Resolving TIA Portal Bool Array Trigger Tag Error in WinCC

Problem Overview

When configuring discrete alarms on a SIMATIC HMI with WinCC Comfort or WinCC Unified in TIA Portal V16 (or later), engineers frequently encounter a configuration rejection when assigning an Array [0..31] of Bool PLC tag as the alarm trigger tag. The TIA Portal editor returns an error similar to "The data type is not permitted for a trigger tag" or rejects the HMI tag during the Trigger tag selection dropdown. This blocks the entire alarm pipeline: no bit can be mapped to a discrete alarm text, no acknowledge variable can be created, and the project cannot be compiled to the HMI runtime.

The symptom typically presents as follows:

• HMI tag is created as Array [0..31] of Bool and successfully linked to the matching PLC tag (e.g., "plcdata".Alarm).
• The HMI tag is reachable in the tag table, online monitoring is functional, and the connection status shows OK.
• When the tag is selected in the Discrete alarms editor and the Trigger tag field is opened, the entry is either greyed out or rejected with a data-type validation error.
• Compiling the HMI produces warnings such as "Tag <name> cannot be used as a trigger tag" or "Invalid data type for trigger".

The root of the failure is not a bug, a firmware defect, or a licensing issue; it is a hard architectural constraint in the WinCC alarm subsystem.

Root Cause: WinCC Trigger Tag Data Type Requirements

WinCC Comfort, WinCC Advanced, and WinCC Unified all require that the trigger tag for a discrete alarm be a fixed-width, bit-addressable integer type. According to the official WinCC Unified V20 alarm configuration documentation, valid trigger tag data types are:

• Bool (single bit, single alarm)
• Byte (8 alarms, bit 0-7)
• Word (16 alarms, bit 0-15)
• DWord (32 alarms, bit 0-31)
• LWord (64 alarms, bit 0-63)
• Array of Byte / Word / DWord / LWord (multi-word alarm blocks)

The reference is documented in the Siemens TIA Portal Help under Configure trigger (RT Unified) - WinCC Unified.

An Array of Bool is not a valid trigger type because the alarm subsystem must address individual bits inside a contiguous, machine-word-aligned memory region. The runtime must be able to compute a single absolute bit offset (byte index × 8 + bit index) and read the value in a single fetch. A boolean array breaks that model: TIA Portal would have to either (a) generate a separate fetch per bit (catastrophic for 32-bit alarm clusters) or (b) maintain a shadow register. Siemens chose to disallow the configuration instead.

This restriction applies regardless of:

• The HMI panel series (Comfort, Unified Comfort Panels, WinCC Runtime Advanced, WinCC Runtime Professional).
• Whether the source PLC tag resides in an optimized or non-optimized DB.
• Whether the HMI tag is internal or linked to a PLC tag via an HMI connection.
• The TIA Portal version (V15.1, V16, V17, V18, V19, V20, and current V21).

Trigger Tag Parameter Reference

Solution 1: Use a Word / DWord Array in a Non-Optimized DB

The cleanest fix is to change the alarm source data type at the PLC level from Array [0..31] of Bool to Array [0..1] of Word (32 bits) or Array [0..0] of DWord (32 bits). This requires modifying the data block from optimized to non-optimized so that the HMI can address each word absolutely.

Step 1 — Recreate the alarm block

1. Open the project in TIA Portal and navigate to the Program blocks tree.
2. Right-click the alarm source DB and select Properties → Attributes.
3. Uncheck Optimized block access. Click OK and confirm the recompile prompt.
4. Replace the existing Array [0..31] of Bool member with Array [0..1] of Word (rename to e.g. AlarmWord) or Array [0..0] of DWord.
5. Recompile the PLC program; fix any code that referenced individual bool elements with new bit syntax (e.g., DB.AlarmWord[0].%X5 instead of DB.AlarmBool[5]).

Step 2 — Map the HMI tag

1. Open the HMI tag table on the Comfort Panel project.
2. Add a new tag (e.g., HMI_AlarmWord) with data type Word or Array of Word.
3. In the Connection column, link to the same PLC, and in the Address field, type the absolute DB address (e.g., DB100.DBX0.0 WORD for the first 16 bits, or use the dropdown to navigate to "MyDB".AlarmWord[0]).
4. Verify the tag compiles without warnings.

Step 3 — Configure the discrete alarm

1. Open HMI alarms → Discrete alarms.
2. Add a new alarm row.
3. Set Trigger tag to HMI_AlarmWord (or HMI_AlarmWord[0]).
4. Set Trigger bit to the desired bit index (0-15 for Word, 0-31 for DWord).
5. Configure the alarm text, class (e.g., Warnings), and acknowledge tag if required.
6. Repeat for each bit. The editor now accepts the configuration because the data type is on the approved list.

Solution 2: SCATTER_BLK and GATHER_BLK Conversion (Optimized DB Compatible)

For projects that must keep the optimized DB attribute (typical for S7-1500 / S7-1200 firmware 4.x and newer), use the SCATTER_BLK and GATHER_BLK instructions to maintain a parallel Word/DWord image of the bool array. The HMI then points to the image, not the source bool array.

Data Flow

The PLC program copies bits from the optimized source array into a non-optimized (or simply non-array-of-bool) Word/DWord that the HMI can poll:

// FB "Alarm_Image" — called cyclically in OB1 or in the alarm-source FB
// Inputs
//   boolSrc : Array[0..31] of Bool   (from optimized DB, e.g. "plcdata".Alarm)
// Outputs
//   dwordImg: DWord                  (HMI trigger image)

// SCATTER: Bool[0..31]  ->  DWord
"GATHER_BLK"(
    IN  := "plcdata".Alarm,           // source: Array[0..31] of Bool
    OUT := #dwordImg                  // dest : DWord
);

For the reverse direction (e.g., HMI writes an acknowledge bit back), use SCATTER_BLK:

// SCATTER: DWord  ->  Bool[0..31]
"SCATTER_BLK"(
    IN  := #dwordAckImg,             // source: DWord from HMI
    OUT := "plcdata".Ack             // dest : Array[0..31] of Bool
);

Step-by-Step Setup

1. Create a new global DB (e.g., DB_HMI_AlarmImage) and leave it optimized.
2. Add the members: TriggerImage : DWord; and AckImage : DWord;
3. Add an FB or FC that calls GATHER_BLK on the bool source array and writes the result into TriggerImage. Call it in OB1 (or in a cyclic interrupt OB30 at 100 ms) to keep the image in sync.
4. If acknowledges are required, call SCATTER_BLK in the same cycle to push AckImage back to a Array[0..31] of Bool.
5. Compile the PLC. Use a watch table to confirm the image updates when the source bits toggle.
6. On the HMI side, add a tag HMI_TriggerImage with data type DWord, link it to "DB_HMI_AlarmImage".TriggerImage.
7. Open the discrete alarm editor, assign HMI_TriggerImage as the trigger tag, and set the trigger bit to 0..31 for each of the 32 alarms.

Solution 3: Per-Bit Bool Tags with Single-Bit Trigger

If the alarm count is small (≤ 16 alarms) and the project does not require a clustered trigger tag, an alternative is to expose each alarm bit as a separate Bool tag at the HMI level and assign it directly to one alarm row per tag.

1. Leave the optimized bool array in the PLC as-is.
2. In the HMI tag table, add 32 individual Bool tags (e.g., HMI_Alarm_00 through HMI_Alarm_31).
3. For each tag, set the access mode to Symbolic access and select the individual array element (e.g., "plcdata".Alarm[5]).
4. Create 32 discrete alarm rows, each using its corresponding HMI_Alarm_NN tag as the trigger.

Trade-off: this multiplies the HMI tag count and the polling load, but it preserves the optimized DB and the original PLC code structure. For 32 bits on a Comfort Panel over EtherNet/IP or PROFINET, the tag count is well within HMI capability (Comfort Panels support up to 4096 tags in the standard model, 32 of which is trivial).

Cross-Version Compatibility Table

Verification Procedure

1. PLC compile: Re-compile the PLC program after any DB change. The output window must show 0 errors for the modified FB/FC and DB.
2. Download: Download the PLC program to the S7-1500 / S7-1200 in STOP-RUN, or perform a download-in-RUN if supported.
3. HMI compile: Compile the HMI project. No warning "Tag cannot be used as a trigger tag" should remain.
4. HMI download: Download the runtime to the Comfort Panel (or start the Unified RT).
5. Online test: From TIA Portal, use Online & diagnostics → Force on a single bit (e.g., plcdata.Alarm[5] or plcdata.AlarmWord[0].%X5). Verify the corresponding alarm row appears in the HMI message view within one HMI poll cycle.
6. Acknowledge test: Press the Acknowledge button on the HMI (or use the touch area wired to the ack tag). Verify the bit in the ack image toggles and the alarm clears from the active list.
7. Cold restart: Power-cycle the PLC and HMI. Reload the project. Confirm the alarm image initializes to zero and no false alarms appear at startup (if needed, pre-initialize the image DB with explicit 0 in its startup values).

Troubleshooting Matrix

Performance and Sizing Considerations

For a single DWord alarm cluster (32 alarms), the runtime cost is negligible: one GATHER_BLK call per PLC cycle reads 32 bools and packs them into a 32-bit word. At a typical 10 ms OB1 cycle, this is sub-microsecond on an S7-1500 CPU 1515 or higher.

For larger alarm counts (256, 1024, 4096), the gather approach scales linearly. The HMI poll load is the more relevant bottleneck: a Comfort Panel polling 128 DWords at 250 ms generates ~512 bit reads/s, which is well within the panel's capacity. Unified RT on a PC scales into the tens of thousands of tags without issue.

Memory layout for the image DB:

• 1 DWord = 4 bytes → 32 alarm bits.
• 16 DWords = 64 bytes → 512 alarm bits.
• 128 DWords = 512 bytes → 4096 alarm bits.

For projects exceeding 4096 bits, split the alarm image into multiple DWords and configure each block in its own discrete alarm group. WinCC allows up to 32 trigger tags per alarm line set, and the editor handles multi-cluster alarms transparently when the trigger is an Array of Word / DWord.

Frequently Asked Questions