Generating WinCC Alarm Messages for Drive Interlock Trip Conditions in SIMATIC PCS 7

In SIMATIC PCS 7 Advanced Process Library (APL), interlock blocks (such as Intlk02, Intlk04, Intlk08) are pure logic blocks and do not contain integrated Alarm_8P message blocks; consequently, configuring alarms directly inside the interlock block is not supported. To generate an alarm in WinCC Runtime when an interlock trips a motor, connect the inverted output of the interlock block (or dedicated trip condition) to an external message input on the motor block (such as MotL.ExtMsg1, ExtMsg2, or ExtMsg3). In the motor block's message configuration dialog, assign the message text (e.g. 'Interlock Tripped: Lubrication Low'). This routes the alarm directly under the motor tag, triggering faceplate alarm badges and group display updates.

Why Interlock Blocks Do Not Generate Native WinCC Alarms

In SIMATIC PCS 7 process automation architectures, software design balances diagnostic visibility against controller memory and Process Object (PO) consumption. Interlock blocks (Intlk02, Intlk04, Intlk08, Intlk16) are designed as lightweight combinational logic blocks.

If every interlock block contained native Alarm_8P message routines, controller execution cycles would slow down and each interlock instance would consume additional AS and OS Process Objects. Therefore, Siemens engineered interlock blocks to evaluate logic (In01..In08) and pass a single consolidated boolean status (Out = 0 or 1) to the technological actuator block.

When an interlock trips a drive, the operator must know which specific field condition tripped the motor without having to open engineering CFC charts. This is accomplished using the actuator block's external message architecture.

Architecture of External Messages (ExtMsg1..3) on Drive Blocks

APL drive and actuator blocks—such as MotL (FB1850), MotRevL (FB1851), MotSpdCl (FB1856), and VlvL (FB1899)—incorporate native Alarm_8P instances with pre-allocated external message channels:

Actuator Input PinInternal Alarm_8P SignalDefault Message ClassConfigurable Event TextExtMsg1SIG4Alarm (or Warning)$$BlockComment$$ External message 1ExtMsg2SIG5Alarm (or Warning)$$BlockComment$$ External message 2ExtMsg3SIG6Alarm (or Warning)$$BlockComment$$ External message 3

By tying interlock trip states to ExtMsg1..3, the generated alarm inherits the technological tag name of the motor (e.g., 101_PUMP_01). This ensures that:

The WinCC alarm line displays the motor tag name and description.
The motor block's Group Display (@PG_GroupDisplay) in the overview area illuminates with an active alarm icon.
Clicking the alarm line in the OS alarm banner opens the motor faceplate directly.

Step-by-Step CFC Interconnection and Message Setup Workflow

To configure an interlock trip alarm for a motor in CFC, execute the following technical procedure:

Connect Interlock Output to Motor Interlock Input: In CFC, connect Intlk04.Out to MotL.Intlock (or Protect / Permit, depending on whether it is a running interlock, safety interlock, or start permissive).
Invert Signal for External Message: An interlock healthy state is Out = 1, and trip state is Out = 0. Connect Intlk04.Out to MotL.ExtMsg1. Click on the connection pin at ExtMsg1 and press Ctrl+I (or right-click and select Invert Input) so that when the interlock trips (0), the input pin reads 1.
Configure Alarm Message Text: Right-click MotL and select Object Properties. Click the 'Messages...' button.
Edit Signal 4 (SIG4): In the message table, locate the row corresponding to SIG4 (External Message 1):
- Set the Message Class (e.g. Alarm - High or AS Process Control Fault).
- In the Event column, replace the default text with the specific interlock description: Interlock Trip: Lubrication Pressure Low.
Compile and Download: Save the dialog. Execute Chart -> Compile -> Chart as Program -> Changes Only and download to the AS. Execute an OS compile (Compile OS -> Changes Only) to update WinCC Alarm Logging.

Verification

Verify that interlock trips generate clear WinCC alarms through these testing steps:

Simulate Interlock Trip: In CFC Test Mode, force the input signal feeding the interlock (Intlk04.In01) to 0.
Actuator Reaction: Verify that MotL.Fwd drops to 0, stopping the motor, and the red interlock badge appears on the faceplate.
WinCC Alarm Display: In WinCC OS Runtime, observe the alarm banner. Confirm that an alarm appears reading:
[Tag: 101_PUMP_01] Interlock Trip: Lubrication Pressure Low
Group Display Hierarchy: Verify that the picture icon containing the motor displays a flashing red alarm badge in the top overview hierarchy.

Limitations and Operational Precautions

When engineering interlock alarms, adhere to these operational and alarm management standards:

Alarm Flood Prevention (ISA-18.2): Do not create separate alarms on both the transmitter (MonAnL) and the motor (MotL.ExtMsg1) if they report the exact same event. If the lubrication pressure transmitter already generates an alarm, consider setting ExtMsg1 to a lower priority or routing it to the process log.
First-Out Interlock Identification: If multiple interlock conditions feed a single Intlk08 block, use the APL First-In First-Out (FirstIn) feature. The interlock faceplate will highlight which exact sensor caused the trip.
Inversion Accuracy: Ensure input inversion is properly applied; failing to invert ExtMsg1 will generate a persistent standing alarm when the interlock is healthy.

Engineer FAQ

Why can I not find a 'Messages' button on the Intlk04 block in CFC?

The Intlk04 block is an algorithmic logic block without Alarm_8P message capability. Only technological blocks (MotL, VlvL) possess the message configuration interface.

Can I pass the name of the failing interlock input dynamically into the alarm?

Yes. By interconnecting the interlock status word to the motor's associated values (ExtVa104..07), dynamic text placeholders can insert the failing signal tag into the alarm text.

What is the difference between Intlock, Permit, and Protect on MotL?

Permit is a start permissive (only checked prior to starting). Intlock is an operating interlock with automatic reset. Protect is a safety interlock requiring manual operator reset.