WinCC Status Tag Bit Mapping in Alarm Class Groups

The status tag of a WinCC Professional (TIA Portal) alarm class group is a packed bitfield, not a numeric counter. The field returns a single UDINT (or DWORD) value where the low half holds the current status of every configured alarm bit and the high half holds the corresponding acknowledgment flag. This reference decodes the bit layout, explains why 65537, 65536, and 1 appear for a single-bit alarm, and documents the configuration rules that prevent collisions when more than one alarm is mapped to the same class group.

1. Overview: What the Status Tag Reports

Each alarm class group in a WinCC Professional HMI screen (e.g. Errors, Warnings, System) owns a user-defined PLC tag — the status tag. Whenever the HMI raises, clears, or acknowledges an alarm belonging to that group, the runtime updates this tag bit-by-bit. The PLC or supervisory script can poll the tag to:

• Detect that a particular alarm is currently active (pending / incoming).
• Detect that an alarm is pending and has been acknowledged by the operator.
• Detect that an alarm has gone out without acknowledgment (chattering / clear).
• Drive external indicators, e-readers, or aggregated status word protocols.

The tag value is not a counter, not an alarm number, and not a sequence ID. It is a logical OR of the bit states assigned to the configured alarms at the time the runtime last updated the group.

2. Supported Data Types and Their Layout

WinCC Professional V12 and later lets you select the data type of the class-group status tag in the alarm configuration of the HMI device. The four supported types and their bit partitions are:

For installations that need to route more than 16 alarms into a single class group, split the alarms across multiple class groups rather than re-using bits.

3. Bit Layout Reference (UDINT example)

For a UDINT status tag the runtime uses the layout below. Bit indices are zero-based, MSB-first on the wire but numbered LSB-first in the configuration UI.

The decimal value of the tag is computed as:

StatusTag = Σ (StatusBit[i] × 2^i) + Σ (AckBit[i] × 2^(i+16))   for i = 0 … 15

This is a flat logical-OR. The runtime does not guarantee any particular bit ordering beyond the i / i+16 pair convention; the order of bits in the word is the order in which the alarms were assigned to the class group.

4. Decoding the Three Reference Values

The three values that appear in a freshly configured single-alarm group (65537, 65536, 1) are not arbitrary — each one is the exact sum of the bit weights that are set in the current state. Working through them for an alarm mapped to bit 0:

4.1 StatusTag = 65537 (alarm incoming, not yet acknowledged)

65537 = 0x00010001
  bit  0  = 1   → alarm pending
  bit 16  = 1   → not yet acknowledged
  all other bits = 0

This is the canonical "I have a new unacknowledged alarm" state. Most SCADA graphics use this as the trigger for an audible horn, an annunciator lamp, or an SMS escalation.

4.2 StatusTag = 65536 (alarm outgoing, never acknowledged)

65536 = 0x00010000
  bit  0  = 0   → alarm is gone
  bit 16  = 1   → still not acknowledged
  all other bits = 0

This is the "chattering" or "ghost" state. The trigger condition is no longer present, but the operator never pressed the ACK button. In plants that require mandatory acknowledgment, this state is logged and the event is escalated.

4.3 StatusTag = 1 (alarm still active, but acknowledged)

1 = 0x00000001
  bit  0  = 1   → alarm still pending
  bit 16  = 0   → operator pressed ACK
  all other bits = 0

The horn is silenced, the message is removed from the active list, but the alarm text remains visible until the underlying process bit returns to the safe state.

5. Bit-Pair Reference Table for the Lowest 8 Bits

The table below lists every combination of (status, ack) for the first 8 alarm slots of a UDINT tag. It is the same data a WinCC alarm log prints, just in tabular form.

For multiple simultaneous alarms, the tag is the bitwise OR of all active rows. For example, alarms 0 and 1 both pending, only alarm 0 acknowledged:

  bit  0 = 1   (alarm 0 pending)
  bit  1 = 1   (alarm 1 pending)
  bit 16 = 0   (alarm 0 acknowledged)
  bit 17 = 1   (alarm 1 not acknowledged)
  → StatusTag = 0b10…10 0000 0000 0000 0011 = 131075

6. Configuring the Status Tag in TIA Portal

6.1 Prerequisites

• TIA Portal V12 SP1 or later (HMI engineering).
• WinCC Professional / Comfort runtime license for the HMI device.
• A PLC tag of type UDINT, DWORD, WORD, or BOOL already declared in the PLC data block or HMI tag table.
• The HMI device must be configured with at least one alarm class group (default groups: Errors, Warnings, System).

6.2 Step-by-step configuration

1. In the project tree open HMI Tags → Default tag table and add a new external tag, e.g. HMI_AlarmStatus_Errors, of type UDINT, pointing at the PLC DB word you have reserved.
2. Open Alarms → Classes. The runtime delivers three predefined classes; double-click the one you want (typically Errors).
3. In the class properties, set the Status tag property to the tag created in step 1.
4. For each individual alarm belonging to the class (in Alarms → Discrete Alarms or Analog Alarms), open the alarm properties and select a free bit in the Status bit dropdown (0 … 15 for UDINT). The acknowledgment bit is automatically assigned to bit n+16.
5. Compile the HMI and download the runtime to the panel or PC.
6. Trigger the alarm from the PLC and verify the value as shown in Section 4.

6.3 Verification

Use a watch table in STEP 7 / TIA Portal to force the alarm trigger bit high. After the alarm is raised by the runtime, read the HMI tag — it should jump from 0 to 65537 within one polling cycle. Acknowledge the alarm on the runtime and the tag should fall to 1. Clear the trigger bit; if the alarm was not acknowledged, the tag should sit at 65536 until the operator ACKs it.

7. Bit Assignment Rules per Class

Within a single class group, each alarm must use a unique status bit. Re-using a bit inside the same class is the supported pattern — both alarms set the same bit, so the bit goes high when either alarm is pending and falls when both are gone. Re-using a bit across different classes is a different topic and is discussed in Section 8.

8. The "Same Bit in Different Classes" Anti-Pattern

It is technically possible in WinCC to assign the same bit number to alarms belonging to two different class groups, because the two groups own independent tags. The danger is conceptual: a maintenance engineer reading a single class status tag will treat the bit as belonging to that class only, and lose track of the cross-coupled alarm.

A second, more concrete failure mode appears when the same physical bit is used as the trigger in two different alarm groups. Consider:

• Group A, alarm 0, trigger bit = M0.0 → mapped to status bit 0 of tag A.
• Group B, alarm 0, trigger bit = M0.0 → mapped to status bit 0 of tag B.

M0.0 going high sets both tags to 65537. When M0.0 falls, both tags fall — but the operator may have intended to acknowledge alarm 0 in group A while the second group still had work to do. The set/reset of the bit works correctly for multiple alarms inside one class, but couples state across different classes when bits are shared. The recommended fix is to give every class its own dedicated bit pool, or, when a single trigger needs to feed two classes, create two separate discrete alarms in the PLC and let each class own a distinct bit.

9. State Machine for One Alarm Slot

The four meaningful states for a single (status, ack) pair can be drawn as the following finite state machine. Transitions are triggered by the alarm source (PLC trigger bit) and by the operator ACK key on the runtime.

The S3 → S0 transition (dashed) only happens when the trigger is already gone and the operator presses ACK. S2 → S0 only happens when the trigger falls and ACK has already been pressed.

10. Diagnostics and Verification in the Runtime

When the bit mapping appears to be wrong, follow the matrix below to localize the fault before assuming a runtime bug.

11. Migration Notes: V11 → V12 → V15 → V16 → V17

• V11 → V12: The class-group status tag was already a bitfield in V11, but the UI for selecting a status bit was reorganized in V12. When migrating, double-check that the bit assigned to each alarm was preserved (TIA Portal sometimes re-numbers the bit list when classes are renamed).
• V12 → V13/14: No change to the bit layout. The runtime performance of alarm updates was improved, but the wire format is identical.
• V15+: The Alarm ACK with a single tag feature lets a single tag of type UDINT acknowledge all alarms in a class. This is unrelated to the status tag, but the two tags are often wired to the same DB word in older projects — be careful not to use the same tag for both functions.
• V16 / V17: The OPC UA Alarms & Conditions (A&C) server exposes the same status bits as event fields. The mapping from WinCC status bit to OPC UA event ConditionId is one-to-one, so the same bit-pair table (Section 5) applies.

12. Practical Coding Patterns

12.1 SCL snippet — "is alarm 0 pending and acknowledged?"

// Inputs:  i_AlarmStatus : UDINT  // status tag of the class group
// Outputs: q_PendingAck  : BOOL   // alarm 0 is pending and ACKed
//          q_Horn        : BOOL   // any pending, not-ACKed alarm in the group

IF (i_AlarmStatus AND 16#00000001) <> 0 AND
   (i_AlarmStatus AND 16#00010000) = 0 THEN
    q_PendingAck := TRUE;   // S2 in the state machine
END_IF;

q_Horn := (i_AlarmStatus AND 16#0000FFFF) <> 0 AND
          (i_AlarmStatus AND 16#FFFF0000) <> 0;
// Horn is on only when at least one status bit AND at least one ACK bit are set,
// i.e. an alarm is pending without acknowledgment.

12.2 VB script — log the bit transitions to a CSV

' Run on the "Alarm Occurred" event of the HMI tag
Dim s : s = ""
Dim i : For i = 0 To 15
    If (SmartTags("HMI_AlarmStatus_Errors") And (2 ^ i)) <> 0 Then _
        s = s & "Alarm" & i & "=pending "
    If (SmartTags("HMI_AlarmStatus_Errors") And (2 ^ (i+16))) <> 0 Then _
        s = s & "Alarm" & i & "=ack "
Next
HMIRuntime.Trace "Alarm update: " & s

13. Related Tags You Will Also Configure

When you set the Status tag of a class, the HMI also lets you assign a number of companion tags. Keep them in a single DB so the PLC logic stays consistent:

14. Field-Proven Caveats