C135 PCS 7 (PCS7)
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Generating Timestamped Alarms from Boolean Variables in SIMATIC PCS 7 (ALARM_8P Integration)

Generating Timestamped Alarms from Boolean Variables in SIMATIC PCS 7 (ALARM_8P Integration)

In SIMATIC PCS 7 V8.1 and WinCC V7.3, generating alarms from a Boolean variable is achieved natively using the APL digital monitor block MonDiL (or EventTS), which encapsulates system block ALARM_8P (SFB35) for sub-millisecond controller timestamping. Standalone ALARM_8P blocks can also be placed in CFC with s7_m_c = 'true', or WinCC internal tags can trigger Discrete Alarms in WinCC Alarm Logging.

Table of Contents

Alarm Architecture in SIMATIC PCS 7: AS Timestamping vs. OS Polling

In industrial automation supervisory systems, process alarms fall into two distinct architectural categories:
1. SCADA Polled Alarms (WinCC Discrete Alarms): An internal WinCC tag is monitored by the OS alarm engine. When the bit transitions to 1, WinCC generates an alarm and stamps it with the current PC computer clock timestamp.
2. Controller-Timestamped Alarms (PCS 7 Native Standard): When an event occurs in the plant (e.g. pump trip, motor overload), the Automation Station (AS) captures the event and stamps it with the controller's high-precision hardware clock down to the millisecond. The controller packages the message and transmits it to WinCC via ALARM_8P (SFB35) telegrams.

In SIMATIC PCS 7, controller-based timestamping is mandatory to ensure precise sequence-of-events (SOE) chronological reconstruction during root-cause incident investigations. Engineers frequently ask how to generate an ALARM_8P message from a custom Boolean variable.

Native APL Integration via MonDiL and EventTS Blocks

In SIMATIC PCS 7 Advanced Process Library (APL), engineers rarely need to instantiate raw ALARM_8P blocks manually. APL provides dedicated, pre-engineered technological blocks that embed ALARM_8P alongside faceplate visualization:

1. Using MonDiL (Digital Process Value Monitoring - Recommended):

  • Drag MonDiL (FB1848) into your CFC chart.
  • Connect your Boolean signal (from field I/O or internal logic) to input In.
  • Configure message text: Right-click MonDiL > Object Properties > Special Properties > Messages.
  • Enter your custom alarm text (e.g. 'Lube Oil Pressure Low - Trip').
  • When compiled, WinCC automatically creates the alarm, links the faceplate, and handles alarm acknowledgement.

2. Using EventTS (Timestamped Event Block):

  • If you have 8 discrete Boolean signals that do not require independent faceplates (e.g. electrical switchgear auxiliary contacts):
  • Drag EventTS (FB1217) into CFC.
  • Connect up to 8 Boolean signals to inputs In1 through In8.
  • EventTS transmits all 8 events to WinCC via an internal ALARM_8P with high-precision hardware timestamps.

Standalone ALARM_8P (SFB35) Configuration in CFC

If engineering requirements dictate using standalone ALARM_8P (SFB35) directly:

  1. Instantiate ALARM_8P in CFC:
    * Open the CFC chart.
    * From the block catalog under System Function Blocks, drag ALARM_8P (SFB35) onto the canvas.
  2. Configure Input Signals:
    * Connect your Boolean alarm bit to input SIG_1.
    * Inputs SIG_2 through SIG_8 can be connected to additional bits or left disconnected.
    * Ensure input EN_R (Enable Reporting) is set to 1 (TRUE).
  3. Set Message Attributes:
    * Right-click the ALARM_8P block, select Object Properties.
    * Verify the block has attribute s7_m_c = 'true'.
    * Click the Messages button.
    * In the Message configuration table:
    • Set Message Class: e.g., Alarm - High, Warning, or Operational Message.
    • Set Message Text: Enter your alarm description for SIG_1.
  4. Compile the OS:
    * In SIMATIC Manager, execute Compile OS.
    * The OS compiler reads the ALARM_8P instance and registers the message definitions into WinCC Alarm Logging automatically.

Alarm Generation Method Comparison Matrix

MethodImplemented InTimestamp LocationFaceplate Available?Recommended ApplicationAPL MonDiLCFC (APL Library)Controller (AS Hardware Clock)YES (Standard Faceplate)Standard field digital alarm (switches, trips)APL EventTSCFC (APL Library)Controller (High-Precision SOE)No (Message list only)High-speed electrical / sequence-of-events signalsStandalone ALARM_8PCFC (System Block)Controller (AS Hardware Clock)No (Requires custom graphics)Specialized controller-to-SCADA messagingWinCC Discrete AlarmWinCC Alarm LoggingOperator Station (PC Clock)No (Standard SCADA alarm)WinCC internal status or non-critical HMI events

Verification

  1. In the CFC chart, connect a test memory bit to MonDiL.In (or ALARM_8P.SIG_1).
  2. Execute Chart > Compile > Changes only and download to S7-PLCSIM.
  3. Execute Compile OS; verify compilation registers the new message.
  4. Launch WinCC Runtime.
  5. In S7-PLCSIM, force the Boolean bit from 0 to 1.
  6. Confirm the WinCC Alarm banner immediately displays the configured message text with the exact controller timestamp.
  7. Reset the bit to 0; confirm the alarm transitions to 'Cleared' (Outgoing).

Limitations and Operational Precautions

  • Communication Buffer Limits: Flooding an S7-400 CPU by triggering dozens of ALARM_8P instances in the same millisecond scan can exhaust AS message communication buffers, generating event buffer overflow diagnostics.
  • Avoid Generating Alarms in Graphic C-Scripts: Never trigger process alarms by writing cyclic C-scripts inside WinCC picture windows. Picture scripts execute only when that specific screen is open by an operator, resulting in missed alarms when operators navigate away.
  • Licensing Notice: APL technological blocks like MonDiL consume standard AS and OS Process Object (PO) licenses, whereas raw ALARM_8P system blocks do not consume PO licenses. Balance architectural standardization against license quotas.

Engineer FAQ

Why is MonDiL preferred over standalone ALARM_8P in PCS 7?

MonDiL provides complete APL integration: signal status evaluation (ST), alarm suppression/locking (MsgLock), maintenance bypass, alarm delay timers, and ready-to-use WinCC faceplates.

Can ALARM_8P send analog process values alongside the digital alarm?

Yes. ALARM_8P provides inputs SD_1 through SD_10 (Associated Values). You can connect process variables (e.g. pressure or temperature at the moment of trip) to embed live values into the alarm text string.

What does the 'P' in ALARM_8P stand for?

The '8' indicates up to 8 discrete signals monitored simultaneously, and the 'P' signifies support for Parameter-associated values (SD_1 to SD_10).

Technical References

Related Articles

Related Tags

  • ALARM_8P
  • MonDiL
  • Alarm Logging
  • WinCC V7.3
  • PCS 7 V8.1
  • SFB35
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