, and authorization slices relevant to its section. Cross-section visibility is provided by reference clients in the multiproject (SIMATIC Manager → OS → Reference Client). When one client fails while the rest of the pool loads, the problem is almost always inside that single client project, not the server pair.

3. Pre-Diagnostic Checklist

1. Identify the PCS 7 version on the engineering station and the OS servers. Open SIMATIC Manager → Help → About and record the build (for example PCS 7 V9.0 SP2, V8.2 SP1, V8.1).
2. Record the OS client operating system and the WinCC Runtime version. The Runtime version must match the OS Server project version. Mismatched Runtime CD versions are a frequent cause of partial loads.
3. Capture the exact download error text from the failing transfer. A screenshot of the "Error while downloading" dialog is mandatory before any server-side change.
4. Confirm that the failing client PC is joined to the same Windows workgroup or domain as the OS servers and that the SIMATIC HMI Windows user (or the domain account used by the WinCC service) has local administrator rights on the client.
5. Verify that the SIMATIC NET and WinCC services are running. Open services.msc and check CCEServer, S7DOS, SIMATIC HMI services, and WinCC Runtime.

4. Step-by-Step Diagnostic Procedure

Run these steps in order. Stop at the first step that changes the behavior and document the result.

4.1 Verify Terminal Bus Connectivity from the Client

1. Open SIMATIC Shell on the failing client (Start → SIMATIC → SIMATIC Shell).
2. Right-click the OS Server entry and select Status. The response must show the server in Primary or Standby state. A timeout or "not found" means the terminal bus is the culprit.
3. From a command prompt run ping <OS_Server_IP> -t for 60 seconds. Loss greater than 0% on a dedicated plant network is abnormal.
4. If the network layer is OK but SIMATIC Shell still cannot enumerate, open the Windows Firewall with Advanced Security and confirm that the SIMATIC HMI and Siemens Automations License Server inbound/outbound rules are enabled on the Domain and Private profiles.

On Windows 7 / Server 2008 (the platform referenced in the original case), the default Windows firewall profile can silently block the dynamic ports used by WinCC. Apply the firewall configuration described in the PCS 7 security configuration manual.

4.2 Compare the Failing Client Project to a Working Client

1. In SIMATIC Manager open the multiproject and expand the OS tree.
2. Right-click the failing OS client → Open Object. Note the assigned OS reference, Hierarchy path, and Server preference.
3. Open a known-good client with the same role (for example another Section A client) and compare every field in the OS Client Editor dialog.
4. Pay particular attention to the Start Picture, Area tab, and the Authorization assignments. A blank top navigation bar in Runtime almost always traces back to a missing or misassigned Process Pictures root in the picture tree.

4.3 Delete and Re-download the Target Project Folder

1. On the failing OS client, stop the WinCC Runtime service.
2. Rename the project folder (default: C:\Program Files (x86)\Siemens\Automation\WinCC\WinCCProjects\<projectname>) to <projectname>_old. Do not delete it yet - the rename preserves evidence.
3. Open WinCC Explorer on the OS server, select the matching client project, and start the download.
4. Watch the WinCC download window for the exact error text. If the download now succeeds and Runtime starts with active navigation, the original project folder on the client was corrupted. Delete the _old folder after one week of stable operation.

4.4 Replace the Client with a Reference Client Copy

The fastest empirical test is to deploy a reference client in place of the failing one. This isolates whether the fault is in the client PC or in the client project definition inside the multiproject.

1. In SIMATIC Manager select the failing OS in the component view.
2. Right-click → OS → Assign OS Server... and temporarily point the failing client to a working reference client project.
3. Download and verify Runtime. If the navigation works under the reference project, the engineering project for the original client is at fault and must be re-built or restored from a backup of the multiproject.

4.5 Inspect the OS Server Package and Redundancy State

1. On the master OS server, open WinCC Explorer and select Server Data → Redundancy. Confirm the partner server is reachable and synchronized.
2. Open Server Data → Packages and verify the failing client's package is listed and timestamped to the last successful compile.
3. If the package is older than the rest, recompile the OS (Options → OS → Compile) and redistribute.

5. Root Cause Catalogue and Targeted Fixes

6. Common Download Error Messages and Resolution

PCS 7 surfaces download errors in two places: the WinCC download dialog on the engineering station and the OS Server → Download log on the target. Map the error to the layer first.

7. Reference Client Validation Procedure

The reference-client swap is the most efficient single test for the symptom set described in the source case (Runtime loads, navigation blank). Use the procedure below to execute it deterministically.

1. Open SIMATIC Manager on the engineering station and load the multiproject.
2. Right-click a working OS client of the same role → Copy.
3. Paste as a new OS under the same project and rename it to TMP_<original_name>.
4. Open the new OS, go to OS Client Editor → Server, and change the assigned OS server to the same pair the failing client uses.
5. Compile the OS (Options → OS → Compile) and download to the failing client PC.
6. If the Runtime on the failing PC now shows the full navigation bar, the fault is in the original client project, not the hardware. Restore the original project from the most recent working backup or rebuild it from the reference client template.
7. If the Runtime on the failing PC still shows an empty navigation bar, the fault is on the client PC itself - re-image or replace the unit, but first verify the network adapter driver, MTU, and any third-party endpoint security agents.

8. Terminal Bus Verification on Windows 7 and Server 2008

Windows 7 and Windows Server 2008 default to stricter DCOM and firewall rules than the older Windows XP / Server 2003 deployments many PCS 7 plants still maintain. If a single client fails on these platforms:

1. Open wf.msc and inspect both inbound and outbound rules for the SIMATIC HMI and Siemens group. Enable all rules on Domain and Private profiles.
2. Open dcomcnfg → Component Services → Computers → My Computer. Under Default Properties confirm Enable Distributed COM on this computer is checked, Default Authentication Level is Connect, and Default Impersonation Level is Identify.
3. On the COM Security tab, give the SIMATIC HMI user group Allow on both Access Permissions and Launch and Activation Permissions.
4. Reset the WinCC DCOM components from the OS server using the PCS 7 setup → Configure PC Station wizard.
5. Reboot the client in the order: OS server pair first, then the failing client, then any remaining clients.

9. Project Configuration Repair

When the diagnostic trail points at a corrupt client project in the multiproject, repair it with this sequence before reaching for a full project restore.

1. Open the OS client in the OS Client Editor. From the Area tab, click Update to repopulate the area hierarchy from the master data library.
2. Re-assign the Start Picture and the Standard Pictures on the Basic Data tab.
3. On the Hierarchy tab, confirm the plant hierarchy root is selected. A missing root node is the most common reason for an empty top navigation bar.
4. Recompile the OS (Options → OS → Compile OS) and distribute the package to the OS server pair.
5. Download the client again and watch the runtime diagnostic window for messages from the area selection module.

10. Verification Checklist

Consider the issue closed only when every item below is satisfied.

• Download completes without an error dialog, and the OS server download log records Download OK with a matching timestamp.
• WinCC Runtime starts and the top navigation bar shows the section-specific buttons (Section A, B, C, or Utilities as configured).
• Clicking each top navigation button opens the assigned process picture without an authorization prompt for the operators in the section.
• The client survives a server switchover: stop the master OS server, wait for the standby to take over, and confirm the client Runtime stays up and redraws data.
• The client survives a local reboot: reboot the client PC, confirm Runtime auto-starts and the picture tree is restored within the configured time window.
• Redundancy is preserved: both OS servers report each other in Redundancy → Status and no Partner not reachable alarm is active.

11. Prevention and Best Practices

• Keep the OS client project count in the multiproject as low as practical. Where section-specific picture trees are required, use a single template OS client and replicate from it instead of hand-building each one.
• Maintain a versioned backup of the multiproject database (*.s7p archive) and the OS project files outside the engineering station. PCS 7 backups of the OS server alone do not capture the client project state.
• Apply the Siemens antivirus exclusion list to every OS client. The standard exclusions cover the WinCC project folder, the Siemens installation root, and the SQL Server data directories.
• Document the service account used by SIMATIC HMI and the local SIMATIC HMI group membership for every OS station. Drift here is the single most common cause of intermittent download failures after a Windows update.
• Before any PCS 7 service pack upgrade, run a controlled OS client download in a test project. The Runtime version on the client must match the OS Server project version exactly.

12. Escalation Path to Siemens Support

If the diagnostic sequence above does not resolve the failure, escalate with the following evidence package to Siemens Industry Online Support:

1. PCS 7 version and installed service pack on the engineering station and on every OS server.
2. WinCC Runtime version on the failing client, and a screenshot of Help → About on the OS server.
3. Screen capture of the download error dialog including any numeric error code in parentheses.
4. OS server download log file from C:\Program Files (x86)\Siemens\Automation\WinCC\WinCCProjects\<project>\<server>\<servername>_<timestamp>.LOG for the failing transfer.
5. Output of SIMATIC Shell → Status for both servers from the failing client.
6. Result of the reference-client swap test (Section 7) - did the navigation bar appear under the reference project?

13. Quick Diagnostic Flow

Start
  |
  v
Can SIMATIC Shell see both OS servers?
  |-- No --> Fix terminal bus, firewall, service account, DCOM
  |
  v
Does the download complete (with or without error)?
  |-- No --> Recompile OS, check OS server package, antivirus, folder ACL
  |
  v
Does Runtime start?
  |-- No --> Check WinCC Runtime version, SQL DB state, license
  |
  v
Is the top navigation bar empty?
  |-- Yes --> Re-apply area hierarchy / start picture in OS Client Editor
  |
  v
Deploy reference client in place of failing one
  |-- Works --> Original client project is corrupt, restore or rebuild
  |-- Fails --> Client PC issue, re-image or replace
  |
  v
Verify all items in Section 10 checklist before closing ticket