idex_modes: Fixed the case when carriages home in the same direction

docs/Config_Changes.md

@@ -8,6 +8,15 @@ All dates in this document are approximate.
 
 ## Changes
 
+20230826: If `safe_distance` is set or calculated to be 0 in `[dual_carriage]`,
+the carriages proximity checks will be disabled as per documentation. A user
+may wish to configure `safe_distance` explicitly to prevent accidental crashes
+of the carriages with each other. Additionally, the homing order of the primary
+and the dual carriage is changed in some configurations (certain configurations
+when both carriages home in the same direction, see
+[[dual_carriage] configuration reference](./Config_Reference.md#dual_carriage)
+for more details).
+
 20230810: The flash-sdcard.sh script now supports both variants of the
 Bigtreetech SKR-3, STM32H743 and STM32H723. For this, the original tag
 of btt-skr-3 now has changed to be either btt-skr-3-h743 or btt-skr-3-h723.

docs/Config_Reference.md

@@ -2044,6 +2044,14 @@ in this section (CARRIAGE=0 will return activation to the primary carriage).
 Dual carriage support is typically combined with extra extruders - the
 SET_DUAL_CARRIAGE command is often called at the same time as the
 ACTIVATE_EXTRUDER command. Be sure to park the carriages during deactivation.
+Note that during G28 homing, typically the primary carriage is homed first
+followed by the carriage defined in the `[dual_carriage]` config section.
+However, the `[dual_carriage]` carriage will be homed first if both carriages
+home in a positive direction and the [dual_carriage] carriage has a
+`position_endstop` greater than the primary carriage, or if both carriages home
+in a negative direction and the `[dual_carriage]` carriage has a
+`position_endstop` less than the primary carriage.
+
 Additionally, one could use "SET_DUAL_CARRIAGE CARRIAGE=1 MODE=COPY" or
 "SET_DUAL_CARRIAGE CARRIAGE=1 MODE=MIRROR" commands to activate either copying
 or mirroring mode of the dual carriage, in which case it will follow the

klippy/kinematics/idex_modes.py

@@ -65,7 +65,13 @@ def toggle_active_dc_rail(self, index, override_rail=False):
         kin.update_limits(self.axis, target_dc.get_rail().get_range())
     def home(self, homing_state):
         kin = self.printer.lookup_object('toolhead').get_kinematics()
-        for i, dc_rail in enumerate(self.dc):
+        enumerated_dcs = list(enumerate(self.dc))
+        if (self.get_dc_order(0, 1) > 0) != \
+                self.dc[0].get_rail().get_homing_info().positive_dir:
+            # The second carriage must home first, because the carriages home in
+            # the same direction and the first carriage homes on the second one
+            enumerated_dcs.reverse()
+        for i, dc_rail in enumerated_dcs:
             self.toggle_active_dc_rail(i, override_rail=True)
             kin.home_axis(homing_state, self.axis, dc_rail.get_rail())
         # Restore the original rails ordering
@@ -78,17 +84,23 @@ def get_kin_range(self, toolhead, mode):
         axes_pos = [dc.get_axis_position(pos) for dc in self.dc]
         dc0_rail = self.dc[0].get_rail()
         dc1_rail = self.dc[1].get_rail()
-        range_min = dc0_rail.position_min
-        range_max = dc0_rail.position_max
+        if mode != PRIMARY or self.dc[0].is_active():
+            range_min = dc0_rail.position_min
+            range_max = dc0_rail.position_max
+        else:
+            range_min = dc1_rail.position_min
+            range_max = dc1_rail.position_max
         safe_dist = self.safe_dist
+        if not safe_dist:
+            return (range_min, range_max)
 
         if mode == COPY:
             range_min = max(range_min,
                             axes_pos[0] - axes_pos[1] + dc1_rail.position_min)
             range_max = min(range_max,
                             axes_pos[0] - axes_pos[1] + dc1_rail.position_max)
         elif mode == MIRROR:
-            if dc0_rail.get_homing_info().positive_dir:
+            if self.get_dc_order(0, 1) > 0:
                 range_min = max(range_min,
                                 0.5 * (sum(axes_pos) + safe_dist))
                 range_max = min(range_max,
@@ -102,14 +114,39 @@ def get_kin_range(self, toolhead, mode):
             # mode == PRIMARY
             active_idx = 1 if self.dc[1].is_active() else 0
             inactive_idx = 1 - active_idx
-            if active_idx:
-                range_min = dc1_rail.position_min
-                range_max = dc1_rail.position_max
-            if self.dc[active_idx].get_rail().get_homing_info().positive_dir:
+            if self.get_dc_order(active_idx, inactive_idx) > 0:
                 range_min = max(range_min, axes_pos[inactive_idx] + safe_dist)
             else:
                 range_max = min(range_max, axes_pos[inactive_idx] - safe_dist)
+        if range_min > range_max:
+            # During multi-MCU homing it is possible that the carriage
+            # position will end up below position_min or above position_max
+            # if position_endstop is too close to the rail motion ends due
+            # to inherent latencies of the data transmission between MCUs.
+            # This can result in an invalid range_min > range_max range
+            # in certain modes, which may confuse the kinematics code.
+            # So, return an empty range instead, which will correctly
+            # block the carriage motion until a different mode is selected
+            # which actually permits carriage motion.
+            return (range_min, range_min)
         return (range_min, range_max)
+    def get_dc_order(self, first, second):
+        if first == second:
+            return 0
+        # Check the relative order of the first and second carriages and
+        # return -1 if the first carriage position is always smaller
+        # than the second one and 1 otherwise
+        first_rail = self.dc[first].get_rail()
+        second_rail = self.dc[second].get_rail()
+        first_homing_info = first_rail.get_homing_info()
+        second_homing_info = second_rail.get_homing_info()
+        if first_homing_info.positive_dir != second_homing_info.positive_dir:
+            # Carriages home away from each other
+            return 1 if first_homing_info.positive_dir else -1
+        # Carriages home in the same direction
+        if first_rail.position_endstop > second_rail.position_endstop:
+            return 1
+        return -1
     def activate_dc_mode(self, index, mode):
         toolhead = self.printer.lookup_object('toolhead')
         toolhead.flush_step_generation()