SIMIT OPC DA Client: Resolving Remote Server Connection Failures

Problem Overview

The SIMATIC SIMIT Simulation Platform OPC DA client coupling is a documented integration path for connecting SIMIT virtual commissioning projects to live process data sources. Field installations routinely fail with the message OPC server cannot be reached when the client attempts to subscribe to a remote OPC Classic server, even when the same workstation can browse and subscribe to that same server through third-party OPC explorers (Matrikon OPC Explorer, Siemens OPC Scout V10) without issue.

This error does not indicate a network problem, a hostname resolution failure, or a missing server. It is a deterministic failure that originates inside the SIMIT OPC DA client runtime and is governed by the OPC Data Access specification compliance of the SIMIT build installed on the engineering workstation.

Symptoms

The failure presents as a fast, near-instantaneous error response. Typical symptoms reported in the field:

• SIMIT project starts in RUN mode and the OPC DA client coupling transitions to Faulted state within 1-2 seconds.
• Message log shows OPC server cannot be reached immediately on coupling activation, before any subscription is attempted.
• Adding a known tag (point) to the coupling produces the identical message when the project is run.
• The remote server is reachable: Matrikon OPC Explorer, Siemens OPC Scout V10, and OPC Expert can browse, subscribe, and read/write tags on the same machine.
• ping, Test-NetConnection on port 135 (RPC endpoint mapper), and telnet to the OPC server's port all succeed.
• Wireshark trace shows no SIMIT-originated DCOM bind traffic to the remote host - the SIMIT client aborts the connection during initial capability negotiation.

Root Cause Analysis

Three independent root causes produce this fault. They are additive: fixing one often exposes the next.

Cause 1 - OPC DA Specification Version Mismatch (Primary)

SIMIT V8.0 and the V8.1 base release ship with an OPC DA client that supports only OPC Data Access 3.0. Earlier specifications (OPC DA 1.0 and 2.0) are intentionally disabled at the COM proxy layer. Many production OPC servers (Siemens WinCC, PCS 7, older Matrikon Simulation Server, custom in-house DCOM servers) advertise themselves as OPC DA 1.0 or 2.0 servers, or as multi-version servers that prioritize legacy handshakes.

When SIMIT V8.0 sends an IOPCCommon::GetStatus or IOPCServer::GetStatus call, the proxy only accepts the DA 3.0 OPCSERVERSTATUS structure. A 1.0/2.0 server returns a structure of a different binary layout, and SIMIT's strict typed unmarshaller rejects the response. The COM runtime then surfaces this as RPC_E_DISCONNECTED (0x80010108) or simply a failed bind, which SIMIT renders as OPC server cannot be reached.

Affected versions and the fix matrix:

Cause 2 - DCOM Security and OPCEnum Permissions

Even with the DA version mismatch resolved, a remote OPC server still requires a fully configured DCOM path on both endpoints. SIMIT must be able to:

1. Resolve the remote host via NetBIOS or DNS.
2. Contact the OPCEnum service on the remote host (registered as a COM catalog enumerator over RPC port 135).
3. Launch the OPC server process on the remote host under a security context that has access to the server's AppID.
4. Establish a callback channel from the server back to the SIMIT client (reverse COM connection).

Disabling DCOM security across the board is a valid diagnostic step on an isolated test network, but it does not resolve the OPCEnum ACL on Windows Server installations where the OpcEnum service runs under the local system or a service account that has not been granted remote launch rights.

Cause 3 - Clock Skew Between Client and Server

OPC DA 3.0 added a server keep-alive and band-based update mechanism that includes a ServerBandwidth negotiation. SIMIT's COM marshalling layer validates the Bandwidth field against a window derived from the local system clock. When the client and server clocks differ by more than the configured tolerance (default 5 minutes in OPC DA 3.0 spec, but some servers enforce tighter windows), the connection is dropped before any tag read is attempted. The error path here also manifests as OPC server cannot be reached in SIMIT because the disconnect occurs during IConnectionPoint::Advise.

Diagnostic Procedure

Run these checks in order. Each one isolates one of the three root causes above.

1. Verify SIMIT build version. Open SIMIT, navigate to Help → About. Confirm whether V8.0, V8.0u1, V8.1 base, V8.1u1, V9.0+, or V10/V11 is installed. Cross-reference against the table in Cause 1. If the build is V8.0/V8.0u1/V8.1 base, stop here and plan an upgrade before continuing.
2. Verify remote server OPC DA compliance. On the engineering workstation, open Matrikon OPC Explorer or OPC Scout V10 and connect to the remote server. Right-click the server entry and read the Specification or Server Capabilities dialog. Note whether it advertises DA 1.0, 2.0, 3.0, or multiple. If it is DA 1.0/2.0 only and SIMIT is pre-V8.1u1, the upgrade is mandatory.
3. Validate DCOM path independently. Use dcomcnfg on both machines, expand Component Services → Computers → My Computer → DCOM Config. Locate the target server's AppID and confirm the launch and access permissions include an account that exists on both machines. Then verify the OPCEnum service is running: sc \\REMOTE_HOST query OpcEnum. If stopped, start it and set the startup type to Automatic.
4. Check clock skew. Run w32tm /monitor /computers:REMOTE_HOST on the SIMIT workstation. A skew greater than 30 seconds between the two systems is a problem. Configure a common NTP source: w32tm /config /manualpeerlist:"time.nist.gov" /syncfromflags:manual /reliable:YES /update and force a resync: w32tm /resync /force.
5. Capture COM traffic. Install Wireshark on the SIMIT machine and filter on dcerpc during the SIMIT coupling activation. You should see: (a) a TCP SYN to port 135, (b) an ep_map (endpoint map) request, (c) a bind to the server's AppID GUID, and (d) an IOPCServer::GetStatus call. The absence of any traffic confirms SIMIT is rejecting the server during the COM proxy selection, which is the signature of the DA version mismatch.

Resolution

Resolution A - Upgrade SIMIT (Permanent Fix)

Apply the upgrade path indicated in the affected-versions table. For V8.x sites that cannot move to V9/V10, install V8.1 Update 1 over the existing V8.1 installation. The update preserves the project files, coupling configurations, and license keys. After installation:

1. Open the project containing the OPC DA client coupling.
2. Re-validate the coupling: open Couplings → OPC DA Client → Properties and confirm the server CLSID resolves to a DA 1.0/2.0/3.0 server.
3. Start the project in RUN mode. The OPC server cannot be reached message should not appear; the coupling should transition to Connected within 2-3 seconds.

Resolution B - Configure DCOM Properly

On the remote OPC server host (Windows machine hosting the OPC server):

1. Open Component Services via dcomcnfg.exe as Administrator.
2. Navigate to Component Services → Computers → My Computer. Right-click and select Properties.
3. On the Default Properties tab: enable Enable Distributed COM on this computer, set the default authentication level to Connect (or Packet Privacy for production), and the default impersonation level to Identify.
4. On the COM Security tab: under Access Permissions and Launch and Activation Permissions, add the user account under which the SIMIT client is running (typically a domain user or a local user mirrored on both machines with the same password). Grant both Local Access and Remote Access.
5. Under DCOM Config, locate the OPC server's AppID. Right-click and open Properties. On the Security tab, override the defaults to give the SIMIT user Launch and Activation rights.
6. On the Endpoints tab, ensure that at least one endpoint uses a static port and that the Windows Firewall allows inbound traffic on that port. For dynamic ports, add the OPC server to the Windows Firewall exception list.
7. Confirm the Remote Registry service is running on the server (sc query RemoteRegistry). It is required for COM activation over the network.

On the SIMIT engineering workstation:

1. Open dcomcnfg and confirm Enable Distributed COM is on.
2. Under COM Security → Access Permissions, add the ANONYMOUS LOGON and

   EVERYONE groups with Allow Local Access and Allow Remote Access. This is a diagnostic concession; tighten it for production.

3. Add the remote OPC server's hostname or IP to the Windows Firewall inbound rule set for %SystemRoot%\System32\dllhost.exe.

Resolution C - Synchronize Clocks

Both the SIMIT workstation and the OPC server host must have a time offset of less than 5 seconds. Use the same NTP source for both:

1. On both machines, set the Windows Time service to automatic: sc config W32Time start= auto and net start w32time.
2. Configure both to peer with the same authoritative time source: w32tm /config /manualpeerlist:"PTB.DE" /syncfromflags:manual /reliable:YES /update (substitute the corporate NTP host as required).
3. Force an immediate resync: w32tm /resync /force.
4. Verify: w32tm /query /status should report Phase: Locked and a small offset (sub-second).

Resolution D - Migrate to OPC UA Client Coupling (V10 / V11.3)

From SIMIT V10 onward, the legacy OPC DA client coupling is replaced by the OPC UA client coupling as described in the official SIMIT V11.3 SP manual (Siemens ID 109996325). The migration path is:

1. Open the SIMIT project. In the project navigation, right-click Couplings and select New coupling.
2. From the coupling type list, select OPC UA client.
3. Enter the OPC UA server endpoint URL: opc.tcp://<remote-host>:4840 (or the manufacturer's published port).
4. Configure security policy. For a non-production network, None is acceptable; for production, use Basic256Sha256 with a signed certificate.
5. Browse the UA address space from the SIMIT coupling editor and import the desired nodes.

The OPC UA path removes DCOM entirely and is not subject to the DA 1.0/2.0/3.0 specification negotiation that causes the legacy OPC server cannot be reached error.

Verification

After applying the resolution, confirm the fix with the following checks:

1. Open the SIMIT project and start it in RUN mode.
2. In the coupling status panel, the OPC DA client (or OPC UA client) coupling should report Connected within 3 seconds of project start.
3. Force a tag read in the SIMIT signal monitor for a known point. The value should update within the configured update rate (default 1000 ms).
4. Check the message log: the OPC server cannot be reached message must be absent. Acceptable log entries are OPC server connected and per-tag Subscription created entries.
5. Disconnect the remote server network cable (or stop the OPC server service) and confirm SIMIT reports a deterministic error such as OPC server lost rather than the generic cannot be reached - this confirms the underlying COM/UA path is functioning.

Error Code Reference

Platform Compatibility Matrix

Field-Proven Caveats

• The OPC server cannot be reached message text in the SIMIT log is generic. It is the same string for DCOM failure, DA version mismatch, clock skew, and outright network unreachability. Always begin with the diagnostic procedure above rather than treating the message as a definitive diagnosis.
• Disabling DCOM security globally (COM Security → Edit Limits → All Anonymous Users, all permissions) is acceptable on a fully isolated engineering VLAN, but it should never be the final state on a machine connected to a production control network.
• The SIMIT OPC DA client uses the workstation's interactive logon identity for DCOM authentication. If SIMIT is run as a service (Windows Service, not interactive), the service account must be granted the same DCOM rights on the remote host.
• For SIMIT V10/V11.3 OPC UA client couplings, the server certificate must be added to the SIMIT trust store. SIMIT does not honor the Windows certificate store for UA trust - it maintains its own PKI directory at %ProgramData%\Siemens\Automation\SIMIT\<version>\pki\trusted.
• When migrating from a SIMIT V8.x OPC DA coupling to a V10/V11.3 OPC UA coupling, the tag names do not transfer automatically. The UA address space uses a different namespace; tags must be re-mapped in the SIMIT signal editor.

Related Diagnostics - Process Simulate 12 Integration

SIMIT is also deployed as an OPC server to upstream tools such as Tecnomatix Process Simulate 12. In that topology, Process Simulate subscribes to a SIMIT OPC server. The same DA version rules apply in reverse: if Process Simulate's OPC client supports only DA 1.0/2.0, the SIMIT V8.0 DA 3.0-only build is incompatible. The OPC UA path is the recommended forward direction for these integrations as well.

FAQ