(*═══════════════════════════════════════════════════════════════════════════════
  FB_WeldSequencer
  S7-1500 SCL — 10-Step Robotic Welding Cell Sequencer
  TIA Portal V18+  |  Optimized Block Access
  ─────────────────────────────────────────────────────────────────────────────
  APPLICATION:  MIG/MAG Welding Cell — Clamp → Weld → Release cycle
  ─────────────────────────────────────────────────────────────────────────────

  MODES:
    AUTO  — Steps advance automatically the moment advance condition is met
    INCR  — Operator must press Cmd_Incr AFTER advance condition is TRUE
            (used for commissioning, fault recovery, or slow-speed prove-out)

  SEQUENCER STATES  (SeqState output INT):
    0 = IDLE            Waiting for Start command
    1 = RUNNING         Sequence actively executing
    2 = PAUSED          Outputs held; resumes from same step on Cmd_Start
    3 = STOPPING_CAT1   Controlled stop: current step finishes then halts
    4 = STOPPED         CAT1/CAT2 halt complete; Reset required
    5 = FAULTED         Hard fault; safety interlock; Reset + clear required
    6 = COMPLETE        All 10 steps done; awaiting restart or Reset

  STOP CATEGORIES (IEC 60204-1):
    CAT0  Immediate: ALL outputs de-energised instantly
          Trigger:  Cmd_EStop = TRUE  AND  StopCategory = 0
    CAT1  Controlled: current step completes its "safe motion" then stops.
          A 10 s timeout (CAT1_TIMEOUT) guards against stuck conditions.
          Trigger:  Cmd_EStop = TRUE  AND  StopCategory = 1
    CAT2  Suspend & hold: outputs frozen at current state until Reset
          Trigger:  Cmd_Stop (always CAT2)
                    Cmd_EStop = TRUE  AND  StopCategory = 2

  PAUSE vs STOP:
    PAUSE  — Soft, temporary. Cmd_Pause latches PAUSED state. All outputs held.
             Cmd_Start resumes from same step / same position.
             Safety interlock (door open) will NOT allow resume.
    STOP   — CAT2 permanent hold. Requires Cmd_Reset to return to IDLE.
             Typically used for planned end-of-shift or material change.

  STEP FEATURES:
    _stepEnabled[N]      = FALSE  → step is silently skipped every cycle
    _stepSafetyLocked[N] = TRUE   → operator Cmd_SkipStep is ignored
    Cmd_SkipStep (rising edge)    → forces advance on UN-locked steps only
    _stepUseTimer[N]     = TRUE   → advance on timer expiry (no sensor required)
    _stepTimerPreset[N]           → IEC time preset for timer steps

  FAULT CODES (FaultCode output INT):
    0   No fault
    10  Safety door opened during run / pause
    11  E-Stop activated  (CAT0/1/2 determined by StopCategory)
    20  CAT1 controlled-stop timeout (step overran CAT1_TIMEOUT)
    30  Sensor conflict: FWD + BWD sensors simultaneously TRUE
    40  Step watchdog timeout: step did not complete in WATCHDOG_TIME

  I/O SUMMARY — WELDING CELL:
  ┌──────┬──────────────────────────┬──────────────┬────────────────────────────┐
  │ Step │ Name                     │ Advance       │ Sensors / Timers           │
  ├──────┼──────────────────────────┼──────────────┼────────────────────────────┤
  │   1  │ Home Position Verify     │ 2 Sensors    │ Sen_HomeFwd + NOT HomeBwd  │
  │   2  │ Clamp Workpiece          │ 1 Sensor     │ Sen_ClampClosed            │
  │   3  │ Extend Weld Head         │ 1 Sensor FWD │ Sen_HeadAtWeldPos (FWD)   │
  │      │                          │ (2 declared) │ Sen_HeadRetracted  (BWD)   │
  │   4  │ Pre-Purge Gas            │ Timer  2.0 s │ None                       │
  │   5  │ Arc Strike               │ 1 Sensor     │ Sen_ArcDetect     🔒LOCKED │
  │   6  │ Weld Travel              │ Timer  4.0 s │ None                       │
  │   7  │ Arc Off / Weld End       │ 1 Sensor     │ Sen_ArcOff        🔒LOCKED │
  │   8  │ Post-Purge Gas           │ Timer  2.5 s │ None                       │
  │   9  │ Retract Weld Head        │ 1 Sensor BWD │ Sen_HeadRetracted (BWD)   │
  │      │                          │ (2 declared) │ Sen_HeadAtWeldPos  (FWD)   │
  │  10  │ Unclamp / Part Eject     │ 1 Sensor     │ Sen_UnclampConf            │
  └──────┴──────────────────────────┴──────────────┴────────────────────────────┘
  🔒 Safety-locked: Step 1 (home verify), Step 5 (arc confirm), Step 7 (arc off)
═══════════════════════════════════════════════════════════════════════════════*)

FUNCTION_BLOCK "FB_WeldSequencer"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1

VAR_INPUT
    //── MODE SELECTION ──────────────────────────────────────────────────────
    Mode_Auto           : Bool;     // TRUE = Automatic continuous advance
    Mode_Incr           : Bool;     // TRUE = Increment step-by-step mode

    //── OPERATOR COMMANDS (all rising-edge triggered unless noted) ──────────
    Cmd_Start           : Bool;     // Start cycle  /  Resume after Pause
    Cmd_Pause           : Bool;     // Pause: hold outputs, resume same step
    Cmd_Stop            : Bool;     // CAT2 Stop: hold & require Reset
    Cmd_EStop           : Bool;     // E-Stop: level signal (TRUE = activated)
    Cmd_Reset           : Bool;     // Clear fault / return to IDLE
    Cmd_Incr            : Bool;     // Increment next step (INCR mode only)
    Cmd_SkipStep        : Bool;     // Skip current step if not safety-locked
    StopCategory        : Int;      // E-Stop category: 0=CAT0, 1=CAT1, 2=CAT2

    //── SAFETY MONITORING ───────────────────────────────────────────────────
    Safety_DoorClosed   : Bool;     // Safety gate (NC contact, TRUE = closed/safe)
    Safety_EStopOK      : Bool;     // Safety relay feedback (TRUE = no E-Stop)

    //── PROCESS SENSORS (DI) ────────────────────────────────────────────────
    // STEP 1 — Home Position  [2 sensors]
    Sen_HomeFwd         : Bool;     // Fixture confirmed at home position
    Sen_HomeBwd         : Bool;     // Fixture NOT in work zone (interlock sensor)

    // STEP 2 — Clamp  [1 sensor]
    Sen_ClampClosed     : Bool;     // Clamp jaw fully closed on part

    // STEP 3 + 9 — Weld Head  [2 sensors: FWD + BWD shared]
    Sen_HeadAtWeldPos   : Bool;     // Head fully EXTENDED to weld start position
    Sen_HeadRetracted   : Bool;     // Head fully RETRACTED to safe position

    // STEP 5 — Arc Strike  [1 sensor]
    Sen_ArcDetect       : Bool;     // Welding arc current detected (TRUE = arc ON)

    // STEP 7 — Arc Off  [1 sensor]
    Sen_ArcOff          : Bool;     // Arc current extinguished (TRUE = arc OFF)

    // STEP 10 — Unclamp  [1 sensor]
    Sen_UnclampConf     : Bool;     // Clamp fully open / part cleared
END_VAR

VAR_OUTPUT
    //── ACTUATORS (DQ) ──────────────────────────────────────────────────────
    Act_Clamp           : Bool;     // Step 2–9:  Clamp solenoid (energise = clamp)
    Act_HeadExtend      : Bool;     // Step 3–8:  Weld head extend solenoid
    Act_HeadRetract     : Bool;     // Step 9:    Weld head retract solenoid
    Act_GasValve        : Bool;     // Step 4–8:  Shielding gas solenoid valve
    Act_WeldEnable      : Bool;     // Step 5–6:  Welder power enable
    Act_Unclamp         : Bool;     // Step 10:   Unclamp solenoid
    Act_PartEject       : Bool;     // Step 10:   Part ejector cylinder

    //── SEQUENCER STATUS ────────────────────────────────────────────────────
    Seq_Idle            : Bool;     // State = IDLE
    Seq_Running         : Bool;     // State = RUNNING
    Seq_Paused          : Bool;     // State = PAUSED
    Seq_Stopping        : Bool;     // State = STOPPING_CAT1
    Seq_Stopped         : Bool;     // State = STOPPED
    Seq_Faulted         : Bool;     // State = FAULTED
    Seq_Complete        : Bool;     // State = COMPLETE

    ActiveStep          : Int;      // Currently executing step (0 = none)
    SeqState            : Int;      // Numeric state code (see header)
    FaultCode           : Int;      // Active fault code (0 = no fault)
    StepAdvanceReady    : Bool;     // TRUE when advance condition is satisfied
    IncrWaiting         : Bool;     // TRUE in INCR mode, condition met, waiting Incr
    CycleCount          : DInt;     // Completed cycle counter (persistent)
END_VAR

VAR
    //── INTERNAL STATE ──────────────────────────────────────────────────────
    _state              : Int;      // Sequencer state machine current state
    _activeStep         : Int;      // Active step number (1..10, 0=none)
    _stepAdvReady       : Bool;     // Advance condition for current step

    //── STEP CONFIGURATION ARRAYS [1..10] ───────────────────────────────────
    _stepEnabled        : Array[1..10] OF Bool; // FALSE = disabled/skip
    _stepSafetyLocked   : Array[1..10] OF Bool; // TRUE = SkipStep ignored
    _stepUseTimer       : Array[1..10] OF Bool; // TRUE = timer-based advance
    _stepTimerPreset    : Array[1..10] OF Time; // Timer preset (relevant if above)

    //── TIMERS ──────────────────────────────────────────────────────────────
    _stepTimer          : Array[1..10] OF TON;  // Per-step IEC timer
    _stepWatchdog       : Array[1..10] OF TON;  // Per-step watchdog timeout
    _cat1Timer          : TON;                  // CAT1 controlled-stop guard timer

    //── EDGE DETECTION (previous scan values) ───────────────────────────────
    _prevStart          : Bool;
    _prevPause          : Bool;
    _prevStop           : Bool;
    _prevReset          : Bool;
    _prevIncr           : Bool;
    _prevSkip           : Bool;
    _prevEStop          : Bool;

    //── LATCHED FLAGS ───────────────────────────────────────────────────────
    _incrRequest        : Bool;     // Latched: set on rising Cmd_Incr edge
    _cat1InProgress     : Bool;     // TRUE during CAT1 controlled-stop sequence
    _forceAdvance       : Bool;     // Set by SkipStep to force one advance

    //── MISCELLANEOUS ───────────────────────────────────────────────────────
    _initDone           : Bool;     // First-scan initialisation flag
    _i                  : Int;      // Loop variable
END_VAR

VAR_TEMP
    //── RISING-EDGE PULSES (computed each scan) ─────────────────────────────
    rStart              : Bool;
    rPause              : Bool;
    rStop               : Bool;
    rReset              : Bool;
    rIncr               : Bool;
    rSkip               : Bool;
    rEStop              : Bool;
    nextStep            : Int;      // Working variable for step advance
END_VAR

VAR CONSTANT
    //── STATE CONSTANTS ─────────────────────────────────────────────────────
    STATE_IDLE          : Int := 0;
    STATE_RUNNING       : Int := 1;
    STATE_PAUSED        : Int := 2;
    STATE_STOPPING_CAT1 : Int := 3;
    STATE_STOPPED       : Int := 4;
    STATE_FAULTED       : Int := 5;
    STATE_COMPLETE      : Int := 6;

    //── SEQUENCE LIMITS ─────────────────────────────────────────────────────
    FIRST_STEP          : Int := 1;
    LAST_STEP           : Int := 10;

    //── TIMING CONSTANTS ────────────────────────────────────────────────────
    CAT1_TIMEOUT        : Time := T#10S;    // Max time for CAT1 controlled stop
    WATCHDOG_TIME       : Time := T#30S;    // Max time any step may take
END_VAR

BEGIN
(*═══════════════════════════════════════════════════════════════════════════
  ░░  S E C T I O N  1  —  F I R S T - S C A N   I N I T I A L I S A T I O N  ░░
═══════════════════════════════════════════════════════════════════════════*)
IF NOT _initDone THEN

    FOR _i := 1 TO 10 DO
        _stepEnabled[_i]      := TRUE;     // All steps ON by default
        _stepSafetyLocked[_i] := FALSE;
        _stepUseTimer[_i]     := FALSE;
        _stepTimerPreset[_i]  := T#0S;
    END_FOR;

    //── Safety-locked steps (operator Cmd_SkipStep has NO effect) ───────────
    // Step 1: Home verify  — must confirm safe start position
    // Step 5: Arc strike   — arc MUST be detected before weld travel
    // Step 7: Arc off      — arc MUST be extinguished before gas purge ends
    _stepSafetyLocked[1] := TRUE;
    _stepSafetyLocked[5] := TRUE;
    _stepSafetyLocked[7] := TRUE;

    //── Timer-advance steps (no sensor required) ────────────────────────────
    _stepUseTimer[4]     := TRUE;   _stepTimerPreset[4] := T#2S;      // Pre-purge
    _stepUseTimer[6]     := TRUE;   _stepTimerPreset[6] := T#4S;      // Weld travel
    _stepUseTimer[8]     := TRUE;   _stepTimerPreset[8] := T#2500MS;  // Post-purge

    _state     := STATE_IDLE;
    _activeStep := 0;
    _initDone  := TRUE;
END_IF;

(*═══════════════════════════════════════════════════════════════════════════
  ░░  S E C T I O N  2  —  E D G E   D E T E C T I O N  ░░
═══════════════════════════════════════════════════════════════════════════*)
rStart := Cmd_Start    AND NOT _prevStart;
rPause := Cmd_Pause    AND NOT _prevPause;
rStop  := Cmd_Stop     AND NOT _prevStop;
rReset := Cmd_Reset    AND NOT _prevReset;
rIncr  := Cmd_Incr     AND NOT _prevIncr;
rSkip  := Cmd_SkipStep AND NOT _prevSkip;
rEStop := Cmd_EStop    AND NOT _prevEStop;     // Rising edge for event logging

_prevStart := Cmd_Start;
_prevPause := Cmd_Pause;
_prevStop  := Cmd_Stop;
_prevReset := Cmd_Reset;
_prevIncr  := Cmd_Incr;
_prevSkip  := Cmd_SkipStep;
_prevEStop := Cmd_EStop;

// Latch increment request — consumed when step actually advances
IF rIncr THEN
    _incrRequest := TRUE;
END_IF;

// Skip step — latch if current step is NOT safety-locked
IF rSkip AND (_activeStep >= FIRST_STEP) THEN
    IF NOT _stepSafetyLocked[_activeStep] THEN
        _forceAdvance := TRUE;
    END_IF;
END_IF;

(*═══════════════════════════════════════════════════════════════════════════
  ░░  S E C T I O N  3  —  S A F E T Y   M O N I T O R I N G  ░░
         Evaluated EVERY SCAN and takes priority over state machine
═══════════════════════════════════════════════════════════════════════════*)

// ── E-Stop (level-triggered, highest priority) ─────────────────────────
IF Cmd_EStop OR NOT Safety_EStopOK THEN
    IF NOT _cat1InProgress THEN        // Prevent re-entry if already handling
        CASE StopCategory OF
            0: // CAT0 — Immediate de-energise (outputs cleared in Section 6)
                _state    := STATE_FAULTED;
                FaultCode := 11;

            1: // CAT1 — Allow current step to reach safe state, then stop
                _cat1InProgress := TRUE;
                _state          := STATE_STOPPING_CAT1;
                FaultCode       := 11;

            2: // CAT2 — Freeze outputs where they are
                _state    := STATE_STOPPED;
                FaultCode := 11;

            ELSE:
                _state    := STATE_FAULTED;  // Unknown category → safe fault
                FaultCode := 11;
        END_CASE;
    END_IF;
END_IF;

// ── Safety door opened while running or paused ─────────────────────────
IF NOT Safety_DoorClosed THEN
    IF _state = STATE_RUNNING OR _state = STATE_PAUSED THEN
        _state    := STATE_FAULTED;
        FaultCode := 10;
    END_IF;
END_IF;

// ── Sensor conflict: FWD and BWD sensors both TRUE simultaneously ──────
IF Sen_HeadAtWeldPos AND Sen_HeadRetracted THEN
    IF _state = STATE_RUNNING OR _state = STATE_STOPPING_CAT1 THEN
        _state    := STATE_FAULTED;
        FaultCode := 30;
    END_IF;
END_IF;

(*═══════════════════════════════════════════════════════════════════════════
  ░░  S E C T I O N  4  —  M A I N   S T A T E   M A C H I N E  ░░
═══════════════════════════════════════════════════════════════════════════*)
CASE _state OF

    //──────────────────────────────────────────────────────────────────────
    // STATE 0: IDLE — Waiting for Start
    //──────────────────────────────────────────────────────────────────────
    STATE_IDLE:
        _activeStep     := 0;
        _cat1InProgress := FALSE;
        _incrRequest    := FALSE;
        _forceAdvance   := FALSE;

        // Start only if safety circuit is healthy
        IF rStart AND Safety_DoorClosed AND Safety_EStopOK AND (FaultCode = 0) THEN
            // Find first enabled step
            nextStep := FIRST_STEP;
            WHILE nextStep <= LAST_STEP AND NOT _stepEnabled[nextStep] DO
                nextStep := nextStep + 1;
            END_WHILE;
            IF nextStep <= LAST_STEP THEN
                _activeStep := nextStep;
                _state      := STATE_RUNNING;
            END_IF;
        END_IF;

    //──────────────────────────────────────────────────────────────────────
    // STATE 1: RUNNING — Sequence executing
    //──────────────────────────────────────────────────────────────────────
    STATE_RUNNING:

        // Pause request
        IF rPause THEN
            _state := STATE_PAUSED;
        END_IF;

        // CAT2 Stop request
        IF rStop THEN
            _state := STATE_STOPPED;
        END_IF;

        // ── Step advance logic ────────────────────────────────────────────
        // Advance condition: sensor/timer TRUE  OR  forced skip
        IF _stepAdvReady OR _forceAdvance THEN

            _forceAdvance := FALSE;

            // INCR mode: advance condition met — wait for operator Incr
            // AUTO mode: advance immediately
            IF Mode_Incr AND NOT _incrRequest THEN
                // Stay on current step — IncrWaiting will be TRUE (see outputs)
                ;
            ELSE
                // Consume the increment request
                _incrRequest := FALSE;

                // Find next enabled step
                nextStep := _activeStep + 1;
                WHILE nextStep <= LAST_STEP AND NOT _stepEnabled[nextStep] DO
                    nextStep := nextStep + 1;
                END_WHILE;

                IF nextStep > LAST_STEP THEN
                    // All steps complete
                    _state      := STATE_COMPLETE;
                    _activeStep := 0;
                    CycleCount  := CycleCount + 1;
                ELSE
                    _activeStep := nextStep;
                END_IF;
            END_IF;
        END_IF;

    //──────────────────────────────────────────────────────────────────────
    // STATE 2: PAUSED — Outputs held, awaiting resume
    //──────────────────────────────────────────────────────────────────────
    STATE_PAUSED:
        // Resume requires door still closed (re-check safety)
        IF rStart AND Safety_DoorClosed AND Safety_EStopOK THEN
            _state := STATE_RUNNING;
        END_IF;

        IF rStop THEN
            _state := STATE_STOPPED;
        END_IF;

    //──────────────────────────────────────────────────────────────────────
    // STATE 3: STOPPING_CAT1 — Controlled stop; current step finishes
    //──────────────────────────────────────────────────────────────────────
    STATE_STOPPING_CAT1:

        // CAT1 guard timer — prevents infinite hang
        _cat1Timer(IN := TRUE, PT := CAT1_TIMEOUT);

        // Step logic still runs (see Section 5) to allow safe motion to complete
        // Advance to STOPPED when step condition met OR timeout
        IF _stepAdvReady THEN
            _state          := STATE_STOPPED;
            _activeStep     := 0;
            _cat1InProgress := FALSE;
            _cat1Timer(IN := FALSE, PT := CAT1_TIMEOUT);

        ELSIF _cat1Timer.Q THEN
            // Timeout — step did not complete safely
            FaultCode       := 20;
            _state          := STATE_FAULTED;
            _cat1InProgress := FALSE;
            _cat1Timer(IN := FALSE, PT := CAT1_TIMEOUT);
        END_IF;

    //──────────────────────────────────────────────────────────────────────
    // STATE 4: STOPPED — Requires Reset to return to IDLE
    //──────────────────────────────────────────────────────────────────────
    STATE_STOPPED:
        // Safe state: weld outputs cleared in Section 6
        IF rReset AND NOT Cmd_EStop AND Safety_EStopOK THEN
            FaultCode   := 0;
            _activeStep := 0;
            _state      := STATE_IDLE;
        END_IF;

    //──────────────────────────────────────────────────────────────────────
    // STATE 5: FAULTED — Hard stop; all outputs cleared (CAT0 / safety fault)
    //──────────────────────────────────────────────────────────────────────
    STATE_FAULTED:
        // Reset only when E-Stop cleared, door closed, and reset pressed
        IF rReset AND NOT Cmd_EStop AND Safety_EStopOK AND Safety_DoorClosed THEN
            FaultCode   := 0;
            _activeStep := 0;
            _state      := STATE_IDLE;
        END_IF;

    //──────────────────────────────────────────────────────────────────────
    // STATE 6: COMPLETE — Cycle finished
    //──────────────────────────────────────────────────────────────────────
    STATE_COMPLETE:
        IF rStart AND Safety_DoorClosed AND Safety_EStopOK THEN
            // Start new cycle
            nextStep := FIRST_STEP;
            WHILE nextStep <= LAST_STEP AND NOT _stepEnabled[nextStep] DO
                nextStep := nextStep + 1;
            END_WHILE;
            IF nextStep <= LAST_STEP THEN
                _activeStep := nextStep;
                _state      := STATE_RUNNING;
            END_IF;
        END_IF;
        IF rReset THEN
            _activeStep := 0;
            _state      := STATE_IDLE;
        END_IF;

END_CASE; // Main state machine

(*═══════════════════════════════════════════════════════════════════════════
  ░░  S E C T I O N  5  —  S T E P   E X E C U T I O N  ░░
         Outputs built up here. All output vars cleared to FALSE first,
         then each active step energises what it needs.
         This ensures outputs drop when step changes — no "stuck on" risk.
═══════════════════════════════════════════════════════════════════════════*)

// ── Clear all actuator outputs every scan (re-energised by step logic) ──
Act_Clamp      := FALSE;
Act_HeadExtend := FALSE;
Act_HeadRetract:= FALSE;
Act_GasValve   := FALSE;
Act_WeldEnable := FALSE;
Act_Unclamp    := FALSE;
Act_PartEject  := FALSE;
_stepAdvReady  := FALSE;

// ── Step logic only active during RUNNING or STOPPING_CAT1 ──────────────
IF _state = STATE_RUNNING OR _state = STATE_STOPPING_CAT1 THEN

    IF _activeStep >= FIRST_STEP AND _activeStep <= LAST_STEP THEN

        //── Per-step timers ─────────────────────────────────────────────────
        // IN driven by (activeStep = this step AND running) — auto-resets on step change
        _stepTimer[1](IN := (_activeStep = 1  AND _state = STATE_RUNNING), PT := _stepTimerPreset[1]);
        _stepTimer[2](IN := (_activeStep = 2  AND _state = STATE_RUNNING), PT := _stepTimerPreset[2]);
        _stepTimer[3](IN := (_activeStep = 3  AND _state = STATE_RUNNING), PT := _stepTimerPreset[3]);
        _stepTimer[4](IN := (_activeStep = 4  AND _state = STATE_RUNNING), PT := _stepTimerPreset[4]);
        _stepTimer[5](IN := (_activeStep = 5  AND _state = STATE_RUNNING), PT := _stepTimerPreset[5]);
        _stepTimer[6](IN := (_activeStep = 6  AND _state = STATE_RUNNING), PT := _stepTimerPreset[6]);
        _stepTimer[7](IN := (_activeStep = 7  AND _state = STATE_RUNNING), PT := _stepTimerPreset[7]);
        _stepTimer[8](IN := (_activeStep = 8  AND _state = STATE_RUNNING), PT := _stepTimerPreset[8]);
        _stepTimer[9](IN := (_activeStep = 9  AND _state = STATE_RUNNING), PT := _stepTimerPreset[9]);
        _stepTimer[10](IN:= (_activeStep = 10 AND _state = STATE_RUNNING), PT := _stepTimerPreset[10]);

        //── Per-step watchdog timers ─────────────────────────────────────────
        _stepWatchdog[1](IN := (_activeStep = 1),  PT := WATCHDOG_TIME);
        _stepWatchdog[2](IN := (_activeStep = 2),  PT := WATCHDOG_TIME);
        _stepWatchdog[3](IN := (_activeStep = 3),  PT := WATCHDOG_TIME);
        _stepWatchdog[4](IN := (_activeStep = 4),  PT := WATCHDOG_TIME);
        _stepWatchdog[5](IN := (_activeStep = 5),  PT := WATCHDOG_TIME);
        _stepWatchdog[6](IN := (_activeStep = 6),  PT := WATCHDOG_TIME);
        _stepWatchdog[7](IN := (_activeStep = 7),  PT := WATCHDOG_TIME);
        _stepWatchdog[8](IN := (_activeStep = 8),  PT := WATCHDOG_TIME);
        _stepWatchdog[9](IN := (_activeStep = 9),  PT := WATCHDOG_TIME);
        _stepWatchdog[10](IN:= (_activeStep = 10), PT := WATCHDOG_TIME);

        // Watchdog trip
        IF _stepWatchdog[_activeStep].Q THEN
            FaultCode := 40;
            _state    := STATE_FAULTED;
        END_IF;

    END_IF;

    //──────────────────────────────────────────────────────────────────────
    CASE _activeStep OF

        //══════════════════════════════════════════════════════════════════
        // STEP 1 — HOME POSITION VERIFY
        //   Purpose : Confirm fixture/robot is at safe home before clamping
        //   Actuators: NONE (verify-only step)
        //   Sensors  : 2 sensors — Sen_HomeFwd (at home) + Sen_HomeBwd (NOT in
        //              work zone). BOTH conditions required for safe start.
        //   Safety   : 🔒 LOCKED — cannot be skipped
        //   Advance  : Sen_HomeFwd = TRUE  AND  Sen_HomeBwd = FALSE
        //══════════════════════════════════════════════════════════════════
        1:
            // No outputs energised — pure monitoring step
            _stepAdvReady := Sen_HomeFwd AND NOT Sen_HomeBwd;

        //══════════════════════════════════════════════════════════════════
        // STEP 2 — CLAMP WORKPIECE
        //   Purpose : Close clamp jaw to secure part for welding
        //   Actuators: Act_Clamp ON
        //   Sensors  : 1 sensor — Sen_ClampClosed (clamp jaw closed)
        //   Advance  : Sen_ClampClosed = TRUE
        //══════════════════════════════════════════════════════════════════
        2:
            Act_Clamp     := TRUE;
            _stepAdvReady := Sen_ClampClosed;

        //══════════════════════════════════════════════════════════════════
        // STEP 3 — EXTEND WELD HEAD TO POSITION
        //   Purpose : Drive weld torch to weld-start co-ordinate
        //   Actuators: Act_HeadExtend ON
        //   Sensors  : 2 sensors declared (FWD + BWD shared with Step 9)
        //              FWD: Sen_HeadAtWeldPos  → advance trigger
        //              BWD: Sen_HeadRetracted  → conflict / interlock check
        //   Advance  : Sen_HeadAtWeldPos = TRUE (FWD sensor)
        //══════════════════════════════════════════════════════════════════
        3:
            Act_Clamp      := TRUE;         // Maintain clamp during all weld steps
            Act_HeadExtend := TRUE;
            _stepAdvReady  := Sen_HeadAtWeldPos;

        //══════════════════════════════════════════════════════════════════
        // STEP 4 — PRE-PURGE GAS  (TIMER ADVANCE — no sensor)
        //   Purpose : Flow shielding gas before arc strike to purge atmosphere
        //   Actuators: Act_GasValve ON
        //   Sensors  : NONE
        //   Advance  : Timer T#2S elapsed
        //══════════════════════════════════════════════════════════════════
        4:
            Act_Clamp    := TRUE;
            Act_HeadExtend := TRUE;
            Act_GasValve := TRUE;
            _stepAdvReady:= _stepTimer[4].Q;

        //══════════════════════════════════════════════════════════════════
        // STEP 5 — ARC STRIKE
        //   Purpose : Enable welder; wait for arc current confirmation
        //   Actuators: Act_WeldEnable ON + Act_GasValve ON
        //   Sensors  : 1 sensor — Sen_ArcDetect (arc current detected)
        //   Safety   : 🔒 LOCKED — arc MUST be confirmed before travel
        //   Advance  : Sen_ArcDetect = TRUE
        //══════════════════════════════════════════════════════════════════
        5:
            Act_Clamp      := TRUE;
            Act_HeadExtend := TRUE;
            Act_GasValve   := TRUE;
            Act_WeldEnable := TRUE;
            _stepAdvReady  := Sen_ArcDetect;

        //══════════════════════════════════════════════════════════════════
        // STEP 6 — WELD TRAVEL  (TIMER ADVANCE — no sensor)
        //   Purpose : Weld torch traverses seam (robot/axis handles motion)
        //             SCL holds outputs stable for weld duration
        //   Actuators: Act_WeldEnable ON + Act_GasValve ON (all weld outputs)
        //   Sensors  : NONE (robot reports completion via external handshake
        //              or fixed weld time used here for demo)
        //   Advance  : Timer T#4S elapsed
        //══════════════════════════════════════════════════════════════════
        6:
            Act_Clamp      := TRUE;
            Act_HeadExtend := TRUE;
            Act_GasValve   := TRUE;
            Act_WeldEnable := TRUE;
            _stepAdvReady  := _stepTimer[6].Q;

        //══════════════════════════════════════════════════════════════════
        // STEP 7 — ARC OFF / WELD END
        //   Purpose : Command welder off; wait for arc extinction confirmation
        //   Actuators: Act_WeldEnable OFF, Act_GasValve ON (gas post-protection)
        //   Sensors  : 1 sensor — Sen_ArcOff (TRUE = current < threshold)
        //   Safety   : 🔒 LOCKED — arc MUST be extinguished before any movement
        //   Advance  : Sen_ArcOff = TRUE
        //══════════════════════════════════════════════════════════════════
        7:
            Act_Clamp      := TRUE;
            Act_HeadExtend := TRUE;
            Act_GasValve   := TRUE;
            Act_WeldEnable := FALSE;    // Welder OFF — waiting for arc to die
            _stepAdvReady  := Sen_ArcOff;

        //══════════════════════════════════════════════════════════════════
        // STEP 8 — POST-PURGE GAS  (TIMER ADVANCE — no sensor)
        //   Purpose : Continue gas flow after arc off to protect hot weld pool
        //   Actuators: Act_GasValve ON
        //   Sensors  : NONE
        //   Advance  : Timer T#2.5S elapsed
        //══════════════════════════════════════════════════════════════════
        8:
            Act_Clamp      := TRUE;
            Act_HeadExtend := TRUE;
            Act_GasValve   := TRUE;     // Post-weld purge only — no WeldEnable
            _stepAdvReady  := _stepTimer[8].Q;

        //══════════════════════════════════════════════════════════════════
        // STEP 9 — RETRACT WELD HEAD
        //   Purpose : Drive torch back to safe retracted position
        //   Actuators: Act_HeadRetract ON, Act_HeadExtend OFF
        //   Sensors  : 2 sensors shared with Step 3 (FWD + BWD)
        //              BWD: Sen_HeadRetracted  → advance trigger
        //              FWD: Sen_HeadAtWeldPos  → conflict check (should be FALSE)
        //   Advance  : Sen_HeadRetracted = TRUE  AND  Sen_HeadAtWeldPos = FALSE
        //══════════════════════════════════════════════════════════════════
        9:
            Act_Clamp       := TRUE;
            Act_HeadExtend  := FALSE;
            Act_HeadRetract := TRUE;
            _stepAdvReady   := Sen_HeadRetracted AND NOT Sen_HeadAtWeldPos;

        //══════════════════════════════════════════════════════════════════
        // STEP 10 — UNCLAMP / PART EJECT
        //   Purpose : Release clamp and eject finished part
        //   Actuators: Act_Clamp OFF, Act_Unclamp ON, Act_PartEject ON
        //   Sensors  : 1 sensor — Sen_UnclampConf (clamp fully open / part clear)
        //   Advance  : Sen_UnclampConf = TRUE  → COMPLETE
        //══════════════════════════════════════════════════════════════════
        10:
            Act_Clamp      := FALSE;    // Release clamp
            Act_Unclamp    := TRUE;     // Drive unclamp solenoid
            Act_PartEject  := TRUE;     // Fire ejector cylinder
            _stepAdvReady  := Sen_UnclampConf;

    END_CASE;

END_IF; // RUNNING or STOPPING_CAT1

(*═══════════════════════════════════════════════════════════════════════════
  ░░  S E C T I O N  6  —  O U T P U T   S A F E - S T A T E   O V E R R I D E  ░░
         These rules override the step logic above for safety states
═══════════════════════════════════════════════════════════════════════════*)

// ── FAULTED (CAT0 or hard safety fault): ALL outputs immediately de-energised
IF _state = STATE_FAULTED THEN
    Act_Clamp       := FALSE;
    Act_HeadExtend  := FALSE;
    Act_HeadRetract := FALSE;
    Act_GasValve    := FALSE;
    Act_WeldEnable  := FALSE;
    Act_Unclamp     := FALSE;
    Act_PartEject   := FALSE;
END_IF;

// ── IDLE / COMPLETE: weld-related outputs OFF; clamp may stay per design
IF _state = STATE_IDLE OR _state = STATE_COMPLETE THEN
    Act_WeldEnable  := FALSE;
    Act_GasValve    := FALSE;
    Act_HeadExtend  := FALSE;
    Act_HeadRetract := FALSE;
    Act_Unclamp     := FALSE;
    Act_PartEject   := FALSE;
    // NOTE: Act_Clamp intentionally left as-is here.
    //       Add Act_Clamp := FALSE if open-on-idle is required.
END_IF;

(*═══════════════════════════════════════════════════════════════════════════
  ░░  S E C T I O N  7  —  S T A T U S   O U T P U T S  ░░
═══════════════════════════════════════════════════════════════════════════*)
ActiveStep       := _activeStep;
SeqState         := _state;
StepAdvanceReady := _stepAdvReady;
IncrWaiting      := Mode_Incr AND _stepAdvReady AND NOT _incrRequest
                    AND (_state = STATE_RUNNING);

Seq_Idle      := (_state = STATE_IDLE);
Seq_Running   := (_state = STATE_RUNNING);
Seq_Paused    := (_state = STATE_PAUSED);
Seq_Stopping  := (_state = STATE_STOPPING_CAT1);
Seq_Stopped   := (_state = STATE_STOPPED);
Seq_Faulted   := (_state = STATE_FAULTED);
Seq_Complete  := (_state = STATE_COMPLETE);

END_FUNCTION_BLOCK


(*═══════════════════════════════════════════════════════════════════════════════
  USAGE CALL EXAMPLE — FC_WeldSeq_Call (or OB_CyclicInterrupt)

  // In TIA Portal create an instance DB:  DB_WeldSeq : FB_WeldSequencer

  "DB_WeldSeq"(
      Mode_Auto           := %I2.0,
      Mode_Incr           := %I2.1,
      Cmd_Start           := %I1.4,
      Cmd_Pause           := %I1.5,
      Cmd_Stop            := %I1.6,
      Cmd_EStop           := %I1.2,
      Cmd_Reset           := %I1.3,
      Cmd_Incr            := %I1.7,
      Cmd_SkipStep        := %I2.2,
      StopCategory        := %MW10,        // 0, 1 or 2 — set by HMI
      Safety_DoorClosed   := %I1.1,
      Safety_EStopOK      := %I1.0,
      Sen_HomeFwd         := %I0.0,
      Sen_HomeBwd         := %I0.1,
      Sen_ClampClosed     := %I0.2,
      Sen_HeadAtWeldPos   := %I0.3,
      Sen_HeadRetracted   := %I0.4,
      Sen_ArcDetect       := %I0.5,
      Sen_ArcOff          := %I0.6,
      Sen_UnclampConf     := %I0.7,
      Act_Clamp           => %Q0.0,
      Act_HeadExtend      => %Q0.1,
      Act_HeadRetract     => %Q0.2,
      Act_GasValve        => %Q0.3,
      Act_WeldEnable      => %Q0.4,
      Act_Unclamp         => %Q0.5,
      Act_PartEject       => %Q0.6,
      Seq_Running         => %Q1.0,
      Seq_Paused          => %Q1.1,
      Seq_Faulted         => %Q1.2,
      Seq_Complete        => %Q1.3,
      ActiveStep          => %MW20,
      SeqState            => %MW22,
      FaultCode           => %MW24,
      StepAdvanceReady    => %M5.0,
      IncrWaiting         => %M5.1,
      CycleCount          => %MD30
  );
═══════════════════════════════════════════════════════════════════════════════*)