Configuring PLC-Driven External Horns and Acoustic Annunciators in WinCC and PCS 7

In SIMATIC PCS 7 and WinCC V7.4, triggering an external physical acoustic horn or siren wired to an S7 controller digital output (such as Q38.7) upon alarm occurrence—rather than playing sound files through a PC audio card—requires properly routing the WinCC Horn option through process tags. When alarms trigger but the PLC horn output remains inactive, the failure occurs because WinCC Horn is configured to write to a local internal tag that is not dispatched to the automation station, the Horn application is omitted from the WinCC computer startup list, or incoming alarms do not match configured message class filters. To resolve this, map the Horn signal to an AS process tag connected to a CFC pulse or latch block, enable Horn in computer startup properties, and configure global Horn acknowledgment.

WinCC Horn Signaling Architecture: PC Audio vs. PLC Field Annunciators

In industrial plants, audible alarm annunciation is vital for alerting operators to critical process excursions, combustible gas leaks, or emergency trip conditions. The WinCC Horn option (Horn.exe) provides centralized acoustic and optical signaling based on active alarm states.

WinCC Horn supports three signaling hardware methods:
1. Sound Cards: Playing local .wav audio files through desktop multimedia speakers (standard for quiet control rooms).
2. Signal Modules: Utilizing hardwired PCI/PCIe annunciator cards installed directly inside the WinCC computer.
3. Tags (Process Tag Assignment): Setting a binary tag in the WinCC tag dictionary that is communicated across Industrial Ethernet to the S7 controller. The controller logic evaluates the tag and energizes an industrial 24V DC or 120V AC siren, horn, or rotating beacon connected to a physical digital output module (DO).

When employing the tag-based method, the Horn engine coordinates alarm evaluation, message class filtering, acknowledgment tracking, and silence commands centrally across multi-client systems.

Root Causes of Inactive PLC Horn Outputs During Alarm Events

When an engineer tests a manual pushbutton in WinCC and successfully sounds the horn, but real process alarms fail to trigger the siren, technical analysis identifies four common configuration gaps:

Tag Scope Mismatch (Internal Tag vs. Process Tag): In WinCC Horn Signal Assignment, engineers frequently create an internal HMI tag named Horn_Trigger. While the Horn engine sets this internal tag to 1, it is never transmitted to the PLC unless explicitly interconnected via WinCC Tag Management to an S7 communication channel (Named Connections).
Omission of Horn from WinCC Computer Startup: The WinCC Horn service (Horn.exe) is an optional background task. If Horn is not checked under Computer Properties -> Startup -> Additional Applications/Tasks, the alarm monitoring daemon never initializes in runtime, ignoring all incoming alarms.
Message Class and Priority Filtering Gaps: In WinCC Horn Message Assignment, the engineer must explicitly check which message classes (e.g., Alarm High, Alarm Low, Warning High, Process Message) activate the horn. If alarms are classified under custom or unassigned classes, the Horn trigger ignores them.
Latch and Reset Logic in CFC: Field horns require pulse stretching or latching in the AS. If the controller logic expects a rising edge pulse rather than a steady boolean level, the horn may fail to stay energized.

WinCC Horn Configuration Parameters and Tag Mapping Matrix

The following table summarizes the required configuration tabs and parameters within the WinCC Horn editor:

Configuration TabParameter NameSetting / ValueOperational FunctionMessage AssignmentMessage Class / PrioritySelect Alarm High, ToleranceDefines which alarm categories trigger acoustic signalingSignal AssignmentTag NameS7 Process Tag (e.g., AS01_Horn_Req)Binary tag written to 1 by Horn upon alarm detectionSignal AssignmentSound / Output TypeSet to Tag (Not Sound Card)Disables local audio playback; routes to process tagComputer PropertiesStartup Task ListCheck HornAutomatically launches Horn.exe on WinCC Runtime startAcknowledgmentHorn AcknowledgmentLink to @HornReset / HORN_ACKClears horn tag when operator clicks Silence buttonCFC Logic BlockTechnological BlockOP_D / RS_FF / Pulse BlockInterfaces HMI tag to physical hardware output Q38.7

Step-by-Step Procedure to Configure PLC-Driven Acoustic Horns

To configure WinCC and PCS 7 to sound an external field horn via an S7 digital output:

Create the Process Tag in WinCC Tag Management:
- In WinCC Explorer, open Tag Management.
- Under SIMATIC S7 Protocol Suite -> Industrial Ethernet (or Named Connection), create a binary tag: AS01_Horn_Active.
- Map the tag address to the controller data block or marker address (e.g., DB100.DBX0.0 or M50.0).
Configure WinCC Horn Options:
- In WinCC Explorer, double-click Horn.
- In the Message Assignment tab, add the message classes you want to annunciate (e.g., Alarm - High, Alarm - Low).
- Assign each class to Signal 1.
- Switch to the Signal Assignment tab.
- For Signal 1, set the output type to Tag, and select the process tag AS01_Horn_Active.
- In the Horn Acknowledgment section, verify that global acknowledgment is enabled using standard internal tag @HornReset.
- Click Apply and close Horn.
Enable Horn in WinCC Computer Startup:
- In WinCC Explorer, right-click Computer, select Properties, and open the Startup tab.
- Under Additional Tasks/Applications, verify that Horn is checked.
- If configuring redundant servers or clients, repeat this startup setting on all stations.
Configure Controller Logic and Digital Output in CFC:
- In SIMATIC Manager, open the CFC chart controlling the acoustic horn.
- Interconnect the tag AS01_Horn_Active to an OP_D block or set input of an RS_FF latch.
- Connect the latch output to the digital output driver (Pcs7DiOu) addressing physical terminal Q38.7.
- Interconnect a reset condition from the WinCC Horn Acknowledge button (@HornReset routed back to PLC, or a physical control room silence pushbutton) to unlatch the horn.
- Compile and download CFC and OS changes.

Verification

Verify acoustic alarm operation:
1. In CFC online mode or via a field sensor, simulate an alarm excursion (e.g., trip a high pressure switch).
2. Confirm that WinCC Alarm Logging receives the alarm, the alarm line flashes red, and tag AS01_Horn_Active immediately transitions from 0 to 1.
3. Verify that physical controller output Q38.7 energizes, sounding the field siren.
4. In the WinCC overview toolbar, click the Horn Silence / Acknowledge button; confirm that AS01_Horn_Active drops to 0 and the siren silences, while the alarm line on screen remains visible until individually acknowledged.

Limitations and Operational Precautions

Separate Silence from Acknowledgment: Operating standards (ISA-18.2) mandate that silencing the acoustic horn must not automatically acknowledge the alarm on screen; the horn silences, but the visual alarm remains.
Continuous Siren Fatigue: Equip physical field horns with an automated timeout (e.g., 2 minutes via TIMER_P) to prevent excessive noise pollution and neighbor disturbances if the control room is temporarily unattended.
Redundant Server Horn Routing: Ensure that only the master OS server (or the client commanding the active console) writes the horn tag, preventing conflicting signals.

Engineer FAQ

Can WinCC Horn trigger a PLC digital output directly?

Yes, by configuring Signal Assignment in WinCC Horn to write to a binary process tag mapped to an S7-400 controller address.

Why did the horn sound with a screen button but not with alarms?

The screen button wrote directly to the PLC tag, whereas the Horn application was either not running in Computer Startup or the alarm message classes were unmapped.