Configuring the PCS 7 APL FirstIn Block to Record First-In Interlock Trip Causes in Alarms

In SIMATIC PCS 7 APL (V9.0 / V9.1), configuring the FirstIn block (FB1888) to capture and alarm the exact cause of trip from an interlock block (such as Intlk08) requires connecting the interlock's 'FirstIn' output to the FirstIn block's 'FirstIn' input, and routing the resulting trip identifier into an EventTS (FB1886) or Event16TS block. To display descriptive trip text (e.g., 'Motor interlock: Low Flow') instead of a raw input integer ('Motor interlock: 2'), configure individual event messages in the Step 7 Message Configuration dialog of EventTS corresponding to each interlock channel, or use auxiliary string parameters.

Executive Summary

In complex process plants, critical equipment (such as feed pumps, compressors, and fired heaters) is protected by multi-channel interlock blocks (e.g., Intlk04, Intlk08, Intlk16 in the Advanced Process Library). When an interlock trips, multiple protective sensors frequently drop out within seconds (for instance, a motor trip causes low pressure, high temperature, and zero flow simultaneously). While the standard APL interlock faceplate displays which channel tripped first ('First-In' detection), operators and plant engineers need a permanent, auditable alarm message in WinCC Alarm Logging recording the exact initiating event.

Engineers attempting to use the APL FirstIn block (FB1888) often encounter a significant roadblock: connecting the block to technological message inputs (such as ExtMsg2 / ExtVal02 on MotL) typically outputs only the channel index integer (e.g., 'Motor interlock: 2') instead of the technological description (e.g., 'Motor interlock: Low Flow'). This guide provides the complete engineering methodology to configure the FirstIn block with Intlk08 and EventTS to generate clear, descriptive trip alarms.

Architecture and Fundamental Mechanics

The First-In diagnostic architecture links logic blocks, message generators, and timestamped event processing:

First-In Detection on Interlock Blocks:
- When multiple interlock inputs (In01 through In08) become invalid, Intlk08 latches the input that transitioned to FALSE first.
- The block outputs this information via the FirstIn output word, where each bit corresponds to a channel, or via an integer index.
Role of the APL FirstIn Block (FB1888):
- The FirstIn block acts as a mediator between interlock blocks and technological message blocks.
- It monitors the FirstIn output of one or more interlock blocks.
- It extracts the tripped channel number and outputs the value on pin ExtVa1FI (associated value for external message).
- It also generates discrete trigger pulses (ActFault / Out) to initiate alarm generation.
The Text vs Number Discrepancy:
- When ExtVa1FI is connected to a technological block's auxiliary value (e.g., MotL.ExtVal02) and formatted as @4%s@ or @4%u@, the compiler evaluates ExtVa1FI as an integer. Hence, WinCC displays the literal number of the input channel (1, 2, 3...).
- ExtVa1FI cannot dynamically extract the user text configured on the Intlk08 input labels because those labels exist in the WinCC CS database, not in controller memory.
Solution via EventTS / Event16TS:
- To produce descriptive alarm texts, the individual fault bits or the decoded First-In outputs are connected to an EventTS (FB1886) or Event16TS block.
- In the STEP 7 Message Configuration of EventTS, each discrete message signal (SIG_1 through SIG_8) is assigned its own descriptive text (e.g., SIG_1: 'Trip: High Pressure', SIG_2: 'Trip: Low Flow').
- When FirstIn detects the initial trip, it triggers the corresponding EventTS signal, which logs the exact descriptive message to WinCC Alarm Logging with millisecond-accurate controller timestamps.

Step-by-Step Implementation Procedure

Follow these steps to configure descriptive first-in interlock alarm messages in PCS 7:

Configure the Interlock Block (Intlk08):
- Place Intlk08 in the CFC chart.
- Connect field interlock signals to inputs In01 through In08.
- In the block properties, ensure 'First-in detection' is enabled.
- Connect the output Out of Intlk08 to the motor or valve interlock input (e.g., MotL.Permit or MotL.Protect).
Insert and Interconnect the FirstIn Block (FB1888):
- From the APL library, insert the FirstIn block into the chart.
- Connect Intlk08.FirstIn to FirstIn.FirstIn.
- Connect Intlk08.Out to FirstIn.InvIn (or relevant invert input to detect trip edges).
Insert EventTS (FB1886) for Timestamped Alarms:
- Insert an EventTS block alongside the FirstIn block.
- Connect the individual decoded outputs of the interlock or First-In signals to In1 through In8 of EventTS.
- (Alternatively): If using FirstIn with a single trigger, connect FirstIn.Out to EventTS.In1 and connect FirstIn.ExtVa1FI to EventTS.ExtVal01.
Configure Message Texts in STEP 7 Message Configuration:
- Right-click the EventTS block > select 'Object Properties' > click 'Messages...'.
- In the Message Configuration table, customize the 'Event' column for each input signal:
- SIG_1: Motor Trip Cause: High Discharge Pressure
- SIG_2: Motor Trip Cause: Low Lubrication Flow
- SIG_3: Motor Trip Cause: High Bearing Temperature
- SIG_4: Motor Trip Cause: Emergency Stop PB
- Click 'OK' to save the message definitions.
Interconnect EventTS with Motor Faceplate (Optional):
- To make these messages appear in the message view of the motor faceplate, connect EventTS.EventTsOut to MotL.EventTsIn.
- This establishes the technological message association between the standalone event block and the drive icon.
Compile and Download:
- Compile CFC charts and execute an AS-OS compilation.
- Download changes to the S7 CPU in RUN mode.

Verification

Open the CFC chart in Test Mode (Online).
Force multiple interlock inputs to FALSE in rapid succession (e.g., force In02 to FALSE, followed immediately by In01).
Open the Intlk08 faceplate in WinCC Runtime: confirm that input 2 is highlighted as the 'First-In' trip cause.
Open the WinCC Alarm List: confirm that an alarm message is logged reading: Motor Trip Cause: Low Lubrication Flow with the precise timestamp of the trip.
Confirm that subsequent secondary trips (In01) are suppressed or logged as secondary events, ensuring unambiguous operator diagnosis.

Limitations and Operational Precautions

Directly passing dynamic strings from controller memory requires significant CPU memory and communication overhead; using EventTS with pre-configured message text strings in STEP 7 is the standard, high-performance PCS 7 recommendation. In safety-critical ESD applications, first-in trip logging must be coordinated with Safety Matrix or F-System sequences. Engineering software and library licenses should be verified through an authorized Siemens distributor or the Siemens Industry Mall.

Engineer FAQ

Why does ExtVa1FI only display a number in my alarm message?

Because ExtVa1FI outputs the integer index of the tripped channel; it does not read technological text labels from WinCC CS.

What block provides descriptive first-in alarm messages?

The EventTS (FB1886) or Event16TS block, where message texts are configured per channel in the STEP 7 Message Configuration dialog.

Can EventTS messages appear inside the motor faceplate alarm list?

Yes, by interconnecting EventTS.EventTsOut to MotL.EventTsIn.

Does FirstIn block work with Intlk04 and Intlk16?

Yes, the FirstIn block supports all standard APL interlock blocks (Intlk02, Intlk04, Intlk08, Intlk16).

Can FirstIn alarms be acknowledged from the motor faceplate?

Yes, when technological message interconnection is established, acknowledging the motor alarm acknowledges associated EventTS messages.

Technical References

Related Tags

FirstIn
Intlk08
EventTS
Interlock Alarms
Cause of Trip
PCS 7 V9.1

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Configuring the PCS 7 APL FirstIn Block to Record First-In Interlock Trip Causes in Alarms