Alarm Suppression Checkboxes in APL Analog Faceplates: MsgLock Configuration and Operator Permissions

In SIMATIC PCS 7 Advanced Process Library (APL), when alarm suppression checkboxes in the Limit Values view of an analog faceplate (@pg_monanl_limit.pdl) do not render in WinCC Runtime while remaining clickable, the cause is an unconfigured message suppression feature bit or insufficient operator authorization. APL blocks (such as MonAnL and PIDConL) require Feature.Bit25 ('Suppression of all messages') or individual message enabling pins (PV_AH_MsgEn, PV_WH_MsgEn) to be active, alongside Operator Permission OS_Perm.Bit25 ('Operator can suppress messages'). Without these parameters enabled in CFC, the faceplate script suppresses the visual checkbox graphic to prevent unauthorized alarm shelving.

Alarm Suppression and Shelving Concepts in APL Faceplates

In industrial process plants conforming to ISA-18.2 and EEMUA 191 alarm management guidelines, alarm floods must be minimized during maintenance or equipment startup. The SIMATIC PCS 7 Advanced Process Library (APL) incorporates standardized alarm suppression mechanisms. In the Limit Values view of analog blocks (MonAnL, PIDConL), operators can selectively suppress alarm generation on specific threshold tiers (e.g. suppressing High-High alarms during planned boiler warm-up) or suppress all block alarms entirely.

A common commissioning issue arises when the official Siemens documentation illustrates alarm suppression checkboxes in the limit dialogue, yet in runtime the checkboxes fail to appear or display as transparent clickable areas.

The Role of MsgLock and Feature.Bit25 in Message Control

Alarm suppression in APL is coordinated at the block algorithm layer through several parameters:

MsgLock: Master suppression input. When MsgLock = 1, all alarm messages from the block are locked in the controller.
Feature.Bit25 (Suppression of all messages): Determines whether the block allows operator suppression via the faceplate. If Feature.Bit25 = 0, the faceplate script hides the master message suppression controls.
Individual Message Enables (PV_AH_MsgEn, PV_WH_MsgEn): Control whether an alarm generates an archive message upon limit violation. If PV_AH_MsgEn = 0, the limit is monitored for interlocks, but audible and visual alarm banner messages are suppressed.

Operator Authorization Requirements for Alarm Suppression (OS_Perm.Bit25)

Suppressing process alarms is a safety-critical action that cannot be permitted for general users:

In the block's OS_Perm configuration, OS_Perm.Bit25 dictates whether the logged-in operator is authorized to suppress messages.
Furthermore, the operator's Windows/WinCC user account must possess Authorization Level 6 (Higher Process Controlling) or Authorization Level 7 (Advanced Engineering) in WinCC User Administrator.
If the logged-in user lacks this permission, the faceplate script treats the checkboxes as restricted and suppresses graphic drawing.

Why Suppression Checkboxes Become Invisible in WinCC Runtime

The visual disappearing act is the result of dynamic faceplate script logic in @pg_monanl_limit.pdl:

When the limit view opens, the script reads OS_PermOut and Feature words.
If OS_PermOut.Bit25 == 0 or Feature.Bit25 == 0, the script sets the image path of the checkbox object to empty or transparent.
Because the underlying invisible Windows button object is not disabled, the operator can still click the empty coordinate, opening a parameter dialogue or generating permission warnings.
Missing client-side vector files (@APL_Check_*.emf) or non-standard Windows DPI scaling above 100% further compounds rendering failures.

Comparative Parameter Matrix: Alarm Suppression and Enabling Controls

The following table compares parameters controlling alarm generation and suppression in APL analog blocks:

Parameter NameData TypeFunction in APL BlockFaceplate Graphic RepresentationFeature.Bit25BOOL (in STRUCT)Enables operator alarm suppressionExposes suppression checkboxes in Limit ViewOS_Perm.Bit25BOOL (in DWORD)Grants operator permission to suppressEnables clicking and displays green checkmarkMsgLockSTRUCT (BOOL+ST)Suppresses all block messages immediatelyYellow bell with padlock on block icon and faceplatePV_AH_EnBOOLEnables evaluation of High-High limitWhen 0, hides limit value completelyPV_AH_MsgEnBOOLEnables alarm generation on High-High limitWhen 0, displays crossed-out red bell icon

Step-by-Step Procedure to Enable Faceplate Alarm Suppression

Follow this configuration procedure in the CFC chart:

Open the CFC chart containing MonAnL (or PIDConL).
Open the block properties -> Properties.
In the Feature parameter, set Bit 25 = 1 (Suppression of all messages).
In the OS_Perm parameter, set Bit 25 = 1 (Operator can suppress messages).
Ensure limit message enable inputs (PV_AH_MsgEn, etc.) are set to 1.
Compile CFC charts and perform a changes download to the AS controller.
Open WinCC User Administrator. Ensure operator accounts have Authorization Level 6 (Higher Process Controlling).
Open the faceplate in WinCC Runtime and verify suppression checkboxes render with full visibility.

Verification

Confirm functionality through these tests:

Visual Checkbox Audit: Open the Limit Values view in WinCC Runtime. Confirm crisp checkboxes appear next to message thresholds.
Suppression Activation: Click the suppression checkbox for High-High alarm. Confirm the icon changes to a crossed-out bell or padlocked symbol.
Alarm Silence Test: Force the input process variable beyond the High-High threshold. Verify the value turns red, but no audible horn or alarm banner message triggers.
Audit Trail: Confirm an operator message is recorded in the WinCC Alarm Journal documenting who suppressed the alarm and at what time.

Limitations and Operational Precautions

Observe the following alarm management safeguards:

Safety Instrumented Systems (SIS): Never suppress alarms originating from Safety Instrumented Functions (SIL rated safety loops).
Unshelving Timers: If alarms are suppressed during maintenance, enforce procedural or automated unshelving timers to ensure critical alarms are restored before returning the unit to normal production.
Commercial Accuracy Disclaimer: Software order numbers (MLFBs), package structures, and hardware capacity prerequisites vary across PCS 7 versions and hardware platforms. System integrators must confirm exact software entitlements and licensing requirements against the official Siemens Catalog CA 01 and obtain an authorized Siemens distributor quotation prior to purchasing.

Engineer FAQ

Can an operator suppress alarms permanently without recording an audit event?

No. Every alarm suppression or shelving action from an APL faceplate automatically generates an operator control log entry in the WinCC Alarm Journal.