Diagnosing 'F' Symbol on PCS 7 APL Block Icons: Bit 24/25 in EventState and AS Process Controlling Messages

An 'F' symbol appearing on a SIMATIC PCS 7 APL block icon indicates an active Control System Fault or Error, originating from bits 24 ('AS Process Controlling Messages: Fault') and 25 ('AS Process Controlling Messages: Error') of the block's .EventState tag. If opening the faceplate displays an empty message list, the message is either suppressed by WinCC message class filters, unacknowledged from an earlier incoming event, or triggered by an external driver block via the CSF (Control System Fault) input. To resolve this, inspect the block's Message Configuration in CFC to identify the specific SIG input, trace the CSF interconnection back to its channel driver (Pcs7AnIn / Pcs7DiIn), and eliminate the underlying field hardware or communication diagnostic fault.

The APL Block Icon 'F' Symbol and EventState Bit Mapping

In SIMATIC PCS 7 Advanced Process Library (APL), block icons (e.g., @MotL/1, @VlvL/1, @PIDConL/1) provide condensed operational status through standard diagnostic badges. A frequent point of confusion for commissioning engineers is the sudden appearance of a prominent 'F' symbol (or wrench/lightning badge in certain icon styles) on the block icon, accompanied by a yellow or red status field.

In WinCC, this status badge is driven dynamically by the 32-bit DWORD tag .EventState, generated automatically during OS compilation. The 'F' badge indicates a Control System Fault (Leittechnik Störung / Fehler), which is asserted when:

Bit 24 is High: AS Process Controlling Message - Fault (Leittechnik Störung)
Bit 25 is High: AS Process Controlling Message - Error (Leittechnik Fehler)

These bits indicate that the controller has detected a failure within the control system infrastructure itself, rather than a normal process limit excursion (such as high level or high temperature).

Why the Faceplate Message List Appears Empty During Faults

When engineers click the faulted block icon to inspect the problem, the standard faceplate message view frequently displays an empty alarm list, deepening the mystery. This discrepancy arises from three standard configuration behaviors:

WinCC Message Class Filtering: The default APL faceplate alarm view is configured to display standard process alarms (Alarm High, Warning Low). The message class 'AS Process Controlling Messages' is often filtered out of the standard view and requires selecting the 'All Messages' tab.
Locked or Suppressed Messages: If message locking (MsgLock = 1) or out-of-service mode (OosAct = 1) was recently toggled, the underlying ALARM_8P state machine in the AS may retain the active error state in .EventState while suppressing the alarm display.
Transferred Channel Driver Faults: The fault was generated by a subordinate channel driver block (Pcs7AnIn), which reported the error into its own faceplate, while passing only a binary error flag (CSF) to the parent control block.

Tracing Signal Assignments in CFC Message Configuration (Alarm_8P)

Every APL technological block incorporates an internal Siemens ALARM_8P function block to manage messaging. To determine the precise cause of the 'F' symbol:

Open the Continuous Function Chart (CFC) containing the affected block in SIMATIC Manager.
Right-click the block (e.g. MotL) and select Object Properties.
Click the Messages button in the lower-right corner of the dialog.
In the message configuration table, locate the rows assigned to the message class AS Process Controlling Message:
- Typically, SIG_2 or SIG_3 is assigned to 'Control System Fault' with identifier $BlockComment$ External Error or CSF.
- Dynamic monitoring errors (MonDynErr) or runtime feedback faults (FbkOpenErr) may also be mapped to control system error classes depending on project library templates.

The Role of Control System Fault (CSF) and Channel Driver Interconnections

In more than 90% of industrial occurrences, the 'F' symbol is triggered by the block's CSF (Control System Fault) input pin:

Standard Driver Wiring: In standard APL engineering, the CSF input of an actuator or control block is interconnected directly to the Bad output of the associated field channel driver (Pcs7AnIn.Bad, Pcs7DiIn.Bad, or FbDrive.Bad).
Trigger Conditions: When the underlying I/O module (ET 200M / ET 200SP) detects an open wire, sensor short circuit, communication failure, or missing module power supply, the channel driver asserts Bad = 1. This immediately drives CSF = 1 on the control block, which triggers ALARM_8P to assert Bit 24 in .EventState and display the 'F' badge on the WinCC block icon.

EventState Bit Mapping and Alarm Class Architecture Matrix

The following table details the standard 32-bit .EventState mapping in SIMATIC PCS 7 APL:

Bit PositionEventState Functional DefinitionAssociated Alarm ClassBlock Icon Visual BadgeBit 16 / 17Alarm High / Alarm LowProcess Alarm (Alarm)Flashing Red ExclamationBit 18 / 19Warning High / Warning LowProcess Warning (Warning)Flashing Yellow ExclamationBit 20 / 21Tolerance High / Tolerance LowProcess ToleranceStatic Yellow DotBit 24AS Process Controlling Message: FaultAS Process Controlling Fault'F' Symbol / Red WrenchBit 25AS Process Controlling Message: ErrorAS Process Controlling Error'F' Symbol / Yellow WrenchBit 28Message AcknowledgedStatus InformationBadge ceases flashingBit 31Message Locked / SuppressedSystem StatusPadlock / Crossed Speaker

Step-by-Step Diagnostic and Troubleshooting Workflow

To isolate and resolve the underlying cause of the 'F' symbol:

Inspect CFC Chart Online: Open the CFC chart in test mode and locate the affected block. Check whether the CSF input pin is 1 (True).
Trace Driver Interconnection: If CSF == 1, trace the connection backwards to the source block (e.g. Pcs7AnIn or Pcs7DiIn). Check the outputs Bad, ModErr, and SimAct.
Check HW Config Diagnostics: Open HW Config, go online, and inspect the ET 200 distributed I/O station and channel module. Look for module fault LEDs (SF/BF) and open the Module Information diagnostic buffer to verify wire break, power loss, or channel overflow.
Inspect WinCC Alarm Logging: In WinCC Runtime, open the central Alarm Journal (Long-Term Archive List) without area filters. Filter specifically for message class 'AS Process Controlling Messages' to view the exact diagnostic message generated by the AS.

Verification

Verify full resolution of the Control System Fault:

Rectify the physical wiring issue or restore power to the affected field instrument.
In CFC test mode, confirm that Pcs7AnIn.Bad resets to 0 and Block.CSF resets to 0.
In WinCC Runtime, acknowledge the outgoing alarm in the alarm banner.
Confirm that Bit 24/25 in .EventState clears to 0 and the 'F' badge vanishes from the block icon.
Open the faceplate and verify that all status indicators show green and the drive/valve is ready for operator control.

Limitations and Operational Precautions

Adhere to the following operational precautions:

Never Disconnect CSF to Hide Alarms: Never disconnect the CSF pin or hard-wire it to 0 to clear the 'F' badge; CSF is a critical safety interlock that prevents automating drives during transmitter failures.
Alarm Flood Prevention: Ensure analog channel drivers have proper hysteresis configured (Hysteresis parameter on Pcs7AnIn) to prevent rapid cycling of Bad flags during intermittent wire breaks.
OS Compile Time Stamps: If block message attributes are modified in CFC, always perform an OS Compilation to ensure WinCC message descriptors and .EventState masks are synchronized.

Engineer FAQ

How do I find which signal input caused an ALARM_8P message in CFC?

In CFC, right-click the block, select Object Properties, click Messages, and examine the message configuration table to see which SIG input corresponds to the fault message class.

Can an 'F' symbol prevent a motor or valve from starting in Auto mode?

Yes. In standard APL logic, asserting CSF (Control System Fault) or an internal monitoring fault causes the block to trip into a fault state, blocking automatic start commands until reset.

Does clearing the physical wire break automatically remove the 'F' symbol?

Once the wire break is fixed and CSF returns to 0, the outgoing alarm message must typically be acknowledged in WinCC before the EventState bit clears and the 'F' icon disappears.