New Feature: Z_DUAL_ENDSTOPS

Z_DUAL_ENDSTOPS is a feature to enable the use of 2 endstops for both Z
steppers - Let's call them Z stepper and Z2 stepper.
That way the machine is capable to align the bed during home, since both
Z steppers are homed.
There is also an implementation of M666 (software endstops adjustment)
to this feature.
After Z homing, this adjustment is applied to just one of the steppers
in order to align the bed.
One just need to home the Z axis and measure the distance difference
between both Z axis and apply the math: Z adjust = Z - Z2.
If the Z stepper axis is closer to the bed, the measure Z > Z2 (yes, it
is.. think about it) and the Z adjust would be positive.
Play a little bit with small adjustments (0.5mm) and check the
behaviour.
The M119 (endstops report) will start reporting the Z2 Endstop as well.

Marlin/ConfigurationStore.cpp

  *
  *  filament_size (x4)
  *
+ * Z_DUAL_ENDSTOPS
+ *  z_endstop_adj
+ *
  */
 #include "Marlin.h"
 #include "language.h"
     EEPROM_WRITE_VAR(i, delta_radius);              // 1 float
     EEPROM_WRITE_VAR(i, delta_diagonal_rod);        // 1 float
     EEPROM_WRITE_VAR(i, delta_segments_per_second); // 1 float
+  #elif defined(Z_DUAL_ENDSTOPS)
+    EEPROM_WRITE_VAR(i, z_endstop_adj);            // 1 floats
+    dummy = 0.0f;
+    for (int q=5; q--;) EEPROM_WRITE_VAR(i, dummy);
   #else
     dummy = 0.0f;
     for (int q=6; q--;) EEPROM_WRITE_VAR(i, dummy);
       EEPROM_READ_VAR(i, delta_radius);               // 1 float
       EEPROM_READ_VAR(i, delta_diagonal_rod);         // 1 float
       EEPROM_READ_VAR(i, delta_segments_per_second);  // 1 float
+    #elif defined(Z_DUAL_ENDSTOPS)
+      EEPROM_READ_VAR(i, z_endstop_adj);
+      dummy = 0.0f;
+      for (int q=5; q--;) EEPROM_READ_VAR(i, dummy);
     #else
+      dummy = 0.0f;
       for (int q=6; q--;) EEPROM_READ_VAR(i, dummy);
     #endif
 
     delta_diagonal_rod =  DELTA_DIAGONAL_ROD;
     delta_segments_per_second =  DELTA_SEGMENTS_PER_SECOND;
     recalc_delta_settings(delta_radius, delta_diagonal_rod);
+  #elif defined(Z_DUAL_ENDSTOPS)
+    z_endstop_adj = 0;
   #endif
 
   #ifdef ULTIPANEL
     SERIAL_ECHOPAIR(" R", delta_radius );
     SERIAL_ECHOPAIR(" S", delta_segments_per_second );
     SERIAL_EOL;
+  #elif defined(Z_DUAL_ENDSTOPS)
+    SERIAL_ECHO_START;
+    if (!forReplay) {
+      SERIAL_ECHOLNPGM("Z2 Endstop adjustement (mm):");
+      SERIAL_ECHO_START;
+    }
+    SERIAL_ECHOPAIR("  M666 Z", z_endstop_adj );
+    SERIAL_EOL;  
   #endif // DELTA
 
   #ifdef PIDTEMP

Marlin/Configuration_adv.h

 // Only a few motherboards support this, like RAMPS, which have dual extruder support (the 2nd, often unused, extruder driver is used
 // to control the 2nd Z axis stepper motor). The pins are currently only defined for a RAMPS motherboards.
 // On a RAMPS (or other 5 driver) motherboard, using this feature will limit you to using 1 extruder.
-//#define Z_DUAL_STEPPER_DRIVERS
+#define Z_DUAL_STEPPER_DRIVERS
+
+#ifdef Z_DUAL_STEPPER_DRIVERS
+
+// Z_DUAL_ENDSTOPS is a feature to enable the use of 2 endstops for both Z steppers - Let's call them Z stepper and Z2 stepper.
+// That way the machine is capable to align the bed during home, since both Z steppers are homed. 
+// There is also an implementation of M666 (software endstops adjustment) to this feature.
+// After Z homing, this adjustment is applied to just one of the steppers in order to align the bed.
+// One just need to home the Z axis and measure the distance difference between both Z axis and apply the math: Z adjust = Z - Z2.
+// If the Z stepper axis is closer to the bed, the measure Z > Z2 (yes, it is.. think about it) and the Z adjust would be positive.
+// Play a little bit with small adjustments (0.5mm) and check the behaviour.
+// The M119 (endstops report) will start reporting the Z2 Endstop as well.
+
+#define Z_DUAL_ENDSTOPS
+
+#ifdef Z_DUAL_ENDSTOPS
+  #define Z2_STEP_PIN E2_STEP_PIN           // Stepper to be used to Z2 axis.
+  #define Z2_DIR_PIN E2_DIR_PIN
+  #define Z2_ENABLE_PIN E2_ENABLE_PIN
+  #define Z2_MAX_PIN 36                     //Endstop used for Z2 axis. In this case I'm using XMAX in a Rumba Board (pin 36)
+  const bool Z2_MAX_ENDSTOP_INVERTING = false;
+  #define DISABLE_XMAX_ENDSTOP              //Better to disable the XMAX to avoid conflict. Just rename "XMAX_ENDSTOP" by the endstop you are using for Z2 axis.
+#endif
+
+
+#endif
 
 // Same again but for Y Axis.
 //#define Y_DUAL_STEPPER_DRIVERS

Marlin/Marlin.h

   extern float delta_diagonal_rod;
   extern float delta_segments_per_second;
   void recalc_delta_settings(float radius, float diagonal_rod);
+#elif defined(Z_DUAL_ENDSTOPS)
+extern float z_endstop_adj;
 #endif
 #ifdef SCARA
   extern float axis_scaling[3];  // Build size scaling

Marlin/Marlin_main.cpp

 float home_offset[3] = { 0, 0, 0 };
 #ifdef DELTA
   float endstop_adj[3] = { 0, 0, 0 };
+#elif defined(Z_DUAL_ENDSTOPS)
+  float z_endstop_adj = 0;
 #endif
 
 float min_pos[3] = { X_MIN_POS, Y_MIN_POS, Z_MIN_POS };
 
   static float x_home_pos(int extruder) {
     if (extruder == 0)
-      return base_home_pos(X_AXIS) + add_homing[X_AXIS];
+    return base_home_pos(X_AXIS) + home_offset[X_AXIS];
     else
       // In dual carriage mode the extruder offset provides an override of the
       // second X-carriage offset when homed - otherwise X2_HOME_POS is used.
       }
     #endif
 #endif // Z_PROBE_SLED
+    #ifdef Z_DUAL_ENDSTOPS
+      if (axis==Z_AXIS) In_Homing_Process(true);
+    #endif
     destination[axis] = 1.5 * max_length(axis) * axis_home_dir;
     feedrate = homing_feedrate[axis];
     plan_buffer_line(destination[X_AXIS], destination[Y_AXIS], destination[Z_AXIS], destination[E_AXIS], feedrate/60, active_extruder);
 
     plan_buffer_line(destination[X_AXIS], destination[Y_AXIS], destination[Z_AXIS], destination[E_AXIS], feedrate/60, active_extruder);
     st_synchronize();
+    #ifdef Z_DUAL_ENDSTOPS
+      if (axis==Z_AXIS)
+      {
+        feedrate = homing_feedrate[axis];
+        plan_set_position(current_position[X_AXIS], current_position[Y_AXIS], current_position[Z_AXIS], current_position[E_AXIS]);
+        if (axis_home_dir > 0)
+        {
+          destination[axis] = (-1) * fabs(z_endstop_adj);
+          if (z_endstop_adj > 0) Lock_z_motor(true); else Lock_z2_motor(true);
+        } else {
+          destination[axis] = fabs(z_endstop_adj);
+          if (z_endstop_adj < 0) Lock_z_motor(true); else Lock_z2_motor(true);        
+        }
+        plan_buffer_line(destination[X_AXIS], destination[Y_AXIS], destination[Z_AXIS], destination[E_AXIS], feedrate/60, active_extruder);
+        st_synchronize();
+        Lock_z_motor(false);
+        Lock_z2_motor(false);
+        In_Homing_Process(false);
+      }
+    #endif
+
 #ifdef DELTA
     // retrace by the amount specified in endstop_adj
     if (endstop_adj[axis] * axis_home_dir < 0) {
 
   enable_endstops(true);
 
-  for (int i = X_AXIS; i <= Z_AXIS; i++) destination[i] = current_position[i];
+  for (int i = X_AXIS; i <= NUM_AXIS; i++) destination[i] = current_position[i];
 
   feedrate = 0.0;
 
     if (code_seen(axis_codes[Z_AXIS]) && code_value_long() != 0)
       current_position[Z_AXIS] = code_value() + home_offset[Z_AXIS];
 
-    #ifdef ENABLE_AUTO_BED_LEVELING
+    #if defined(ENABLE_AUTO_BED_LEVELING) && (Z_HOME_DIR < 0)
       if (home_all_axis || code_seen(axis_codes[Z_AXIS]))
         current_position[Z_AXIS] += zprobe_zoffset;  //Add Z_Probe offset (the distance is negative)
     #endif
     SERIAL_PROTOCOLPGM(MSG_Z_MAX);
     SERIAL_PROTOCOLLN(((READ(Z_MAX_PIN)^Z_MAX_ENDSTOP_INVERTING)?MSG_ENDSTOP_HIT:MSG_ENDSTOP_OPEN));
   #endif
+  #if defined(Z2_MAX_PIN) && Z2_MAX_PIN > -1
+    SERIAL_PROTOCOLPGM(MSG_Z2_MAX);
+    SERIAL_PROTOCOLLN(((READ(Z2_MAX_PIN)^Z2_MAX_ENDSTOP_INVERTING)?MSG_ENDSTOP_HIT:MSG_ENDSTOP_OPEN));
+  #endif
+  
 }
 
 /**
       }
     }
   }
+#elif defined(Z_DUAL_ENDSTOPS)
+  /**
+   * M666: For Z Dual Endstop setup, set z axis offset to the z2 axis.
+   */
+  inline void gcode_M666() {
+   if (code_seen('Z')) z_endstop_adj = code_value();
+   SERIAL_ECHOPAIR("Z Endstop Adjustment set to (mm):", z_endstop_adj );
+   SERIAL_EOL;
+  }
+  
 #endif // DELTA
 
 #ifdef FWRETRACT
         case 666: // M666 set delta endstop adjustment
           gcode_M666();
           break;
+      #elif defined(Z_DUAL_ENDSTOPS)
+        case 666: // M666 set delta endstop adjustment
+          gcode_M666();
+          break;
       #endif // DELTA
 
       #ifdef FWRETRACT

Marlin/language.h

 #define MSG_Y_MAX                           "y_max: "
 #define MSG_Z_MIN                           "z_min: "
 #define MSG_Z_MAX                           "z_max: "
+#define MSG_Z2_MAX                          "z2_max: "
 #define MSG_M119_REPORT                     "Reporting endstop status"
 #define MSG_ENDSTOP_HIT                     "TRIGGERED"
 #define MSG_ENDSTOP_OPEN                    "open"

Marlin/pins.h

   #define Z_MIN_PIN          -1
 #endif
 
+#ifdef DISABLE_XMAX_ENDSTOP
+  #undef X_MAX_PIN
+  #define X_MAX_PIN          -1
+#endif
+
+#ifdef DISABLE_XMIN_ENDSTOP
+  #undef X_MIN_PIN 
+  #define X_MIN_PIN          -1
+#endif
+
+#ifdef DISABLE_YMAX_ENDSTOP
+  #define Y_MAX_PIN          -1
+#endif
+
+#ifdef DISABLE_YMIN_ENDSTOP
+  #undef Y_MIN_PIN
+  #define Y_MIN_PIN          -1
+#endif
+
+#ifdef DISABLE_ZMAX_ENDSTOP
+  #undef Z_MAX_PIN
+  #define Z_MAX_PIN          -1
+#endif
+
+#ifdef DISABLE_ZMIN_ENDSTOP
+  #undef Z_MIN_PIN 
+  #define Z_MIN_PIN          -1
+#endif
+
 #define SENSITIVE_PINS { 0, 1, X_STEP_PIN, X_DIR_PIN, X_ENABLE_PIN, X_MIN_PIN, X_MAX_PIN, Y_STEP_PIN, Y_DIR_PIN, Y_ENABLE_PIN, Y_MIN_PIN, Y_MAX_PIN, Z_STEP_PIN, Z_DIR_PIN, Z_ENABLE_PIN, Z_MIN_PIN, Z_MAX_PIN, PS_ON_PIN, \
                         HEATER_BED_PIN, FAN_PIN, \
                         _E0_PINS _E1_PINS _E2_PINS _E3_PINS \

Marlin/stepper.cpp

 static unsigned char out_bits;        // The next stepping-bits to be output
 static unsigned int cleaning_buffer_counter;  
 
+#ifdef Z_DUAL_ENDSTOPS
+  static bool performing_homing = false, 
+              locked_z_motor = false, 
+              locked_z2_motor = false;
+#endif
+
 // Counter variables for the bresenham line tracer
 static long counter_x, counter_y, counter_z, counter_e;
 volatile static unsigned long step_events_completed; // The number of step events executed in the current block
             old_y_min_endstop = false,
             old_y_max_endstop = false,
             old_z_min_endstop = false,
+            #ifndef Z_DUAL_ENDSTOPS
             old_z_max_endstop = false;
+            #else
+              old_z_max_endstop = false,
+              old_z2_min_endstop = false,
+              old_z2_max_endstop = false;
+            #endif
 
 static bool check_endstops = true;
 
 
 #ifdef Z_DUAL_STEPPER_DRIVERS
   #define Z_APPLY_DIR(v,Q) { Z_DIR_WRITE(v); Z2_DIR_WRITE(v); }
-  #define Z_APPLY_STEP(v,Q) { Z_STEP_WRITE(v); Z2_STEP_WRITE(v); }
+  #ifdef Z_DUAL_ENDSTOPS
+    #define Z_APPLY_STEP(v,Q) \
+    if (performing_homing) { \
+      if (Z_HOME_DIR > 0) {\
+        if (!(old_z_max_endstop && (count_direction[Z_AXIS] > 0)) && !locked_z_motor) Z_STEP_WRITE(v); \
+        if (!(old_z2_max_endstop && (count_direction[Z_AXIS] > 0)) && !locked_z2_motor) Z2_STEP_WRITE(v); \
+      } else {\
+        if (!(old_z_min_endstop && (count_direction[Z_AXIS] < 0)) && !locked_z_motor) Z_STEP_WRITE(v); \
+        if (!(old_z2_min_endstop && (count_direction[Z_AXIS] < 0)) && !locked_z2_motor) Z2_STEP_WRITE(v); \
+      } \
+    } else { \
+      Z_STEP_WRITE(v); \
+      Z2_STEP_WRITE(v); \
+    }
+  #else
+    #define Z_APPLY_STEP(v,Q) Z_STEP_WRITE(v), Z2_STEP_WRITE(v)
+  #endif
 #else
   #define Z_APPLY_DIR(v,Q) Z_DIR_WRITE(v)
   #define Z_APPLY_STEP(v,Q) Z_STEP_WRITE(v)
     }
 
     if (TEST(out_bits, Z_AXIS)) {   // -direction
-      Z_DIR_WRITE(INVERT_Z_DIR);
-      #ifdef Z_DUAL_STEPPER_DRIVERS
-        Z2_DIR_WRITE(INVERT_Z_DIR);
-      #endif
-
+      Z_APPLY_DIR(INVERT_Z_DIR,0);
       count_direction[Z_AXIS] = -1;
-      if (check_endstops) {
-        #if defined(Z_MIN_PIN) && Z_MIN_PIN >= 0
-          UPDATE_ENDSTOP(z, Z, min, MIN);
+      if (check_endstops) 
+      {
+        #if defined(Z_MIN_PIN) && Z_MIN_PIN > -1
+          #ifndef Z_DUAL_ENDSTOPS
+            UPDATE_ENDSTOP(z, Z, min, MIN);
+          #else
+            bool z_min_endstop=(READ(Z_MIN_PIN) != Z_MIN_ENDSTOP_INVERTING);
+            #if defined(Z2_MIN_PIN) && Z2_MIN_PIN > -1
+              bool z2_min_endstop=(READ(Z2_MIN_PIN) != Z2_MIN_ENDSTOP_INVERTING);
+            #else
+              bool z2_min_endstop=z_min_endstop;
+            #endif
+            if(((z_min_endstop && old_z_min_endstop) || (z2_min_endstop && old_z2_min_endstop)) && (current_block->steps[Z_AXIS] > 0))
+            {
+              endstops_trigsteps[Z_AXIS] = count_position[Z_AXIS];
+              endstop_z_hit=true;
+              if (!(performing_homing) || ((performing_homing)&&(z_min_endstop && old_z_min_endstop)&&(z2_min_endstop && old_z2_min_endstop))) //if not performing home or if both endstops were trigged during homing...
+              {
+                step_events_completed = current_block->step_event_count;
+              } 
+            }
+            old_z_min_endstop = z_min_endstop;
+            old_z2_min_endstop = z2_min_endstop;
+          #endif
         #endif
       }
     }
     else { // +direction
-      Z_DIR_WRITE(!INVERT_Z_DIR);
-      #ifdef Z_DUAL_STEPPER_DRIVERS
-        Z2_DIR_WRITE(!INVERT_Z_DIR);
-      #endif
-
+      Z_APPLY_DIR(!INVERT_Z_DIR,0);
       count_direction[Z_AXIS] = 1;
       if (check_endstops) {
         #if defined(Z_MAX_PIN) && Z_MAX_PIN >= 0
-          UPDATE_ENDSTOP(z, Z, max, MAX);
+          #ifndef Z_DUAL_ENDSTOPS
+            UPDATE_ENDSTOP(z, Z, max, MAX);
+          #else
+            bool z_max_endstop=(READ(Z_MAX_PIN) != Z_MAX_ENDSTOP_INVERTING);
+            #if defined(Z2_MAX_PIN) && Z2_MAX_PIN > -1
+              bool z2_max_endstop=(READ(Z2_MAX_PIN) != Z2_MAX_ENDSTOP_INVERTING);
+            #else
+              bool z2_max_endstop=z_max_endstop;
+            #endif
+            if(((z_max_endstop && old_z_max_endstop) || (z2_max_endstop && old_z2_max_endstop)) && (current_block->steps[Z_AXIS] > 0))
+            {
+              endstops_trigsteps[Z_AXIS] = count_position[Z_AXIS];
+              endstop_z_hit=true;
+
+//              if (z_max_endstop && old_z_max_endstop) SERIAL_ECHOLN("z_max_endstop = true");
+//              if (z2_max_endstop && old_z2_max_endstop) SERIAL_ECHOLN("z2_max_endstop = true");
+
+            
+              if (!(performing_homing) || ((performing_homing)&&(z_max_endstop && old_z_max_endstop)&&(z2_max_endstop && old_z2_max_endstop))) //if not performing home or if both endstops were trigged during homing...
+              {
+                step_events_completed = current_block->step_event_count;
+              } 
+            }
+            old_z_max_endstop = z_max_endstop;
+            old_z2_max_endstop = z2_max_endstop;
+          #endif
         #endif
       }
     }
     #endif
   #endif
 
+  #if defined(Z2_MAX_PIN) && Z2_MAX_PIN >= 0
+    SET_INPUT(Z2_MAX_PIN);
+    #ifdef ENDSTOPPULLUP_ZMAX
+      WRITE(Z2_MAX_PIN,HIGH);
+    #endif
+  #endif  
+  
   #define AXIS_INIT(axis, AXIS, PIN) \
     AXIS ##_STEP_INIT; \
     AXIS ##_STEP_WRITE(INVERT_## PIN ##_STEP_PIN); \
     SERIAL_PROTOCOLLN(digitalRead(E1_MS2_PIN));
   #endif
 }
+
+#ifdef Z_DUAL_ENDSTOPS
+  void In_Homing_Process(bool state) { performing_homing = state; }
+  void Lock_z_motor(bool state) { locked_z_motor = state; }
+  void Lock_z2_motor(bool state) { locked_z2_motor = state; }
+#endif

Marlin/stepper.h

 void microstep_init();
 void microstep_readings();
 
+#ifdef Z_DUAL_ENDSTOPS
+  void In_Homing_Process(bool state);
+  void Lock_z_motor(bool state);
+  void Lock_z2_motor(bool state);
+#endif
+
 #ifdef BABYSTEPPING
   void babystep(const uint8_t axis,const bool direction); // perform a short step with a single stepper motor, outside of any convention
 #endif