Sinumerik 840Dsl OPC UA Alarm Subscription Troubleshooting

Problem Overview

The Sinumerik 840Dsl NCU (Numeric Control Unit) exposes its alarms and events through a dedicated OPC UA endpoint distinct from the standard tag namespace. A common field failure pattern is the following: the OPC UA license has been loaded onto the controller, browsing of the tag (data) address space succeeds, but alarm/event subscription either returns an empty list, hangs the client at the Create Subscription call, or causes the server-side handler to crash when the monitored items return no items. The third-party quick-start Alarm & Condition clients (UA Expert, Ignition OPC UA Client, Prosys OPC UA Browser) report Connected in the session status but never display the SINUMERIK server node under the alarm view, because the client has not navigated to the event-notification node ns=0;i=2253 (Server) or the Sinumerik-specific SinumerikEventServer object that publishes condition instances.

This article documents the root cause, the firmware/license preconditions, the correct client-side configuration of the OPC Classic A&E tool AEClient.exe, the OPC UA event subscription model used by Sinumerik, the severity/condition mapping, and the known connection-break issue observed when the event buffer is empty.

Prerequisites and Licensing

Before any alarm can be retrieved from a 840Dsl controller over OPC UA, the following hardware and software preconditions must be satisfied:

• NCU software version: Sinumerik Operate for PC/PCU base software V4.5 SP2 or higher; the OPC UA server functionality is integrated from V4.5 onward but the full alarm/event model requires the V2.2 generation of the embedded OPC UA server stack (delivered with Sinumerik Operate V4.7+ or with the Access MyMachine / OPC UA add-on V2.2).
• License key: The option bit for OPC UA server (6FC5800-0AP76-0YB0) and the alarm option bit (6FC5800-0AP76-0YB1). The license is keyed against the CF card serial number; verify with sc start /S ShowLic in the SINUMERIK service shell or via Commissioning > Licenses in the HMI.
• Network reachability: TCP port 4840 (OPC UA TCP binary) must be reachable from the client. Confirm with:
  telnet <NCU_IP> 4840
• User accounts: The SINUMERIK-side account auduser (audit user) with read access on alarms and the standard operator or service account for tag read.
• Firewall/host firewall on PCU50 / TCU: The Windows firewall rule Siemens OPC UA Server must be enabled.

Root Cause Analysis

The symptom of “tags visible but no alarms” can be traced to one of four underlying causes, in order of frequency observed in the field:

1. OPC UA server version too old. The alarm/event support was extended in OPC UA server V2.2. Older stacks (1.0.x, 1.4.x) only expose tags; subscriptions to Server for EventType notifications return Bad_ServiceUnsupported or no events.
2. Alarm license bit not loaded. The OPC UA option without alarm subscription right returns Bad_UserAccessDenied when the client invokes CreateSubscription on the event notifier.
3. Client not calling the correct event notifier node. A naive Browse of the address space reveals only the tag folder. Event notifications are not anchored to a tag node; they are emitted by the Server object (NodeId ns=0;i=2253) and by a Sinumerik-specific EventManager object. Clients that only subscribe to a tag folder receive no events.
4. Empty subscription buffer crash. A documented defect in the V2.2/V2.3 server stack causes the server process to terminate when an event subscription is opened against the controller, the alarm queue is momentarily empty, and the client does not pre-populate any monitored item. The workaround is to keep at least one monitored item alive in parallel to the event subscription.

OPC UA Server Stack Versions

The embedded OPC UA server on Sinumerik 840Dsl evolves through these major generations:

Identify the running version with opc.tcp://<NCU_IP>:4840 and the GetEndpoints service. The endpoint description contains Server.ApplicationUri and the ServerCertificate that includes the build timestamp. A typical V2.2 banner returned via Discovery is:

Configuring AEClient.exe for OPC Classic A&E

Siemens ships the standalone test client AEClient.exe with the Access MyMachine / OPC UA toolkit. It speaks OPC Classic Alarms & Events against the Sinumerik server and is the fastest way to verify that the controller-side event delivery is alive before introducing a UA-only client. It is located in C:\Program Files\Siemens\Automation\AccessMyMachine\AEClient\ on the development host.

Step-by-Step: Launch AEClient.exe

1. Hold Shift and right-click AEClient.exe, then choose Run as different user.
2. Enter the credentials in the form .\auduser (dot + backslash + username) followed by the password. The leading dot forces the local SAM database; the auduser account is the audit account created during the Sinumerik installation and is the only account authorized to subscribe to alarms.
3. In the dialog, enter the DNS hostname or IP address of the NCU in the textbox to the right of [Connect].
4. Click [Connect]. A successful connection is shown by the server name SINUMERIK appearing in the Server tab.
5. If the server tab stays empty, jump to the Troubleshooting Matrix below. The most common cause at this stage is the AEClient.exe process running as the wrong user; kill the process and retry.

Interpreting the Event List

Once connected, AEClient.exe displays active alarms in the upper pane. Each row contains the fields defined by the OPC A&E 1.0 specification, mapped to Sinumerik-specific values:

OPC UA Event Subscription Model

Modern OPC UA clients must subscribe to the Alarms & Conditions model defined in OPC UA Part 9, not to the legacy Events (Part 5) model. On the Sinumerik 840Dsl the event notifier nodes are:

• ns=0;i=2253 — Server object; receives all condition instances system-wide.
• ns=4;s=Sinumerik.EventManager — Sinumerik-specific event manager; can be filtered by Source (PLC / NCK / DRIVES).
• ns=4;s=Sinumerik.EventManager.PLC — narrowed event manager for PLC alarms only.
• ns=4;s=Sinumerik.EventManager.NCK — narrowed event manager for NCK alarms only.

The client must call CreateSubscription with the event notifier set to one of the above node IDs, then call CreateMonitoredItems with the Event filter. The event filter selects the fields that will appear in the EventNotificationList. A minimal filter is:

SelectClause[0]: /EventId
SelectClause[1]: /EventType
SelectClause[2]: /SourceName
SelectClause[3]: /Time
SelectClause[4]: /Severity
SelectClause[5]: /Message
SelectClause[6]: /ConditionId

WhereClause for Filtering

Add a WhereClause to limit the notification flood. Examples:

// Only alarms with severity >= 500
ofType(Severity) >= 500

// Only NCK alarms
SourceName == 'NCK'

// Only unacknowledged alarms
ofType(Retain) == true

Event Filter and Severity Settings

In AEClient.exe the menu Subscription → Filter Settings defines which event types are acquired and the minimum/maximum severity bounds:

1. Open Subscription → Filter Settings.
2. Enable Simple Event, Tracking Event, and Condition Event (the three event categories used by Sinumerik).
3. Set Minimum Severity to 1 (include everything) during commissioning, then raise it later for production.
4. Set Maximum Severity to 1000 (the OPC A&E ceiling).
5. Click OK. The subscription is updated with ModifySubscription on the running session.

Connection Break After Subscribing to Events

A documented defect in OPC UA servers based on certain pre-V2.3 stacks causes the server to drop the TCP connection when a client subscribes to events on the Server object and the controller's alarm queue is empty at the moment of subscription. The client receives Bad_ConnectionClosed and must reconnect. The defect originates from a null-pointer dereference inside the alarm handler when no condition instance is present in the snapshot.

Workaround

1. Before opening the event subscription, write a benign write to a tag the operator is guaranteed to monitor, e.g. ns=4;s=Channel/MachineAxis/ee_actualPos[0]. This forces at least one monitored item to be present.
2. Create the tag monitored items first, then the event subscription, both inside the same session.
3. On the client side, implement automatic reconnection with exponential backoff (1 s, 2 s, 4 s, max 30 s) and resubscribe on reconnect.
4. Alternatively, upgrade to OPC UA server stack V2.3 or higher, where the null check is fixed.

Verification Procedure

1. Discovery. From the client, call opc.tcp://<NCU_IP>:4840 via Discovery URL and confirm Server.ApplicationUri = Siemens.Sinumerik.840Dsl.OpcUaServer with version >= 2.2.
2. Session. Open a session with auduser. Verify the server returns ServiceResult = Good for ActivateSession.
3. Browse event notifier. Browse ns=0;i=2253 and confirm that EventNotifier bitmask has SubscribeToEvents (0x01) set.
4. Subscribe. Create a subscription with publishing interval 500 ms, keep-alive count 10.
5. Monitor. Create a monitored item on the event notifier with the filter shown above. Trigger a known alarm, e.g. by issuing NCK stop via DB31..DBX0.2 = 1; confirm a RefreshStartEventType followed by the alarm condition appears within 2 s.
6. Acknowledge. Send the Acknowledge service call against the ConditionId and confirm the Severity stays at the configured value while Retain = false.

Troubleshooting Matrix

Known Limitations and Field Notes

• The Alarms & Conditions support in Sinumerik 840Dsl does not include exclusive alarms; a single condition can be active on multiple physical axes without instance differentiation.
• The EventId returned by the server is a 16-byte binary blob that includes the NCK alarm number plus a sequence counter; persist it in the historian for de-duplication.
• Acknowledgement from a remote OPC UA client is supported from V2.2 onward. Prior versions expose read-only conditions and reject the Acknowledge call with Bad_NotSupported.
• When multiple OPC UA clients (e.g. Ignition prod/test/dev) connect concurrently, the server allocates a subscription per session; the default maximum is 32. Exceeding it returns Bad_ResourceUnavailable. Increase the MaxSubscriptionsPerSession via the server configuration file opcua_server.conf on the NCU.
• The localization of the alarm Message field follows the active HMI language at the time of subscription. Switch the HMI language before opening the subscription if a fixed locale is required for the historian.

Commissioning Checklist

1. Confirm NCU software build >= Sinumerik Operate V4.7 with OPC UA server stack V2.2.
2. Load both license options: OPC UA base + alarm subscription.
3. Create local Windows user auduser with the password configured in the NCU.
4. Open Windows Firewall rule for TCP 4840 inbound.
5. Run AEClient.exe as .\auduser, connect, verify SINUMERIK server tab populates.
6. From the OPC UA client of choice, call Discovery on opc.tcp://<NCU_IP>:4840 and read the endpoint description.
7. Open session, browse ns=0;i=2253, confirm SubscribeToEvents bit is set.
8. Create subscription with publishing interval 500 ms.
9. Create a single tag monitored item before the event monitored item.
10. Trigger a test alarm (e.g. NCK 21612 by forcing an axis drive fault), confirm receipt within 2 s.
11. Acknowledge the alarm from the client and confirm the state transition.

FAQ