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Diagnosing Missing PROFIBUS DP Rack and Station Failure Alarms in WinCC After PCS 7 V8.0 Upgrades

Diagnosing Missing PROFIBUS DP Rack and Station Failure Alarms in WinCC After PCS 7 V8.0 Upgrades

When PROFIBUS DP failure alarms (such as ET 200M station dropout or rack failure) do not appear in WinCC Alarm Logging after upgrading to PCS 7 V8.0 SP2, the fault is caused by desynchronized AS-OS message data, missing module driver generation, or WinCC AlarmControl filter settings. In PCS 7 V8.0, diagnostic alarms are created by system block OB_DIAG1 and assigned to the 'Process Control System' message class. To restore alarms, open CFC, execute compilation with 'Generate module drivers' enabled, perform a full OS compilation with 'Tags and Messages' checked, and verify that the WinCC AlarmControl display filter includes unacknowledged system messages.

Table of Contents

Executive Summary

Following software upgrades from legacy PCS 7 versions (such as V7.1) to PCS 7 V8.0 SP2, operations teams often report that critical hardware failure alarms—specifically PROFIBUS DP slave disconnections, optical link faults, and ET 200M rack failures—no longer trigger visual or audible banners in the WinCC Alarm List. While the field bus physical fault LEDs actuate and I/O values freeze, the operator interface remains silent. This issue originates from desynchronization between the compiled CFC module drivers and the WinCC Alarm Logging database schema, or from unintentional message class filtering in modernized WinCC AlarmControl templates. This guide details the technical pathway required to regenerate diagnostic drivers and restore alarm annunciation.

PCS 7 Diagnostic Alarm Architecture: OB_DIAG1 and WinCC Message Classes

In SIMATIC PCS 7, peripheral bus faults are detected by CPU organization blocks (OB82 for diagnostic interrupts, OB86 for rack/substation failures, and OB122 for I/O access errors). When the CFC Driver Generator processes the hardware configuration, it automatically inserts system diagnostic blocks:

  • OB_DIAG1 (FB118): Monitors cyclic execution and OB interrupt events, constructing diagnostic messages.
  • MOD_D1 / MOD_HA: Evaluates channel health and module status words.
  • RACK / SUBNET: Evaluates master system availability and transmission line integrity.

These blocks transmit structured alarms via ALARM_8P (SFB35) directly into the WinCC Alarm Logging engine under the standardized message class Process Control System Messages (Message Type: AS Dispatch Fault / Maintenance).

Root Causes for Missing PROFIBUS Alarms Post-Migration

Three distinct issues suppress PROFIBUS alarms following an upgrade to PCS 7 V8.0 SP2:

  1. Bypassed Module Driver Generation: If the AS was compiled using 'Changes Only' without selecting 'Generate module drivers', the compiler maintains outdated driver structures that fail to communicate with updated APL libraries.
  2. Incomplete OS Compilation: If the Operator Station is compiled without selecting 'Tags and Messages', WinCC retains the previous message table pointers, causing incoming ALARM_8P event packets to be discarded as unassigned raw events.
  3. AlarmControl Message Filter Overlap: Modern WinCC V7.2 AlarmControl objects introduce user filter configurations. If the operator screen filter is configured to show only 'Process Alarms' (Class 1), system messages (Class 10) are hidden.

WinCC AlarmControl Filter Configurations

The WinCC AlarmControl object (@alarmline.pdl and @screen.pdl) evaluates runtime message classes. To verify visibility:

  • Open the AlarmControl properties in Graphic Designer.
  • Navigate to Message Lists > Selection.
  • Ensure that message classes System, requires acknowledgment and Process Control System are checked.
  • Verify that SQL filter strings do not contain restrictive criteria (e.g., CLASS IN (1, 2)).

Alarm Diagnostic Pipeline and Parameter Comparison

The following table compares the diagnostic blocks, message classes, and compilation settings governing bus alarms:

Diagnostic FunctionUnderlying AS BlockWinCC Message ClassEvent TriggerRequired Engineering SettingPROFIBUS Station DropoutOB_DIAG1 / RACKProcess Control SystemOB86 (Rack Failure)CFC: Generate module drivers enabledModule Wire Break / FaultMOD_D1 / MOD_HAProcess Control SystemOB82 (Diagnostic Interrupt)Channel Driver Pcs7DiIn connected to hardwareMaster System FaultSUBNETProcess Control SystemOB86 (DP Master failure)Hardware Config: Diagnostic addresses validAlarm PresentationWinCC AlarmControlClass 10 (System)ALARM_8P telegramWinCC: 'Tags and Messages' compiledOperator FilterSelection CriteriaAll Active ClassesRuntime Filter BarEnsure System Messages are not unchecked

Step-by-Step Restoration Procedure

Method 1: Regenerating Module Drivers in CFC

  1. Open the target AS project in SIMATIC Manager.
  2. Open any CFC chart and select Chart > Compile > Charts as Program.
  3. In the compilation dialog, select the radio button Entire program.
  4. Check the box Generate module drivers.
  5. Click OK to execute compilation. Verify in the compilation log that OB_DIAG1 and related driver blocks are compiled without errors.

Method 2: Compiling OS Messages and Verifying AlarmControl

  1. In SIMATIC Manager, select the OS object under the target PC Station.
  2. Right-click and choose Compile.
  3. In the OS Compilation Wizard, select Tags and messages and ensure the connection to the AS is checked.
  4. Execute the OS compilation and verify that message generation completes with 0 errors.
  5. Open WinCC Explorer, launch Alarm Logging, and confirm that messages for OB_DIAG1 are populated.
  6. Open the main alarm screen in Graphic Designer, check the AlarmControl selection filter, and save the picture.

Verification

To verify that PROFIBUS DP failure messages are annunciated correctly:

  1. Simulation / Physical Test: Disconnect the PROFIBUS Sub-D connector from an ET 200M remote I/O station (or simulate OB86 via S7-PLCSIM).
  2. Alarm Banner Inspection: Confirm that an incoming alarm appears immediately in the WinCC Alarm Banner with text 'Rack Failure' or 'Station Disconnected'.
  3. Alarm Acknowledgment: Reconnect the bus connector. Confirm that the outgoing event transitions to 'Gone' and clears upon operator acknowledgment.

Limitations and Operational Precautions

  • CPU STOP Risk: Performing a full compilation with 'Generate module drivers' may alter OB execution orders. Delta downloads are typically supported, but verify that the CPU does not require a cold restart.
  • Third-Party GSD Modules: Standard module drivers cannot evaluate proprietary diagnostic structures of third-party DP slaves unless customized diagnostic blocks or generic MOD_PAL blocks are implemented.
  • System Message Flooding: In plants with marginal physical cabling, flapping PROFIBUS drops can generate alarm storms. Ensure alarm suppression or flutter suppression is parameterized in the channel drivers.

Engineer FAQ

Why did PROFIBUS failure alarms work in V7.1 but disappear after upgrading to V8.0?

In V8.0, message numbers and APL driver blocks were updated. If the OS is not compiled with 'Tags and Messages' after generating module drivers, the OS retains stale message pointers.

Can I generate module drivers for a single CFC chart?

No. Module driver generation is a global system compilation process that evaluates the entire hardware configuration and all CFC charts simultaneously.

Where are OB86 rack failure events logged if WinCC is offline?

Events are retained in the CPU circular Diagnostic Buffer, accessible via SIMATIC Manager under PLC > Module Information > Diagnostic Buffer.

Technical References

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  • https://industrialautomation.net/search?query=simatic-pcs7
  • https://industrialautomation.net/search?query=profibus-dp
  • https://industrialautomation.net/search?query=wincc-scada
  • https://industrialautomation.net/search?query=alarm-logging
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