Resolving Alarm Acknowledgment and Message Lock Failures on SIMATIC AS-RTX in PCS 7

In SIMATIC PCS 7 V8.0 installations utilizing AS-RTX PC-based controllers (WinAC RTX), failures where alarms do not trigger until toggling the message lock icon, or faceplates remain in a fault state after acknowledgment, are caused by NetBIOS computer name length violations exceeding 15 characters, or unpatched message synchronization defects in the base V8.0 release. To resolve this, restrict all PC station names strictly to 15 uppercase characters without special symbols, and install PCS 7 V8.0 SP1 or SP2.

Architecture of Alarm Synchronization on AS-RTX PC-Based Controllers

In SIMATIC PCS 7 environments, automation processing can be hosted on hardware controllers (S7-400 / S7-410) or industrial PC-based software controllers: the SIMATIC Microbox AS-RTX (running Windows Embedded Standard and the real-time software PLC WinAC RTX).

Alarm generation in PCS 7 utilizes the system function block ALARM_8P (SFB35) embedded within technological blocks such as MonAnS (FB1845) and MonAnL (FB1838). When a process limit is violated:
1. The controller captures the timestamp and generates an alarm frame.
2. The alarm is transmitted over the Plant Bus to redundant OS Servers (WinCC Alarm Logging).
3. Operators acknowledge the alarm from OS Clients.
4. WinCC transmits an acknowledgment telegram back to the controller, which clears the unacknowledged alarm flag and updates the faceplate state.

In early PCS 7 V8.0 installations with redundant OS Servers, engineers encountered an erratic defect on AS-RTX: alarms failed to appear upon limit violation. Only when the operator clicked the Message Lock/Unlock (Sperren/Freigeben) icon on the faceplate did the alarm suddenly flood into the alarm list. Furthermore, acknowledging the alarm left the faceplate in a permanent faulted state until the lock icon was toggled again.

Message Processing and Station Name Compatibility Matrix

Understand how software releases and station naming conventions govern alarm synchronization:

System Parameter / ComponentPermissible ConfigurationProhibited / Defective ConfigurationImpact on AS-RTX Alarm ProcessingWindows Computer Name Length1 to 15 characters16 characters or longerTruncation in NetBIOS buffer breaks OS-AS message lock handshakeCharacter Casing / SymbolsUppercase alphanumeric (A-Z, 0-9, hyphen)Lowercase letters, underscores, spacesDCOM / SIMATIC NET authentication token mismatchPCS 7 V8.0 Software LevelV8.0 SP1 / SP2 + Latest UpdV8.0 Base Release (Initial delivery)Known internal timing bug in WinAC RTX Alarm_8P queueController Hardware PlatformAS-RTX (Microbox 427C/427D)Unconfigured virtual machineRequires dedicated real-time RTX core allocationWinCC Redundancy StateFully synchronized (@RM_MASTER healthy)Redundancy partner desynchronizedAcknowledgment telegrams discarded by standby server

System configuration note: Maintain strict adherence to RFC 1035 and NetBIOS naming standards across all PCS 7 nodes.

Root Causes: 15-Character NetBIOS Limit and V8.0 RTX Synchronization Defect

This behavior results from two compounding architectural defects:
1. NetBIOS Computer Name Length Violation: In Windows and SIMATIC NET, network station identification relies on NetBIOS names, which have a hard architectural ceiling of 15 characters. If an OS Client, OS Server, or AS-RTX has a computer name such as OS_CLIENT_PACKAGING_01 (22 characters), the operating system truncates the identifier. The WinCC Message Lock manager (CCMsgLock.exe) uses the full string while the communication driver uses the truncated 15-character string. The resulting token mismatch causes the controller to believe messages are permanently locked (MsgLock = 1), suppressing incoming alarms until the lock state is manually refreshed.
2. WinAC RTX Alarm Queue Synchronization Timing Defect: In the initial release of PCS 7 V8.0, an asynchronous timing bug in the WinAC RTX real-time kernel delayed ALARM_8P state transitions when operating with redundant OS servers. Toggling the lock button forced a complete buffer rebuild, masking the underlying timing deadlock.

Step-by-Step Resolution and Station Renaming Procedure

Follow this verified workflow to eliminate alarm synchronization failures:

Verify and Rename Computer Names to <= 15 Characters:
* On all OS Servers, OS Clients, and AS-RTX units, open Windows System Properties.
* Verify the Computer Name.
* If the name exceeds 15 characters, shorten it:
- Example: Change OS_SERVER_PRIMARY_UNIT1 (23 chars) to OS-SRV-PRI-01 (13 chars).
- Ensure the name uses only uppercase letters, numbers, and hyphens (no underscores).
- In SIMATIC Manager, update the PC Station names in the project to match the new Windows names exactly.
Apply PCS 7 V8.0 Service Pack (SP1 or SP2):
* Download and install PCS 7 V8.0 SP1 (or upgrade to SP2) on the Engineering Station, OS Servers, OS Clients, and AS-RTX.
* This service pack updates the WinAC RTX communication driver and resolves the ALARM_8P buffer synchronization bug.
Execute Complete OS and AS Download:
* In SIMATIC Manager, right-click the AS-RTX station and execute PLC > Download > Scope: Entire program.
* Recompile all OS projects: Compile OS > Entire OS with memory reset.
* Download the complete server packages to redundant OS Servers and Clients.

Verification

Verify alarm processing and acknowledgment:
1. In CFC, force an analog input on MonAnS above its alarm threshold.
2. Confirm that the WinCC Alarm Logging list immediately registers an Incoming (+) Alarm without touching the lock icon.
3. Confirm that the motor/analog block icon displays the active alarm indicator.
4. Click the Acknowledge (Quittieren) button in WinCC: Confirm that the alarm transitions to acknowledged state.
5. Force the input back to normal: Confirm that the faceplate immediately returns to the normal green/grey operational state without toggling the message lock.

Limitations and Operational Precautions

Changing Windows computer names on existing PCS 7 stations requires updating SQL Server database instance names and re-running the WinCC Project Migrator / Security Controller; perform renaming during planned commissioning windows.
WinAC RTX requires continuous real-time subsystem execution (IntervalZero RTX); never install unauthorized office software, web browsers, or third-party backup tools on the AS-RTX host OS.
Always ensure that PV_AH_En (Alarm High Enable) and MsgEn (Message Enable) remain set to 1 on the monitoring blocks.

Engineer FAQ

Why did clicking the lock icon suddenly make the alarm appear?

Clicking the message lock icon sent an explicit command to the controller to change the block's message state. This event forced WinAC RTX to flush its internal ALARM_8P message buffer, releasing the queued alarms that were stalled by the synchronization bug.

Can I use underscores (_) in PCS 7 computer names?

Siemens guidelines strongly recommend using hyphens (-) instead of underscores (_). DNS and NetBIOS standards treat underscores as non-standard characters, which can disrupt WinCC web and redundancy handshakes.

Does this problem occur on hardware S7-400 CPUs?

The specific buffer timing bug was unique to WinAC RTX in PCS 7 V8.0 base release; however, the 15-character computer name truncation rule applies equally to all S7-400, S7-410, and WinCC architectures.