Configuring Analog Alarm Limits and Messages in PCS 7 CFC vs WinCC

In SIMATIC PCS 7, analog alarm thresholds and message parameters must always be configured directly on the Continuous Function Chart (CFC) monitoring block (such as MonAnL or MonAnalog) rather than inside WinCC Alarm Logging. Any modifications made directly in WinCC Alarm Logging are completely overwritten during subsequent OS compilations. To configure alarm and warning setpoints, adjust block input parameters (AH_Lim, WH_Lim, WL_Lim, AL_Lim) in the CFC editor or via the faceplate Limits view. To configure message texts, priorities, and alarm classes, open the block's Object Properties in CFC, select the 'Messages' dialog, and execute an OS compilation to deploy the changes.

Single Source of Truth: CFC vs WinCC Alarm Logging

In classic standalone SCADA systems (such as standalone WinCC V7), alarm definitions, limits, and texts are engineered directly inside the Alarm Logging editor. In SIMATIC PCS 7, however, the architecture is strictly hierarchical: the automation controller's Continuous Function Chart (CFC) database is the authoritative Single Source of Truth.

When a pressure transmitter monitored by MonAnL (or legacy MonAnalog in PCS 7 RTX systems) experiences high pressure, the evaluation occurs cyclically inside the controller program. The block calls system message block ALARM_8P to transmit timestamped event packets to the Operator Station. Therefore, all alarm attributes—both operating limits and message text definitions—originate in the CFC.

Why Direct Edits in WinCC Alarm Logging Are Overwritten

Automation engineers unfamiliar with PCS 7 often attempt to tweak message texts, priority classes, or archive filters directly within the WinCC Explorer Alarm Logging editor. While changes function temporarily in runtime, they are wiped out as soon as the next OS compilation is executed:

OS Compilation Mechanism: During OS Compilation, the compiler scans all CFC charts in the multiproject, generates message instances from ALARM_8P calls, and rebuilds the WinCC Alarm Logging database tables.
Overwriting Existing Records: The compiler matches message IDs generated by STEP 7. Any record whose text, class, or priority was modified in WinCC is overwritten with the parameters stored in the CFC block properties.
Database Consistency: Enforcing CFC as the master database guarantees that all redundant OS servers and clients maintain 100% identical alarm catalogs without manual reconciliation.

Configuring Alarm Limits vs Configuring Message Texts

Distinguishing between numerical alarm setpoints and alarm message texts is essential for proper administration:

Alarm Limits (Numerical Thresholds): Managed via block I/O pins:
- AH_Lim: High-High Alarm limit.
- WH_Lim: High Warning limit.
- WL_Lim: Low Warning limit.
- AL_Lim: Low-Low Alarm limit.
- Hyst: Deadband percentage to prevent alarm chattering.
These limits can be pre-configured offline in CFC or adjusted dynamically by operators via the WinCC faceplate Limits view.
Alarm Messages (Texts and Classes): Managed via the block's internal ALARM_8P properties. Configuring text tokens, process tag comments, and batch headers requires opening Object Properties -> Messages in CFC.

Comparison of Alarm Parameter Locations in PCS 7

The table below clarifies where specific alarm parameters must be engineered and their persistence across OS compilations.

Alarm Configuration ElementWhere to ConfigureMechanism / DialogOverwritten by OS Compilation?Numerical Limits (AH, WH, WL, AL)CFC Block Pins or OS FaceplateCFC parameter value or WinCC runtime inputNo (Retentive in AS instance DB)Alarm Deadband / HysteresisCFC Block Pin HystCFC block properties or faceplateNo (Retentive in AS instance DB)Message Event TextCFC EditorObject Properties -> MessagesNo (Master definition in CFC)Message Class & PriorityCFC EditorObject Properties -> MessagesNo (Master definition in CFC)Direct Edits in WinCC Alarm LoggingWinCC ExplorerWinCC Alarm Logging EditorYES (Overwritten on every OS compile)

Step-by-Step Procedure to Configure Analog Alarms and Messages

Follow this procedure to properly configure analog alarms and customize message texts for an analog pressure sensor:

Set Numerical Alarm Limits in CFC:
- Open the CFC chart containing the MonAnL block.
- Set nominal operating limits on pins AH_Lim, WH_Lim, WL_Lim, and AL_Lim (e.g., AH_Lim = 10.0 bar, WH_Lim = 8.5 bar).
- Set hysteresis on pin Hyst (e.g., 1.0 %).
Configure Message Texts and Classes:
- Right-click the MonAnL block and select Object Properties.
- In the General tab, click the Messages... button.
- A message configuration window opens listing all 8 signals of the internal ALARM_8P block.
- Select Signal 1 (High Alarm) or Signal 2 (High Warning): edit the Event text (e.g., 'Reactor Pressure High-High').
- Assign message class (e.g., 'Alarm - High') and set priority (1 to 16).
- Click OK to save the message parameters.
Compile and Download:
- Download CFC changes to the AS controller via delta download.
- In SIMATIC Manager, right-click the OS object and select Compile (select Changes Only).
- Download the updated OS project to the target OS Server.

Verification

Verify alarm configuration through the following tests:
1. In WinCC Explorer on the OS Server, open Alarm Logging: verify that the newly compiled message lines display the custom event text and priority configured in CFC.
2. In WinCC Runtime, open the faceplate for the analog pressure sensor: confirm that alarm and warning limit indicators reflect configured values.
3. Simulate an analog pressure excursion above AH_Lim: verify that an audible and visual alarm appears in the WinCC alarm banner with the exact custom text.
4. Acknowledge the alarm: confirm that acknowledgment status updates smoothly across faceplate and alarm lists.

Limitations and Operational Precautions

When customizing message texts in CFC, avoid hardcoding static tag names directly into the message text string. Instead, use standard Siemens string placeholders (such as $$AKZ$$ or @1%s@ for block comments) to ensure that messages automatically adapt if the plant hierarchy or technological tag name is renamed. Additionally, ensure that limits obey logical ordering (AL_Lim < WL_Lim < WH_Lim < AH_Lim); configuring overlapping limits causes diagnostic warnings and unpredictable alarm annunciation.

Engineer FAQ

Can an operator change alarm limits from the WinCC faceplate?

Yes. By opening the 'Limits' view on the APL faceplate, authorized operators (with appropriate WinCC user permissions) can modify numerical setpoints online without modifying CFC charts.

Why did my custom alarm message colors disappear after OS compilation?

Message colors are determined by the Message Class in WinCC Alarm Logging. In PCS 7, message classes are standardized. To change alarm colors, customize the global PCS 7 Message Classes in the Master Data Library.