Configuring Alarm Behavior and Suppressing Default Messages on MonDiL in PCS 7 APL

In SIMATIC PCS 7 Advanced Process Library (APL), the digital monitoring block MonDiL (FB1848) and MonDiS (FB1847) generate the default alarm 'Out - Binary value set' whenever the monitored input transitions to 1. To resolve or customize this, invert the alarm trigger using parameter MsgInvert or InvIn, suppress message generation entirely using MsgLock = 1 or MsgEn = 0, or customize the message text and alarm class in the block's Special Object Properties > Messages dialog in CFC.

Operating Principles of MonDiL Digital Signal Monitoring in APL

In SIMATIC PCS 7 Advanced Process Library (APL), the function blocks MonDiL (FB1848) (Digital Monitoring Large) and MonDiS (FB1847) (Digital Monitoring Small) monitor binary process signals (such as limit switches, valve position feedback, motor run confirmations, and interlock contacts).

By default design, MonDiL is pre-configured to generate an operator alarm whenever the monitored process value transitions into the active state (In.Value = 1). In the embedded ALARM_8P configuration, Signal 1 is assigned the factory default text:
Out - Binary value set

In many process applications, reaching the active state (1) is normal operational behavior (e.g., Pump Running, Valve Open), rather than an abnormal process condition. Operators and commissioning engineers find the alarm list flooded with nuisance alarms.

To configure MonDiL correctly, engineers must understand the interaction between input inversion (InvIn), message inversion (MsgInvert), message locking (MsgLock), and the WinCC Message Configuration database.

MonDiL Alarm Control Parameters and Signal Inversion Matrix

Understand how specific block parameters govern alarm generation and display:

Parameter NameData TypeDefault ValueFunctional Impact on Alarm ProcessingApplication Use CaseInStruct_Digital0 (ST=16#80)Primary process input (connected to channel driver or logic)Base signal sourceInvInBOOL0Inverts both the output Out and the message evaluationNormally-closed (NC) fail-safe field contactsMsgInvertBOOL0Inverts alarm trigger without inverting Out.Value (1 = Alarm when Out=0)Alarming on loss of signal (e.g., loss of run confirmation)MsgLockStruct_Digital0Suppresses all message generation when Value = 1Interlocked unit shutdown; maintenance bypassMsgEnBOOL1Enables (1) or disables (0) message generationPermanent suppression of alarmsFeature.Bit29BOOL0Disables signaling when MsgLock = 1NAMUR compliant maintenance alarm suppression

Engineering notice: Modifying InvIn changes the logic output; modifying MsgInvert affects ONLY message evaluation.

Customizing Alarm Event Text in S7 Message Configuration

To replace the generic text Out - Binary value set with meaningful technological descriptions:

Open Message Configuration in CFC:
* Open the CFC chart containing the MonDiL instance.
* Right-click the block and select Special Object Properties > Messages.
Modify Message Properties:
* Locate the row for Signal 1.
* In the Event column, replace Out - Binary value set with the plant-specific description (e.g., High Tank Level Switch Tripped or Cooling Fan Offline).
* Review the Message Class:
- If this is an informational event, change the class from Alarm - High to Operating Message - Standard.
- If no alarm should be logged in WinCC, select No Message.
Compile OS:
* Run Compile OS (Scope: Changes only) to update the WinCC Alarm Logging database.

Suppressing Unwanted Alarms via MsgLock and Feature Bits

If MonDiL is used purely for status visualization on faceplates without requiring any WinCC alarm:

Permanent Message Suppression:
* In CFC, set input parameter MsgEn = 0.
* This permanently disables the internal call to ALARM_8P, preventing any alarm frames from being dispatched to the OS.
Dynamic Maintenance Suppression (MsgLock):
* Connect input MsgLock.Value to an operational interlock or maintenance mode tag.
* When maintenance is active (MsgLock.Value = 1), MonDiL suppresses alarm generation while continuing to update live status indicators and faceplate graphics.
* Configure Feature.Bit29 = 1 if the block icon must display the blue lock (suppressed) symbol during maintenance.

Verification

Verify alarm behavior in WinCC Runtime:
1. In CFC, force input In.Value = 1.
2. Confirm that the technological faceplate displays the active status without generating the generic Out - Binary value set alarm.
3. If customized message text was configured, confirm that the alarm list displays High Tank Level Switch Tripped with correct priority and color.
4. Set MsgLock.Value = 1: Confirm that toggling In.Value generates zero alarms in the WinCC Alarm List.

Limitations and Operational Precautions

Never disable alarms (MsgEn = 0) on critical safety transmitters or emergency interlock switches.
Inverting InvIn affects downstream logic interconnections to pumps or valves; verify logic consequences before inverting inputs.
Ensure time synchronization across the automation system is within 1 ms if using timestamped Sequence of Events (SOE) blocks (MonDi08 / EventTs).

Engineer FAQ

What is the difference between MonDiL and MonDiS?

MonDiL (Large) provides full APL functionality including flutter suppression, preview views, maintenance states, and extended interlocks. MonDiS (Small) is a lightweight block with minimal memory footprint and simplified parameters.

Can MonDiL filter out bouncing or fluttering digital inputs?

Yes. MonDiL features a built-in flutter suppression filter (parameters FlutEn, FlutTmIn, FlutFactor) to prevent fluttering limit switches from flooding the alarm log.

Why does the alarm still trigger after changing the message text in CFC?

Modifying message text in CFC updates the STEP 7 project database, but the text is only transferred to WinCC Runtime after executing an OS Compilation.