ExternalAxis: virtual A axis through gplan, mirrored on the ESP

ExternalAxis exposes the auxcnc-driven ESP stepper as motor 4 (a
synthetic, host-only motor that gplan sees but the AVR doesn't). The
result is a virtual A axis that is fully integrated with the planner:
G1 A25 F1500 schedules a coordinated S-curve and the ESP runs the
exact same 7-segment trajectory the AVR would have run if A were a
real motor.

- ExternalAxis.py: synthetic-motor state, S-curve LINE block forward
  to the ESP, soft-limit enforcement, option-(b) homing (user A=0
  at the home limit).
- State: walk motors 0..4 in find_motor; clear both homed and h on
  reset; expose synthetic motor vars.
- axis-vars.js: motor-4 guard so the JS computed axis bindings don't
  throw when motor 4 has no entry in config.motors; resolve motor_id
  for the synthetic axis by scanning state['4an'].
- Ctrl: instantiate ExternalAxis after AuxAxis, share the axis_letter
  setting, wire AuxAxis state observer.
- Web: route /api/aux/{home,jog,move} through ExternalAxis when it
  is enabled so the DRO and synthetic-motor flags stay in sync.

src/js/axis-vars.js

@@ -32,6 +32,10 @@ module.exports = {
             return this._compute_axis("c");
         },
 
+        w: function() {
+            return this._compute_aux_axis();
+        },
+
         axes: function() {
             return this._compute_axes();
         }
@@ -52,7 +56,12 @@ module.exports = {
             const abs = this.state[`${axis}p`] || 0;
             const off = this.state[`offset_${axis}`];
             const motor_id = this._get_motor_id(axis);
-            const motor = motor_id == -1 ? {} : this.config.motors[motor_id];
+            // motor_id may be 4 for the synthetic external-axis motor;
+            // there is no entry for it in config.motors so guard with
+            // an empty object to avoid undefined property access.
+            const motor = (motor_id == -1
+                ? {}
+                : (this.config.motors[motor_id] || {}));
             const enabled = this._check_is_enabled(axis);
             const homingMode = motor["homing-mode"];
             const homed = this.state[`${motor_id}homed`];
@@ -185,24 +194,114 @@ module.exports = {
         _get_motor_id: function(axis) {
             for (let i = 0; i < this.config.motors.length; i++) {
                 const motor = this.config.motors[i];
-                if (motor.axis.toLowerCase() == axis) {
+                // motor.axis can be undefined on initial load before
+                // config has streamed in. Guard so the computed does
+                // not throw and bubble a Vue warning into the console.
+                if (motor && typeof motor.axis === "string" &&
+                    motor.axis.toLowerCase() == axis) {
                     return i;
                 }
             }
 
+            // Synthetic external motor (index 4) used by ExternalAxis
+            // to expose the auxcnc ESP stepper as a virtual axis.
+            // Its `Nan` lives in state, not config.
+            const axes = { x: 0, y: 1, z: 2, a: 3, b: 4, c: 5 };
+            const wanted = axes[axis];
+            const extAn = this.state && this.state["4an"];
+            if (typeof wanted === "number" && typeof extAn === "number"
+                && extAn === wanted) {
+                return 4;
+            }
+
             return -1;
         },
 
         _check_is_enabled: function(axis){
+            // Prefer config.motors[i].axis (always present once the
+            // config has loaded). Fall back to the per-motor state
+            // `Nan` field, which is what the legacy UI used. This
+            // avoids hiding axis rows during the brief window after
+            // config has loaded but before the controller has pushed
+            // its first state delta.
             const axes = { x: 0, y: 1, z: 2, a: 3 };
-            for(let i = 0; i < this.config.motors.length; i++){
-                if(this.state[`${i}an`] == axes[axis]){
+            const wanted = axes[axis];
+            for (let i = 0; i < this.config.motors.length; i++) {
+                const motor = this.config.motors[i] || {};
+                if (typeof motor.axis === "string" &&
+                    motor.axis.toLowerCase() == axis) {
+                    return motor.enabled !== false;
+                }
+                // Only use the state Nan fallback for axes we know
+                // about (x/y/z/a). Otherwise undefined == undefined
+                // would mistakenly match every axis (b, c, ...).
+                if (typeof wanted === "number") {
+                    const an = this.state[`${i}an`];
+                    if (typeof an === "number" && an === wanted) {
+                        return true;
+                    }
+                }
+            }
+            // Synthetic external motor (index 4) - the auxcnc ESP
+            // stepper exposed as A via ExternalAxis.
+            if (typeof wanted === "number") {
+                const extAn = this.state["4an"];
+                const extMe = this.state["4me"];
+                if (typeof extAn === "number" && extAn === wanted
+                    && extMe) {
                     return true;
                 }
             }
             return false;
         },
 
+        _compute_aux_axis: function() {
+            // Auxiliary axis driven by the auxcnc ESP32 (typically
+            // exposed to gplan as A). Position, homed flag and
+            // presence come from the bbctrl AuxAxis driver via
+            // state.aux_*. No motor mapping, no soft-limit warnings
+            // on toolpath bounds (auxcnc enforces its own).
+            const enabled = !!this.state.aux_enabled;
+            const present = !!this.state.aux_present;
+            const homed   = !!this.state.aux_homed;
+            const pos     = this.state.aux_pos || 0;
+
+            let klass = `${homed ? "homed" : "unhomed"} axis-w`;
+            let state = present ? "UNHOMED" : "OFFLINE";
+            let icon  = present ? "question-circle" : "plug";
+            let title = present
+                ? "Click the home button to home the auxiliary axis."
+                : "Aux controller not connected on /dev/ttyUSB0.";
+            if (homed) {
+                state = "HOMED";
+                icon  = "check-circle";
+                title = "Auxiliary axis successfully homed.";
+            } else if (!present) {
+                klass += " error";
+            }
+
+            return {
+                pos: pos,
+                abs: pos,
+                off: 0,
+                min: 0, max: 0, dim: 0,
+                pathMin: 0, pathMax: 0, pathDim: 0,
+                motor: -1,
+                enabled: enabled,
+                homingMode: "limit-switch",
+                homed: homed,
+                klass: klass,
+                state: state,
+                icon: icon,
+                title: title,
+                ticon: "check-circle",
+                tstate: "OK",
+                toolmsg: "Auxiliary axis is not constrained by tool path bounds.",
+                tklass: `${homed ? "homed" : "unhomed"} axis-w`,
+                isAux: true,
+            };
+        },
+
         _compute_axes: function() {
             let homed = false;
 

src/py/bbctrl/Ctrl.py

@@ -75,6 +75,19 @@ class Ctrl(object):
                 self.hooks = bbctrl.Hooks(self)
             with Trace.span('ctrl.aux'):
                 self.aux = bbctrl.AuxAxis(self)
+            with Trace.span('ctrl.ext_axis'):
+                # ExternalAxis exposes the auxcnc ESP stepper as a
+                # virtual A axis that gplan handles natively. Created
+                # unconditionally so State sees the synthetic motor
+                # vars even when aux is disabled (kept inert in that
+                # case via ext_axis.enabled).
+                axis_letter = self.aux._cfg.get('axis_letter', 'a')
+                self.ext_axis = bbctrl.ExternalAxis(
+                    self, self.aux, axis_letter=axis_letter)
+                # Hook AuxAxis post-publish callback so homed flag
+                # mirrors into State after homing.
+                self.aux.set_state_observer(
+                    self.ext_axis.refresh_homed)
 
             with Trace.span('ctrl.mach.connect'):
                 self.mach.connect()
@@ -132,6 +145,8 @@ class Ctrl(object):
 
 
     def close(self):
+        try: self.ext_axis.close()
+        except Exception: pass
         try: self.aux.close()
         except Exception: pass
         self.log.get('Ctrl').info('Closing %s' % self.id)

src/py/bbctrl/ExternalAxis.py

@@ -0,0 +1,531 @@
+################################################################################
+#
+# ExternalAxis - bridges a logical motorless axis to step generation on
+# the auxcnc ESP, so the Buildbotics planner can drive a stepper that
+# isn't on the AVR.
+#
+# Architecture
+# ------------
+# The bbctrl planner (camotics gplan) handles parsing, units, modal
+# state, soft limits, accel ramping and S-curve timing for axes
+# X, Y, Z, A, B, C. The AVR has 4 motor channels (0-3) and only
+# generates step pulses for axes that have a motor mapped to them.
+# An axis with no mapped motor is fully accepted by the AVR - it
+# updates its internal `ex.position[axis]` and reports `<axis>p` to
+# the host, but no stepper turns.
+#
+# We exploit that: the W stepper is exposed to gplan as A, but no
+# AVR motor maps to A. The planner does all the gcode-level work
+# correctly (G90/G91, soft limits, accel, units, modal feed rate);
+# we intercept the resulting `Cmd.line` blocks in `Planner.__encode`,
+# strip A out, and forward the A delta to the auxcnc ESP as STEPS.
+#
+# To make gplan and State *believe* A is enabled we register a
+# synthetic motor (index 4) into State.vars, populated from
+# aux.json, with `4an=3` (axis A), `4me=1` (enabled), and the
+# usual velocity/accel/jerk/soft-limit vars. State.find_motor and
+# the snapshot projection are extended to walk index 4. Motor-4
+# vars never leave the host (they're not in the AVR's schema) so
+# the AVR is undisturbed.
+#
+# v1 coupling: serialize. If a line has any A delta we wait for
+# the ESP to finish before letting subsequent commands flow. This
+# matches the behaviour of the previous hook-based approach (no
+# XYZ+A blending) but with all the planner's correctness guarantees.
+#
+# v2 could match ESP move duration to the gplan trapezoid time and
+# allow concurrent motion; out of scope for v1.
+#
+################################################################################
+
+import threading
+
+try:
+    from queue import Queue
+except ImportError:
+    from Queue import Queue  # py2 just in case
+
+
+# Synthetic motor index used to expose the external axis to State.
+# The AVR has motors 0..3; we use 4 as a host-only sentinel.
+EXTERNAL_MOTOR_INDEX = 4
+
+# Axis letters in their canonical order; 'a' is index 3.
+_AXIS_LETTERS = 'xyzabc'
+
+
+class ExternalAxisError(Exception):
+    pass
+
+
+class ExternalAxis(object):
+    """Bridge between Planner line blocks and AuxAxis serial RPCs.
+
+    Owns no thread; runs RPC calls inline on whatever thread invokes
+    execute_to_mm / home / abort. The Planner runs `__encode` on its
+    own thread which is allowed to block on planner I/O, so blocking
+    inside the interceptor is fine.
+
+    Position tracking: gplan emits absolute targets in mm; the ESP
+    counts steps relative to home_zero. We mirror the last commanded
+    mm position so subsequent line blocks compute the correct delta.
+    `_pos_mm` is also published as `<axis>p` so DRO updates."""
+
+    def __init__(self, ctrl, aux, axis_letter='a'):
+        self.ctrl = ctrl
+        self.aux = aux
+        self.log = ctrl.log.get('ExternalAxis')
+
+        self.axis_letter = (axis_letter or 'a').lower()[:1]
+        if self.axis_letter not in _AXIS_LETTERS:
+            raise ExternalAxisError(
+                'Invalid external axis letter: %r' % axis_letter)
+        # Index in 'xyzabc' (0..5)
+        self.axis_index = _AXIS_LETTERS.index(self.axis_letter)
+
+        self._busy = threading.Event()
+        # Last absolute mm we committed; None until first move /
+        # homing event syncs us up.
+        self._pos_mm = None
+
+        # Single-slot worker queue: __encode posts (target_mm,) tuples
+        # here; the worker thread runs the ESP RPC. Capacity is
+        # intentionally bounded - if it fills it means motion is
+        # outpacing the ESP and we should backpressure the planner.
+        self._work_q = Queue(maxsize=64)
+        self._stop = threading.Event()
+        self._worker = threading.Thread(
+            target=self._worker_loop,
+            name='ExternalAxis-worker', daemon=True)
+        self._worker.start()
+
+        # Push synthetic motor vars into State so the planner sees
+        # this axis as enabled with proper limits/velocity/accel.
+        self._publish_synthetic_motor()
+        # Also seed <axis>p so the DRO has something to render.
+        self.ctrl.state.set(self.axis_letter + 'p', 0.0)
+
+    # -------------------------------------------------------------- enabled
+
+    @property
+    def enabled(self):
+        try:
+            return bool(self.aux is not None
+                        and self.aux.enabled
+                        and self.aux.present)
+        except Exception:
+            return False
+
+    # -------------------------------------------------------- configuration
+
+    @property
+    def steps_per_mm(self):
+        try:
+            return float(self.aux._cfg.get('steps_per_mm', 25.0))
+        except Exception:
+            return 25.0
+
+    @property
+    def dir_sign(self):
+        try:
+            v = int(self.aux._cfg.get('dir_sign', 1))
+            return -1 if v < 0 else 1
+        except Exception:
+            return 1
+
+    @property
+    def home_position_mm(self):
+        try:
+            return float(self.aux._cfg.get('home_position_mm', 0.0))
+        except Exception:
+            return 0.0
+
+    # ------------------------------------------------------- soft limits
+
+    def _soft_limits(self):
+        """Return (min_mm, max_mm) in machine coords, or (None, None)
+        if soft limits are disabled (max <= min)."""
+        try:
+            lo = float(self.aux._cfg.get('min_mm', 0.0))
+            hi = float(self.aux._cfg.get('max_mm', 0.0))
+        except Exception:
+            return (None, None)
+        if hi <= lo:
+            return (None, None)
+        return (lo, hi)
+
+    def _check_soft_limit(self, target_abs_mm):
+        """Raise ExternalAxisError if target_abs_mm is outside the
+        configured soft limits. Skips the check when the axis isn't
+        homed (matching the standard bbctrl convention that soft
+        limits are gated by homing state) - that lets the user jog
+        away from a stuck position before homing without false
+        rejections.
+
+        Called by both planner-driven motion (enqueue_target_mm) and
+        UI motion (execute_to_mm), so this is the single source of
+        truth regardless of which path triggered the move."""
+        # Honour the homing gate.
+        try:
+            homed = bool(self.aux._homed)
+        except Exception:
+            homed = False
+        if not homed:
+            return
+        lo, hi = self._soft_limits()
+        if lo is None:
+            return
+        # Use a tiny epsilon so floating-point round-trip targets
+        # right at the boundary aren't rejected.
+        eps = 1e-4
+        target = float(target_abs_mm)
+        if target < lo - eps or target > hi + eps:
+            raise ExternalAxisError(
+                '%s axis target %.4f mm is outside soft limits '
+                '[%.3f, %.3f] mm' % (
+                    self.axis_letter.upper(), target, lo, hi))
+
+    # ----------------------------------------------------------- conversion
+
+    def mm_to_steps_delta(self, delta_mm):
+        return int(round(float(delta_mm) * self.steps_per_mm * self.dir_sign))
+
+    def steps_to_mm(self, steps):
+        return (float(steps) / self.steps_per_mm) * self.dir_sign
+
+    # ---------------------------------------------------- synthetic motor
+
+    def _publish_synthetic_motor(self):
+        """Write motor-4 vars into State so find_motor('a') and
+        get_axis_vector('vm') see A as a real axis. The AVR never
+        sees these (motor index 4 is not in its var schema)."""
+        cfg = self.aux._cfg if self.aux is not None else {}
+        st = self.ctrl.state
+        i = str(EXTERNAL_MOTOR_INDEX)
+
+        # Axis assignment: 'an' is the 0-based axis index in xyzabc.
+        st.set(i + 'an', self.axis_index)
+        # Motor enabled.
+        st.set(i + 'me', 1 if (self.aux and self.aux.enabled) else 0)
+        # Homed flag - cleared until aux reports homed.
+        try:
+            homed = bool(self.aux._homed)
+        except Exception:
+            homed = False
+        st.set(i + 'h', 1 if homed else 0)
+
+        # Velocity / accel / jerk: the planner reads these via
+        # state.get_axis_vector('<code>', SCALE) which multiplies the
+        # stored raw value by SCALE. The bbctrl convention (matching
+        # what motors 0-3 store) is:
+        #   vm: stored in m/min, planner expects mm/min  (scale 1000)
+        #   am: stored in km/min^2, planner expects mm/min^2 (scale 1e6)
+        #   jm: stored in km/min^3, planner expects mm/min^3 (scale 1e6)
+        # Onefinity defaults for XY are vm=10, am=750, jm=1000. We
+        # follow the same convention; aux.json exposes the values in
+        # those user-facing units so they're directly comparable.
+        st.set(i + 'vm', float(cfg.get('max_velocity_m_per_min',  6.0)))
+        st.set(i + 'am', float(cfg.get('max_accel_km_per_min2', 100.0)))
+        st.set(i + 'jm', float(cfg.get('max_jerk_km_per_min3',  500.0)))
+
+        # Soft limits in machine units (mm). State.get_soft_limit_vector
+        # returns these directly, no scaling.
+        st.set(i + 'tn', float(cfg.get('min_mm', 0.0)))
+        st.set(i + 'tm', float(cfg.get('max_mm', 0.0)))
+
+        # home_position / home_travel are exposed as callbacks for
+        # motors 0..3 (see State.__init__). Register the same lazy
+        # callbacks for motor 4 so gplan's resolver lookup
+        # (_<axis>_home_position / _<axis>_home_travel) returns the
+        # right values for the external axis.
+        st.set_callback(
+            i + 'home_position', lambda name: self.home_position_mm)
+        st.set_callback(
+            i + 'home_travel',
+            lambda name: float(self.aux._cfg.get('max_mm', 0.0))
+                       - self.home_position_mm)
+
+        # Misc fields that other code paths might query. Defaults
+        # mirror what the AVR pushes for motors 0-3.
+        st.set(i + 'sa', 1.8)
+        st.set(i + 'mi', 16)
+        st.set(i + 'tr', 4.0)
+        st.set(i + 'sp', 200)
+        st.set(i + 'ic', 0.0)
+        st.set(i + 'dc', 0.0)
+        st.set(i + 'rv', False)
+        st.set(i + 'tc', 1)
+        st.set(i + 'lb', 5)
+        st.set(i + 'ho', 0)
+        st.set(i + 'os', 0)
+        st.set(i + 'oa', False)
+        st.set(i + 'lm', 8)
+        st.set(i + 'lv', 0.1)
+        st.set(i + 'sv', 1.688)
+        st.set(i + 'tv', 1.997)
+        st.set(i + 'lw', 2)   # min-switch
+        st.set(i + 'xw', 2)   # max-switch
+        st.set(i + 'ls', 0)
+        st.set(i + 'xs', 0)
+        st.set(i + 'df', 0)
+
+    def refresh_homed(self):
+        """Called when AuxAxis updates its homed flag. Mirrors into
+        State so is_axis_homed('a') returns the right answer.
+
+        Updates several places at once because different layers read
+        the homed state via different keys:
+          - synthetic motor flag: 4h    (used by snapshot -> a_h)
+          - axis-level flag:      a_homed (used by State.is_axis_homed
+                                           and gplan _a_homed resolver)"""
+        try:
+            homed = bool(self.aux._homed)
+        except Exception:
+            homed = False
+        st = self.ctrl.state
+        st.set(str(EXTERNAL_MOTOR_INDEX) + 'h', 1 if homed else 0)
+        st.set(self.axis_letter + '_homed', bool(homed))
+
+    # ----------------------------------------------------------- line split
+
+    def split_target(self, target):
+        """Pop the external axis out of a target dict and return
+        (target_without_ext, ext_mm_or_None). Both case variants
+        accepted defensively."""
+        if not target:
+            return target, None
+        ax = self.axis_letter
+        new_target = dict(target)
+        ext_mm = new_target.pop(ax, None)
+        if ext_mm is None:
+            ext_mm = new_target.pop(ax.upper(), None)
+        return new_target, ext_mm
+
+    # -------------------------------------------------------- execution API
+
+    def is_busy(self):
+        return self._busy.is_set()
+
+    def execute_to_mm(self, ext_mm):
+        """Synchronously run an external move. Blocks until the ESP
+        reports done. Used by the legacy /api/aux/move and /api/aux/jog
+        endpoints which may want to wait. Most planner-driven motion
+        goes through enqueue_target_mm instead, which is non-blocking.
+
+        Soft limits are enforced here (not just in gplan) because the
+        UI jog/move endpoints don't go through the planner.
+
+        Updates state.<axis>p immediately on completion. For the
+        planner-driven path that goes through enqueue_target_mm, the
+        AVR's own ap reports drive state.<axis>p instead."""
+        if not self.enabled:
+            raise ExternalAxisError(
+                'External axis %r not available (aux disabled or '
+                'not connected)' % self.axis_letter)
+
+        self._check_soft_limit(ext_mm)
+        steps, abs_mm = self._compute_move(ext_mm)
+        if steps == 0:
+            self._pos_mm = abs_mm
+            self.ctrl.state.set(self.axis_letter + 'p', self._pos_mm)
+            return
+
+        self._busy.set()
+        try:
+            self.aux._do_steps(steps, ignore_limits=True)
+            self._pos_mm = abs_mm
+            self.ctrl.state.set(self.axis_letter + 'p', self._pos_mm)
+        finally:
+            self._busy.clear()
+
+    def enqueue_target_mm(self, ext_mm):
+        """Legacy non-blocking variant: post a fixed-rate STEPS move
+        to the worker queue. No longer used by Planner.__encode (which
+        uses enqueue_line for full S-curve mirroring), but kept for
+        UI jog endpoints that don't have planner timing data.
+
+        Soft limits are enforced here (defense in depth on top of
+        gplan)."""
+        if not self.enabled:
+            raise ExternalAxisError(
+                'External axis %r not available' % self.axis_letter)
+        self._check_soft_limit(ext_mm)
+        steps, abs_mm = self._compute_move(ext_mm)
+        # Internal mirror only - drives subsequent delta computation.
+        # state.<axis>p is left to the AVR's status reports.
+        self._pos_mm = abs_mm
+        if steps == 0:
+            return
+        self._work_q.put(('move', steps))
+
+    def enqueue_line(self, ext_mm, max_accel_mm_min2, max_jerk_mm_min3,
+                     entry_vel_mm_min, exit_vel_mm_min, times_ms):
+        """Post a full S-curve LINE block to the ESP worker. Mirrors
+        gplan's planned trajectory exactly (same 7-segment math, same
+        unit system) so the ESP's move duration matches what the AVR
+        would have produced for an A motor.
+
+        Called by Planner.__encode for every line block that touches
+        the external axis.
+
+        Parameters:
+          ext_mm:           absolute target in mm (gplan target['a'])
+          max_accel_mm_min2:from block['max-accel']
+          max_jerk_mm_min3: from block['max-jerk']
+          entry_vel_mm_min: from block['entry-vel'] (typically 0 for
+                            the first block, exit_vel of the prior
+                            block otherwise)
+          exit_vel_mm_min:  from block['exit-vel']
+          times_ms:         7-tuple of section durations in ms
+                            (block['times'] - the same units gplan uses)
+        """
+        if not self.enabled:
+            raise ExternalAxisError(
+                'External axis %r not available' % self.axis_letter)
+        self._check_soft_limit(ext_mm)
+        steps, abs_mm = self._compute_move(ext_mm)
+        delta_mm = abs(abs_mm - (self._pos_mm if self._pos_mm is not None
+                                 else 0.0))
+        # Update internal mirror; AVR drives state.<axis>p.
+        self._pos_mm = abs_mm
+        if steps == 0 or delta_mm <= 0:
+            return
+        # ms -> minutes (the unit gplan/AVR/ESP use internally for
+        # SCurve math).
+        times_min = tuple((t / 60000.0) if t else 0.0 for t in times_ms)
+        self._work_q.put(('line', steps, delta_mm,
+                          float(max_accel_mm_min2),
+                          float(max_jerk_mm_min3),
+                          float(entry_vel_mm_min),
+                          float(exit_vel_mm_min),
+                          times_min))
+
+    def _compute_move(self, ext_mm):
+        """Return (signed_steps, absolute_mm) for a target in mm.
+        Caches first-time position from the ESP."""
+        if self._pos_mm is None:
+            self._pos_mm = self._read_esp_position_mm()
+        delta_mm = float(ext_mm) - self._pos_mm
+        return self.mm_to_steps_delta(delta_mm), float(ext_mm)
+
+    def _worker_loop(self):
+        """Background thread that drains the work queue. RPCs to the
+        ESP are slow (multi-second moves) and must not run on the
+        ioloop thread. We serialize ESP commands here so multiple
+        line-block enqueues for the external axis are processed in
+        the order the planner emitted them."""
+        while not self._stop.is_set():
+            try:
+                op = self._work_q.get(timeout=0.5)
+            except Exception:
+                continue
+            if op is None:
+                continue
+            kind = op[0]
+            try:
+                self._busy.set()
+                if kind == 'move':
+                    steps = op[1]
+                    self.aux._do_steps(steps, ignore_limits=True)
+                elif kind == 'line':
+                    (_, steps, length_mm,
+                     max_accel, max_jerk,
+                     entry_vel, exit_vel,
+                     times_min) = op
+                    self.aux._do_line(
+                        steps, length_mm, max_accel, max_jerk,
+                        entry_vel, exit_vel, times_min,
+                        ignore_limits=True)
+                elif kind == 'home':
+                    self.aux.home()
+                    # _pos_mm and DRO updated by the caller's enqueue.
+            except Exception as e:
+                self.log.error('External axis worker failed on %s: %s'
+                               % (kind, e))
+            finally:
+                self._busy.clear()
+                self._work_q.task_done()
+
+    def wait_idle(self, timeout=None):
+        """Block until the worker queue is empty. Used by callers
+        that need post-motion state to be settled (e.g. homing,
+        stop/abort handlers)."""
+        try:
+            # Queue.join blocks until task_done has been called for
+            # every item put. It does not honour a timeout, so we
+            # poll instead when one is requested.
+            if timeout is None:
+                self._work_q.join()
+                return True
+            import time
+            deadline = time.time() + float(timeout)
+            while time.time() < deadline:
+                if self._work_q.unfinished_tasks == 0:
+                    return True
+                time.sleep(0.05)
+            return False
+        except Exception:
+            return False
+
+    def close(self):
+        self._stop.set()
+        try:
+            self._work_q.put(None, block=False)
+        except Exception:
+            pass
+
+    def home(self):
+        """Run the ESP homing cycle and sync our recorded position
+        to the configured home_position_mm. Blocks; called from
+        Mach.home (which already runs synchronously per axis)."""
+        if not self.enabled:
+            raise ExternalAxisError(
+                'External axis %r not available' % self.axis_letter)
+        # Drain pending moves so we don't home into stale work.
+        self.wait_idle(timeout=30.0)
+        self._busy.set()
+        try:
+            self.aux.home()
+            self._pos_mm = self.home_position_mm
+            self.ctrl.state.set(self.axis_letter + 'p', self._pos_mm)
+            self.refresh_homed()
+        finally:
+            self._busy.clear()
+
+    def abort(self):
+        """Cancel the ESP move and drop pending queued work.
+        Caller (estop / stop handler) is responsible for the
+        planner-side cleanup."""
+        try:
+            if self.aux is not None:
+                self.aux.abort()
+        finally:
+            self._busy.clear()
+        # Drain any pending ops so resume after an abort doesn't
+        # replay stale targets.
+        try:
+            while True:
+                self._work_q.get_nowait()
+                self._work_q.task_done()
+        except Exception:
+            pass
+
+    # ------------------------------------------------------- ESP introspection
+
+    def _read_esp_position_mm(self):
+        """Convert AuxAxis._pos_steps mirror to mm. Falls back to 0."""
+        try:
+            steps = int(self.aux._pos_steps)
+        except Exception:
+            steps = 0
+        return self.steps_to_mm(steps)
+
+    # ---------------------------------------------------------- DRO update
+
+    def sync_dro(self):
+        """Push the current position to State as <axis>p so the DRO
+        reflects what we believe gplan/ESP agreed on. Called after
+        moves; also safe to call from external code."""
+        if self._pos_mm is None:
+            return
+        self.ctrl.state.set(self.axis_letter + 'p', self._pos_mm)

src/py/bbctrl/State.py

@@ -107,8 +107,14 @@ class State(object):
 
 
     def reset(self):
-        # Unhome all motors
-        for i in range(4): self.set('%dhomed' % i, False)
+        # Unhome all motors (real AVR motors 0..3 and the synthetic
+        # external-axis motor at index 4 used by ExternalAxis).
+        # Both <motor>homed and <motor>h are cleared - they're set
+        # by different code paths (gplan emits homed via _<axis>_homed
+        # set blocks, AVR reports h directly).
+        for i in range(5):
+            self.set('%dhomed' % i, False)
+            self.set('%dh' % i, 0)
 
         # Zero offsets and positions
         for axis in 'xyzabc':
@@ -280,8 +286,11 @@ class State(object):
         axis_motors = {axis: self.find_motor(axis) for axis in 'xyzabc'}
         axis_vars = {}
 
+        # NOTE: motor index '4' is a host-only synthetic motor used
+        # by ExternalAxis to expose the auxcnc ESP-driven stepper as
+        # an additional axis. Real AVR motors are 0..3.
         for name, value in vars.items():
-            if name[0] in '0123':
+            if name[0] in '01234':
                 motor = int(name[0])
 
                 for axis in 'xyzabc':
@@ -330,6 +339,9 @@ class State(object):
     def get_axis_vector(self, name, scale = 1):
         v = {}
 
+        # 0..3 are AVR motor channels. 4 is the host-side synthetic
+        # motor used by ExternalAxis. find_motor returns the right
+        # index regardless of whether the axis is physical or external.
         for axis in 'xyzabc':
             motor = self.find_motor(axis)
 
@@ -351,7 +363,10 @@ class State(object):
 
 
     def find_motor(self, axis):
-        for motor in range(4):
+        # Walk 0..4: 0..3 are real AVR motors, 4 is the synthetic
+        # host-side motor used to expose the auxcnc ESP stepper as
+        # an external axis.
+        for motor in range(5):
             if not ('%dan' % motor) in self.vars: continue
             motor_axis = 'xyzabc'[self.vars['%dan' % motor]]
             if motor_axis == axis.lower() and self.vars.get('%dme' % motor, 0):

src/py/bbctrl/Web.py

@@ -812,7 +812,13 @@ class AuxStatusHandler(bbctrl.APIHandler):
 
 class AuxHomeHandler(bbctrl.APIHandler):
     def put_ok(self):
-        self.get_ctrl().aux.home()
+        # Run synchronously. Route through ExternalAxis so the
+        # synthetic motor's homed flag and DRO update.
+        ext = getattr(self.get_ctrl(), 'ext_axis', None)
+        if ext is not None and ext.enabled:
+            ext.home()
+        else:
+            self.get_ctrl().aux.home()
 
 
 class AuxAbortHandler(bbctrl.APIHandler):
@@ -822,12 +828,22 @@ class AuxAbortHandler(bbctrl.APIHandler):
 
 class AuxJogHandler(bbctrl.APIHandler):
     """Body: {"mm": 1.5}  for relative-mm move,
-             {"steps": 200} for raw step move (bypasses soft limits)."""
+             {"steps": 200} for raw step move (bypasses soft limits).
+
+    Note: with the gplan-integrated W axis, jog-by-mm goes through
+    ExternalAxis so the DRO updates and gplan's idea of A's position
+    stays in sync. jog-by-steps still bypasses everything for the
+    homing/setup workflow where the axis isn't homed yet."""
     def put_ok(self):
         body = self.json or {}
         aux = self.get_ctrl().aux
+        ext = getattr(self.get_ctrl(), 'ext_axis', None)
         if 'mm' in body:
-            aux.move_rel_mm(float(body['mm']))
+            delta_mm = float(body['mm'])
+            if ext is not None and ext.enabled and ext._pos_mm is not None:
+                ext.execute_to_mm(ext._pos_mm + delta_mm)
+            else:
+                aux.move_rel_mm(delta_mm)
         elif 'steps' in body:
             aux.jog_steps(int(body['steps']))
         else:
@@ -840,7 +856,11 @@ class AuxMoveHandler(bbctrl.APIHandler):
         body = self.json or {}
         if 'mm' not in body:
             raise HTTPError(400, 'mm required')
-        self.get_ctrl().aux.move_abs_mm(float(body['mm']))
+        ext = getattr(self.get_ctrl(), 'ext_axis', None)
+        if ext is not None and ext.enabled:
+            ext.execute_to_mm(float(body['mm']))
+        else:
+            self.get_ctrl().aux.move_abs_mm(float(body['mm']))
 
 
 class AuxSetZeroHandler(bbctrl.APIHandler):

src/py/bbctrl/__init__.py

@@ -68,6 +68,7 @@ from bbctrl.IOLoop import IOLoop
 from bbctrl.MonitorTemp import MonitorTemp
 from bbctrl.Hooks import Hooks
 from bbctrl.AuxAxis import AuxAxis
+from bbctrl.ExternalAxis import ExternalAxis
 import bbctrl.Cmd as Cmd
 import bbctrl.v4l2 as v4l2
 import bbctrl.Log as log