Understanding the $$BlockComment$$ Message Parameter and AS-OS Delta Download Dependencies in SIMATIC PCS 7

In SIMATIC PCS 7, adding or modifying the $$BlockComment$$ identifier in the message properties of a function block forces an AS Changes Only download (Delta Download) because message definitions are embedded inside the block instance data block (DB) in controller memory. The CFC compiler resolves $$BlockComment$$ by replacing it with the chart block comment text, which updates the internal Alarm_8P message structure parameters. Because the instance DB's initial values are altered, the controller program must be updated via an AS delta download, followed by an OS compilation and download.

The Role of $$BlockComment$$ in PCS 7 Message Formatting

In the SIMATIC PCS 7 Advanced Process Library (APL), alarms displayed in WinCC Alarm Logging must convey meaningful process context without requiring manual text entry for thousands of blocks:
* The $$BlockComment$$ Macro:
When configuring messages on a block type (e.g., MonDiL, MotL, PIDConL) under Special Properties -> Messages, the string $$BlockComment$$ is placed in the message text definition (e.g., $$BlockComment$$ - High Alarm).
* The Compilation Resolution:
During CFC chart compilation, the compiler dynamically substitutes $$BlockComment$$ with the text entered in the Block Comment field of the CFC block (e.g., Feedwater Pump 101).
* The Operator Result:
In WinCC Runtime, the alarm banner displays: Feedwater Pump 101 - High Alarm.

Why Message Text Changes Affect the Automation Station (AS) Memory

Engineers often assume that modifying message text strings is an exclusively Operator Station (OS) task:
* The AS Connection:
Unlike traditional SCADA systems where alarms are generated on the PC based on polled boolean tags, PCS 7 executes time-stamped alarms directly inside the controller using Alarm_8P (SFB35).
* Instance DB Parameterization:
The message text identifier, message classes, and associated value format parameters are compiled directly into the Instance Data Block (DB) associated with that function block.
* Why an AS Download is Mandatory:
Altering $$BlockComment$$ alters the parameter configuration of the instance DB. To make the controller aware of the modified message parameters, the instance DB must be updated in the CPU load and work memory via an AS Changes Only Download (Download in RUN).

The Alarm_8P Parameter Structure Inside Instance Data Blocks

Each Alarm_8P block instance reserves internal memory words:
* EV_ID: The unique 32-bit Message ID assigned by SIMATIC Manager.
* Message Attribute Flags: Defines whether the message requires acknowledgment, its priority, and format string pointers.
* Associated Values (SD_1 to SD_10): Pointers to analog process values stamped into the alarm telegram.
When the controller detects an alarm condition, it packages the EV_ID and associated values into an S7 message frame. If the instance DB parameters in the controller do not match the compiled OS database, message decoding discrepancies occur.

Comparison Table: AS vs OS Impact of CFC Engineering Changes

The table below outlines which engineering changes require AS downloads vs OS compilations:

Engineering Action in CFCRequires AS Delta Download?Requires OS Compilation?Plant Impact in RUNModifying Block Comment ($$BlockComment$$)Yes (Instance DB altered)Yes (Updates Text Library)Bumpless (Changes in RUN)Modifying Message Class / PriorityYes (Alarm parameters changed)YesBumplessChanging CFC Interconnections (Wiring)Yes (Logic modified)No (Unless OCM pin linked)BumplessAltering WinCC Faceplate Graphic (.pdl)NoNo (Direct save in Graphic Designer)Zero AS ImpactAdding New OCM Parameter (S7_m_c)Yes (Attribute flags updated)Yes (Generates WinCC tags)Bumpless

Step-by-Step Procedure to Update Message Texts and Execute Delta Downloads

Modify Block Comment in CFC:
Open the target CFC chart. Double-click the block (e.g., MonDiL). In the Comment field, enter the updated descriptor (e.g., Primary Chilled Water Pump).
Compile CFC Charts (Changes Only):
In SIMATIC Manager, select the chart folder and click Charts -> Compile -> Changes only. The compiler resolves $$BlockComment$$ and updates the instance DB template.
Execute AS Changes Only Download:
* Click PLC -> Download -> Changes only.
* Confirm that the download executes in RUN mode without interrupting controller execution.
Compile WinCC OS (Changes Only):
Right-click the OS Server object and select Compile -> Changes only. The AS-OS compiler updates the WinCC Text Library and Alarm Logging database.
Download OS Server:
Download changes to redundant OS Servers. WinCC Alarm Logging refreshes its active message texts.

Verification

To verify that the updated block comment is active:
1. Open the WinCC OS Runtime alarm banner.
2. Trigger an alarm on the modified block in the controller.
3. Confirm that the incoming alarm message displays the updated comment text instead of the previous descriptor.
4. Open the AS diagnostic buffer (Ctrl+D) and verify that entry 1 confirms a successful delta download in RUN without parameter assignment faults.

Limitations and Operational Precautions

Single Station Exception: On an ES/OS Single Station, an OS download is not required because the Engineering Station shares the project database directly; only an OS compile is needed.
Avoid Full Compilations in Production: Never select "Entire program" compilation and download during production runs. Full downloads require stopping the CPU (STOP mode). Always verify that the Changes only option is available and selected.
Licensing Disclaimer: Utilizing Advanced Process Library components with Alarm_8P messaging requires valid PCS 7 AS and OS runtime licensing verified via Automation License Manager.