diff --git a/doc/csv/alarm_codes_en_US.csv b/doc/csv/alarm_codes_en_US.csv
index 1ab2b8ae..685b61e6 100644
--- a/doc/csv/alarm_codes_en_US.csv
+++ b/doc/csv/alarm_codes_en_US.csv
@@ -8,3 +8,4 @@
 "7","Homing fail","Homing fail. Safety door was opened during homing cycle."
 "8","Homing fail","Homing fail. Pull off travel failed to clear limit switch. Try increasing pull-off setting or check wiring."
 "9","Homing fail","Homing fail. Could not find limit switch within search distances. Try increasing max travel, decreasing pull-off distance, or check wiring."
+"10","Homing fail","Homing fail. Second dual axis limit switch failed to trigger within configured search distance after first. Try increasing trigger fail distance or check wiring."
diff --git a/doc/csv/build_option_codes_en_US.csv b/doc/csv/build_option_codes_en_US.csv
index ab05ef29..54514bb6 100644
--- a/doc/csv/build_option_codes_en_US.csv
+++ b/doc/csv/build_option_codes_en_US.csv
@@ -19,4 +19,5 @@ $,Restore EEPROM `$` settings command,Disabled
 I,Build info write user string command,Disabled
 E,Force sync upon EEPROM write,Disabled
 W,Force sync upon work coordinate offset change,Disabled
-L,Homing initialization auto-lock,Disabled
\ No newline at end of file
+L,Homing initialization auto-lock,Disabled
+2,Dual axis motors,Enabled
\ No newline at end of file
diff --git a/doc/log/commit_log_v1.1.txt b/doc/log/commit_log_v1.1.txt
index 2aa110de..e875ee3b 100644
--- a/doc/log/commit_log_v1.1.txt
+++ b/doc/log/commit_log_v1.1.txt
@@ -1,3 +1,40 @@
+----------------
+Date: 2018-11-12
+Author: Sonny Jeon
+Subject: Update grbl.h
+
+----------------
+Date: 2018-11-12
+Author: Sonny Jeon
+Subject: Update system.c
+
+Correct control pin state checking within pin change interrupt. Improper if-else statements could lead to missed signal.
+
+----------------
+Date: 2018-11-12
+Author: Sonny Jeon
+Subject: Update gcode.c
+
+If statement bug fix related to jog motion modal group error checking.
+
+----------------
+Date: 2018-06-14
+Author: Sonny Jeon
+Subject: Spindle/coolant rare bug fixes. Free more flash.
+
+[new] Altered the way default settings are stored and restored. Saved about 300 bytes(!) of flashed size. Should free up enough for certain configurations of CoreXY machines.
+
+[fix] When the optional M7 mist coolant IO was enabled, coolant overrides was not disabling correctly.
+
+[fix] Coolant override states was not restored correctly after a parking motion in certain situations. It would restore programmed state, rather than current overridden state.
+
+[fix] Now allow coolant overrides to operate during jogging motion.
+
+[fix] Invert control pin mask typo.
+
+[new] Added a new build info feedback mechanism for enabling the safety door input pin.
+
+
 ----------------
 Date: 2018-06-09
 Author: Jon
diff --git a/grbl/config.h b/grbl/config.h
index 4d36b54e..f48d9586 100644
--- a/grbl/config.h
+++ b/grbl/config.h
@@ -586,9 +586,11 @@
 // to ensure the laser doesn't inadvertently remain powered while at a stop and cause a fire.
 #define DISABLE_LASER_DURING_HOLD // Default enabled. Comment to disable.
 
-// Enables a piecewise linear model of the spindle PWM/speed output. Requires a solution by the
-// 'fit_nonlinear_spindle.py' script in the /doc/script folder of the repo. See file comments 
-// on how to gather spindle data and run the script to generate a solution.
+// This feature alters the spindle PWM/speed to a nonlinear output with a simple piecewise linear
+// curve. Useful for spindles that don't produce the right RPM from Grbl's standard spindle PWM 
+// linear model. Requires a solution by the 'fit_nonlinear_spindle.py' script in the /doc/script
+// folder of the repo. See file comments on how to gather spindle data and run the script to
+// generate a solution.
 // #define ENABLE_PIECEWISE_LINEAR_SPINDLE  // Default disabled. Uncomment to enable.
 
 // N_PIECES, RPM_MAX, RPM_MIN, RPM_POINTxx, and RPM_LINE_XX constants are all set and given by
@@ -610,6 +612,68 @@
 #define RPM_LINE_A4  1.203413e-01  // Used N_PIECES = 4. A and B constants of line 4.
 #define RPM_LINE_B4  1.151360e+03
 
+/* --------------------------------------------------------------------------------------- 
+  This optional dual axis feature is primarily for the homing cycle to locate two sides of 
+  a dual-motor gantry independently, i.e. self-squaring. This requires an additional limit
+  switch for the cloned motor. To self square, both limit switches on the cloned axis must
+  be physically positioned to trigger when the gantry is square. Highly recommend keeping  
+  the motors always enabled to ensure the gantry stays square with the $1=255 setting.
+
+  For Grbl on the Arduino Uno, the cloned axis limit switch must to be shared with and 
+  wired with z-axis limit pin due to the lack of available pins. The homing cycle must home
+  the z-axis and cloned axis in different cycles, which is already the default config.
+
+  The dual axis feature works by cloning an axis step output onto another pair of step
+  and direction pins. The step pulse and direction of the cloned motor can be set 
+  independently of the main axis motor. However to save precious flash and memory, this
+  dual axis feature must share the same settings (step/mm, max speed, acceleration) as the 
+  parent motor. This is NOT a feature for an independent fourth axis. Only a motor clone.
+
+  WARNING: Make sure to test the directions of your dual axis motors! They must be setup
+  to move the same direction BEFORE running your first homing cycle or any long motion!
+  Motors moving in opposite directions can cause serious damage to your machine! Use this 
+  dual axis feature at your own risk.
+*/
+// NOTE: This feature requires approximately 400 bytes of flash. Certain configurations can
+// run out of flash to fit on an Arduino 328p/Uno. Only X and Y axes are supported. Variable
+// spindle/laser mode IS supported, but only for one config option. Core XY, spindle direction
+// pin, and M7 mist coolant are disabled/not supported.
+// #define ENABLE_DUAL_AXIS	// Default disabled. Uncomment to enable.
+
+// Select the one axis to mirror another motor. Only X and Y axis is supported at this time.
+#define DUAL_AXIS_SELECT  X_AXIS  // Must be either X_AXIS or Y_AXIS
+
+// To prevent the homing cycle from racking the dual axis, when one limit triggers before the
+// other due to switch failure or noise, the homing cycle will automatically abort if the second 
+// motor's limit switch does not trigger within the three distance parameters defined below. 
+// Axis length percent will automatically compute a fail distance as a percentage of the max
+// travel of the other non-dual axis, i.e. if dual axis select is X_AXIS at 5.0%, then the fail 
+// distance will be computed as 5.0% of y-axis max travel. Fail distance max and min are the 
+// limits of how far or little a valid fail distance is.
+#define DUAL_AXIS_HOMING_FAIL_AXIS_LENGTH_PERCENT  5.0  // Float (percent)
+#define DUAL_AXIS_HOMING_FAIL_DISTANCE_MAX  25.0  // Float (mm)
+#define DUAL_AXIS_HOMING_FAIL_DISTANCE_MIN  2.5  // Float (mm)
+
+// Dual axis pin configuration currently supports two shields. Uncomment the shield you want,
+// and comment out the other one(s).
+// NOTE: Protoneer CNC Shield v3.51 has A.STP and A.DIR wired to pins A4 and A3 respectively.
+// The variable spindle (i.e. laser mode) build option works and may be enabled or disabled.
+// Coolant pin A3 is moved to D13, replacing spindle direction.
+#define DUAL_AXIS_CONFIG_PROTONEER_V3_51    // Uncomment to select. Comment other configs.
+
+// NOTE: Arduino CNC Shield Clone (Originally Protoneer v3.0) has A.STP and A.DIR wired to 
+// D12 and D13, respectively. With the limit pins and stepper enable pin on this same port,
+// the spindle enable pin had to be moved and spindle direction pin deleted. The spindle
+// enable pin now resides on A3, replacing coolant enable. Coolant enable is bumped over to
+// pin A4. Spindle enable is used far more and this pin setup helps facilitate users to 
+// integrate this feature without arguably too much work. 
+// Variable spindle (i.e. laser mode) does NOT work with this shield as configured. While
+// variable spindle technically can work with this shield, it requires too many changes for
+// most user setups to accomodate. It would best be implemented by sharing all limit switches
+// on pins D9/D10 (as [X1,Z]/[X2,Y] or [X,Y2]/[Y1,Z]), home each axis independently, and 
+// updating lots of code to ensure everything is running correctly.
+// #define DUAL_AXIS_CONFIG_CNC_SHIELD_CLONE  // Uncomment to select. Comment other configs.
+
 
 /* ---------------------------------------------------------------------------------------
    OEM Single File Configuration Option
diff --git a/grbl/cpu_map.h b/grbl/cpu_map.h
index 0e2b25e5..9f43a3fc 100644
--- a/grbl/cpu_map.h
+++ b/grbl/cpu_map.h
@@ -67,39 +67,13 @@
   #else
     #define Z_LIMIT_BIT    3  // Uno Digital Pin 11
   #endif
-  #define LIMIT_MASK       ((1<<X_LIMIT_BIT)|(1<<Y_LIMIT_BIT)|(1<<Z_LIMIT_BIT)) // All limit bits
+  #if !defined(ENABLE_DUAL_AXIS)
+    #define LIMIT_MASK     ((1<<X_LIMIT_BIT)|(1<<Y_LIMIT_BIT)|(1<<Z_LIMIT_BIT)) // All limit bits
+  #endif
   #define LIMIT_INT        PCIE0  // Pin change interrupt enable pin
   #define LIMIT_INT_vect   PCINT0_vect
   #define LIMIT_PCMSK      PCMSK0 // Pin change interrupt register
 
-  // Define spindle enable and spindle direction output pins.
-  #define SPINDLE_ENABLE_DDR    DDRB
-  #define SPINDLE_ENABLE_PORT   PORTB
-  // Z Limit pin and spindle PWM/enable pin swapped to access hardware PWM on Pin 11.
-  #ifdef VARIABLE_SPINDLE
-    #ifdef USE_SPINDLE_DIR_AS_ENABLE_PIN
-      // If enabled, spindle direction pin now used as spindle enable, while PWM remains on D11.
-      #define SPINDLE_ENABLE_BIT    5  // Uno Digital Pin 13 (NOTE: D13 can't be pulled-high input due to LED.)
-    #else
-      #define SPINDLE_ENABLE_BIT    3  // Uno Digital Pin 11
-    #endif
-  #else
-    #define SPINDLE_ENABLE_BIT    4  // Uno Digital Pin 12
-  #endif
-  #ifndef USE_SPINDLE_DIR_AS_ENABLE_PIN
-    #define SPINDLE_DIRECTION_DDR   DDRB
-    #define SPINDLE_DIRECTION_PORT  PORTB
-    #define SPINDLE_DIRECTION_BIT   5  // Uno Digital Pin 13 (NOTE: D13 can't be pulled-high input due to LED.)
-  #endif
-
-  // Define flood and mist coolant enable output pins.
-  #define COOLANT_FLOOD_DDR   DDRC
-  #define COOLANT_FLOOD_PORT  PORTC
-  #define COOLANT_FLOOD_BIT   3  // Uno Analog Pin 3
-  #define COOLANT_MIST_DDR   DDRC
-  #define COOLANT_MIST_PORT  PORTC
-  #define COOLANT_MIST_BIT   4  // Uno Analog Pin 4
-
   // Define user-control controls (cycle start, reset, feed hold) input pins.
   // NOTE: All CONTROLs pins must be on the same port and not on a port with other input pins (limits).
   #define CONTROL_DDR       DDRC
@@ -122,30 +96,156 @@
   #define PROBE_BIT       5  // Uno Analog Pin 5
   #define PROBE_MASK      (1<<PROBE_BIT)
 
-  // Variable spindle configuration below. Do not change unless you know what you are doing.
-  // NOTE: Only used when variable spindle is enabled.
-  #define SPINDLE_PWM_MAX_VALUE     255 // Don't change. 328p fast PWM mode fixes top value as 255.
-  #ifndef SPINDLE_PWM_MIN_VALUE
-    #define SPINDLE_PWM_MIN_VALUE   1   // Must be greater than zero.
+  #if !defined(ENABLE_DUAL_AXIS)
+
+    // Define flood and mist coolant enable output pins.
+    #define COOLANT_FLOOD_DDR   DDRC
+    #define COOLANT_FLOOD_PORT  PORTC
+    #define COOLANT_FLOOD_BIT   3  // Uno Analog Pin 3
+    #define COOLANT_MIST_DDR   DDRC
+    #define COOLANT_MIST_PORT  PORTC
+    #define COOLANT_MIST_BIT   4  // Uno Analog Pin 4
+
+    // Define spindle enable and spindle direction output pins.
+    #define SPINDLE_ENABLE_DDR    DDRB
+    #define SPINDLE_ENABLE_PORT   PORTB
+    // Z Limit pin and spindle PWM/enable pin swapped to access hardware PWM on Pin 11.
+    #ifdef VARIABLE_SPINDLE
+      #ifdef USE_SPINDLE_DIR_AS_ENABLE_PIN
+        // If enabled, spindle direction pin now used as spindle enable, while PWM remains on D11.
+        #define SPINDLE_ENABLE_BIT    5  // Uno Digital Pin 13 (NOTE: D13 can't be pulled-high input due to LED.)
+      #else
+        #define SPINDLE_ENABLE_BIT    3  // Uno Digital Pin 11
+      #endif
+    #else
+      #define SPINDLE_ENABLE_BIT    4  // Uno Digital Pin 12
+    #endif
+    #ifndef USE_SPINDLE_DIR_AS_ENABLE_PIN
+      #define SPINDLE_DIRECTION_DDR   DDRB
+      #define SPINDLE_DIRECTION_PORT  PORTB
+      #define SPINDLE_DIRECTION_BIT   5  // Uno Digital Pin 13 (NOTE: D13 can't be pulled-high input due to LED.)
+    #endif
+
+    // Variable spindle configuration below. Do not change unless you know what you are doing.
+    // NOTE: Only used when variable spindle is enabled.
+    #define SPINDLE_PWM_MAX_VALUE     255 // Don't change. 328p fast PWM mode fixes top value as 255.
+    #ifndef SPINDLE_PWM_MIN_VALUE
+      #define SPINDLE_PWM_MIN_VALUE   1   // Must be greater than zero.
+    #endif
+    #define SPINDLE_PWM_OFF_VALUE     0
+    #define SPINDLE_PWM_RANGE         (SPINDLE_PWM_MAX_VALUE-SPINDLE_PWM_MIN_VALUE)
+    #define SPINDLE_TCCRA_REGISTER    TCCR2A
+    #define SPINDLE_TCCRB_REGISTER    TCCR2B
+    #define SPINDLE_OCR_REGISTER      OCR2A
+    #define SPINDLE_COMB_BIT          COM2A1
+
+    // Prescaled, 8-bit Fast PWM mode.
+    #define SPINDLE_TCCRA_INIT_MASK   ((1<<WGM20) | (1<<WGM21))  // Configures fast PWM mode.
+    // #define SPINDLE_TCCRB_INIT_MASK   (1<<CS20)               // Disable prescaler -> 62.5kHz
+    // #define SPINDLE_TCCRB_INIT_MASK   (1<<CS21)               // 1/8 prescaler -> 7.8kHz (Used in v0.9)
+    // #define SPINDLE_TCCRB_INIT_MASK   ((1<<CS21) | (1<<CS20)) // 1/32 prescaler -> 1.96kHz
+    #define SPINDLE_TCCRB_INIT_MASK      (1<<CS22)               // 1/64 prescaler -> 0.98kHz (J-tech laser)
+
+    // NOTE: On the 328p, these must be the same as the SPINDLE_ENABLE settings.
+    #define SPINDLE_PWM_DDR   DDRB
+    #define SPINDLE_PWM_PORT  PORTB
+    #define SPINDLE_PWM_BIT   3    // Uno Digital Pin 11
+  
+  #else
+
+    // Dual axis feature requires an independent step pulse pin to operate. The independent direction pin is not 
+    // absolutely necessary but facilitates easy direction inverting with a Grbl $$ setting. These pins replace 
+    // the spindle direction and optional coolant mist pins.
+
+    #ifdef DUAL_AXIS_CONFIG_PROTONEER_V3_51
+      // NOTE: Step pulse and direction pins may be on any port and output pin.
+      #define STEP_DDR_DUAL       DDRC
+      #define STEP_PORT_DUAL      PORTC
+      #define DUAL_STEP_BIT       4  // Uno Analog Pin 4
+      #define STEP_MASK_DUAL      ((1<<DUAL_STEP_BIT))
+      #define DIRECTION_DDR_DUAL  DDRC
+      #define DIRECTION_PORT_DUAL PORTC
+      #define DUAL_DIRECTION_BIT  3  // Uno Analog Pin 3
+      #define DIRECTION_MASK_DUAL ((1<<DUAL_DIRECTION_BIT))
+
+      // NOTE: Dual axis limit is shared with the z-axis limit pin by default. Pin used must be on the same port
+      // as other limit pins.
+      #define DUAL_LIMIT_BIT    Z_LIMIT_BIT
+      #define LIMIT_MASK        ((1<<X_LIMIT_BIT)|(1<<Y_LIMIT_BIT)|(1<<Z_LIMIT_BIT)|(1<<DUAL_LIMIT_BIT))
+
+      // Define coolant enable output pins.
+      // NOTE: Coolant flood moved from A3 to A4. Coolant mist not supported with dual axis feature on Arduino Uno.
+      #define COOLANT_FLOOD_DDR   DDRB
+      #define COOLANT_FLOOD_PORT  PORTB
+      #define COOLANT_FLOOD_BIT   5  // Uno Digital Pin 13
+
+      // Define spindle enable output pin.
+      // NOTE: Spindle enable moved from D12 to A3 (old coolant flood enable pin). Spindle direction pin is removed.
+      #define SPINDLE_ENABLE_DDR    DDRB
+      #define SPINDLE_ENABLE_PORT   PORTB
+      #ifdef VARIABLE_SPINDLE
+        // NOTE: USE_SPINDLE_DIR_AS_ENABLE_PIN not supported with dual axis feature.
+        #define SPINDLE_ENABLE_BIT    3  // Uno Digital Pin 11
+      #else
+        #define SPINDLE_ENABLE_BIT    4  // Uno Digital Pin 12
+      #endif
+
+      // Variable spindle configuration below. Do not change unless you know what you are doing.
+      // NOTE: Only used when variable spindle is enabled.
+      #define SPINDLE_PWM_MAX_VALUE     255 // Don't change. 328p fast PWM mode fixes top value as 255.
+      #ifndef SPINDLE_PWM_MIN_VALUE
+        #define SPINDLE_PWM_MIN_VALUE   1   // Must be greater than zero.
+      #endif
+      #define SPINDLE_PWM_OFF_VALUE     0
+      #define SPINDLE_PWM_RANGE         (SPINDLE_PWM_MAX_VALUE-SPINDLE_PWM_MIN_VALUE)
+      #define SPINDLE_TCCRA_REGISTER    TCCR2A
+      #define SPINDLE_TCCRB_REGISTER    TCCR2B
+      #define SPINDLE_OCR_REGISTER      OCR2A
+      #define SPINDLE_COMB_BIT          COM2A1
+
+      // Prescaled, 8-bit Fast PWM mode.
+      #define SPINDLE_TCCRA_INIT_MASK   ((1<<WGM20) | (1<<WGM21))  // Configures fast PWM mode.
+      // #define SPINDLE_TCCRB_INIT_MASK   (1<<CS20)               // Disable prescaler -> 62.5kHz
+      // #define SPINDLE_TCCRB_INIT_MASK   (1<<CS21)               // 1/8 prescaler -> 7.8kHz (Used in v0.9)
+      // #define SPINDLE_TCCRB_INIT_MASK   ((1<<CS21) | (1<<CS20)) // 1/32 prescaler -> 1.96kHz
+      #define SPINDLE_TCCRB_INIT_MASK      (1<<CS22)               // 1/64 prescaler -> 0.98kHz (J-tech laser)
+
+      // NOTE: On the 328p, these must be the same as the SPINDLE_ENABLE settings.
+      #define SPINDLE_PWM_DDR   DDRB
+      #define SPINDLE_PWM_PORT  PORTB
+      #define SPINDLE_PWM_BIT   3    // Uno Digital Pin 11
+    #endif
+
+    // NOTE: Variable spindle not supported with this shield.
+    #ifdef DUAL_AXIS_CONFIG_CNC_SHIELD_CLONE
+      // NOTE: Step pulse and direction pins may be on any port and output pin.
+      #define STEP_DDR_DUAL       DDRB
+      #define STEP_PORT_DUAL      PORTB
+      #define DUAL_STEP_BIT       4  // Uno Digital Pin 12
+      #define STEP_MASK_DUAL      ((1<<DUAL_STEP_BIT))
+      #define DIRECTION_DDR_DUAL  DDRB
+      #define DIRECTION_PORT_DUAL PORTB
+      #define DUAL_DIRECTION_BIT  5  // Uno Digital Pin 13
+      #define DIRECTION_MASK_DUAL ((1<<DUAL_DIRECTION_BIT))
+
+      // NOTE: Dual axis limit is shared with the z-axis limit pin by default.
+      #define DUAL_LIMIT_BIT    Z_LIMIT_BIT
+      #define LIMIT_MASK        ((1<<X_LIMIT_BIT)|(1<<Y_LIMIT_BIT)|(1<<Z_LIMIT_BIT)|(1<<DUAL_LIMIT_BIT))
+
+      // Define coolant enable output pins.
+      // NOTE: Coolant flood moved from A3 to A4. Coolant mist not supported with dual axis feature on Arduino Uno.
+      #define COOLANT_FLOOD_DDR   DDRC
+      #define COOLANT_FLOOD_PORT  PORTC
+      #define COOLANT_FLOOD_BIT   4  // Uno Analog Pin 4
+
+      // Define spindle enable output pin.
+      // NOTE: Spindle enable moved from D12 to A3 (old coolant flood enable pin). Spindle direction pin is removed.
+      #define SPINDLE_ENABLE_DDR    DDRC
+      #define SPINDLE_ENABLE_PORT   PORTC
+      #define SPINDLE_ENABLE_BIT    3  // Uno Analog Pin 3
+    #endif
+
   #endif
-  #define SPINDLE_PWM_OFF_VALUE     0
-  #define SPINDLE_PWM_RANGE         (SPINDLE_PWM_MAX_VALUE-SPINDLE_PWM_MIN_VALUE)
-  #define SPINDLE_TCCRA_REGISTER	  TCCR2A
-  #define SPINDLE_TCCRB_REGISTER	  TCCR2B
-  #define SPINDLE_OCR_REGISTER      OCR2A
-  #define SPINDLE_COMB_BIT	        COM2A1
-
-  // Prescaled, 8-bit Fast PWM mode.
-  #define SPINDLE_TCCRA_INIT_MASK   ((1<<WGM20) | (1<<WGM21))  // Configures fast PWM mode.
-  // #define SPINDLE_TCCRB_INIT_MASK   (1<<CS20)               // Disable prescaler -> 62.5kHz
-  // #define SPINDLE_TCCRB_INIT_MASK   (1<<CS21)               // 1/8 prescaler -> 7.8kHz (Used in v0.9)
-  // #define SPINDLE_TCCRB_INIT_MASK   ((1<<CS21) | (1<<CS20)) // 1/32 prescaler -> 1.96kHz
-  #define SPINDLE_TCCRB_INIT_MASK      (1<<CS22)               // 1/64 prescaler -> 0.98kHz (J-tech laser)
-
-  // NOTE: On the 328p, these must be the same as the SPINDLE_ENABLE settings.
-  #define SPINDLE_PWM_DDR	  DDRB
-  #define SPINDLE_PWM_PORT  PORTB
-  #define SPINDLE_PWM_BIT	  3    // Uno Digital Pin 11
 
 #endif
 
diff --git a/grbl/defaults.h b/grbl/defaults.h
index dbbbb934..2e461300 100644
--- a/grbl/defaults.h
+++ b/grbl/defaults.h
@@ -369,6 +369,84 @@
   #define DEFAULT_HOMING_PULLOFF 1.0 // mm
 #endif
 
+#ifdef DEFAULTS_BOBSCNC_E3
+  // Grbl settings for Bob's CNC E3 Machine
+  // https://www.bobscnc.com/products/e3-cnc-engraving-kit
+  #define DEFAULT_X_STEPS_PER_MM 80.0
+  #define DEFAULT_Y_STEPS_PER_MM 80.0
+  #define DEFAULT_Z_STEPS_PER_MM 2267.717
+  #define DEFAULT_X_MAX_RATE 10000.0 // mm/min
+  #define DEFAULT_Y_MAX_RATE 10000.0 // mm/min
+  #define DEFAULT_Z_MAX_RATE 500.0 // mm/min
+  #define DEFAULT_X_ACCELERATION (500.0*60*60) // 10*60*60 mm/min^2 = 10 mm/sec^2
+  #define DEFAULT_Y_ACCELERATION (500.0*60*60) // 10*60*60 mm/min^2 = 10 mm/sec^2
+  #define DEFAULT_Z_ACCELERATION (300.0*60*60) // 10*60*60 mm/min^2 = 10 mm/sec^2
+  #define DEFAULT_X_MAX_TRAVEL 450.0 // mm NOTE: Must be a positive value.
+  #define DEFAULT_Y_MAX_TRAVEL 390.0 // mm NOTE: Must be a positive value.
+  #define DEFAULT_Z_MAX_TRAVEL 85.0 // mm NOTE: Must be a positive value.
+  #define DEFAULT_SPINDLE_RPM_MAX 1000.0 // rpm
+  #define DEFAULT_SPINDLE_RPM_MIN 0.0 // rpm
+  #define DEFAULT_STEP_PULSE_MICROSECONDS 5
+  #define DEFAULT_STEPPING_INVERT_MASK 0
+  #define DEFAULT_DIRECTION_INVERT_MASK 0
+  #define DEFAULT_STEPPER_IDLE_LOCK_TIME 25 // msec (0-254, 255 keeps steppers enabled)
+  #define DEFAULT_STATUS_REPORT_MASK 1 // MPos enabled
+  #define DEFAULT_JUNCTION_DEVIATION 0.01 // mm
+  #define DEFAULT_ARC_TOLERANCE 0.002 // mm
+  #define DEFAULT_REPORT_INCHES 1 // true
+  #define DEFAULT_INVERT_ST_ENABLE 0 // false
+  #define DEFAULT_INVERT_LIMIT_PINS 1 // true
+  #define DEFAULT_SOFT_LIMIT_ENABLE 1 // true
+  #define DEFAULT_HARD_LIMIT_ENABLE 0  // false
+  #define DEFAULT_INVERT_PROBE_PIN 0 // false
+  #define DEFAULT_LASER_MODE 0 // false
+  #define DEFAULT_HOMING_ENABLE 1  // true
+  #define DEFAULT_HOMING_DIR_MASK 3 // move xy -dir, z dir
+  #define DEFAULT_HOMING_FEED_RATE 500.0 // mm/min
+  #define DEFAULT_HOMING_SEEK_RATE 4000.0 // mm/min
+  #define DEFAULT_HOMING_DEBOUNCE_DELAY 250 // msec (0-65k)
+  #define DEFAULT_HOMING_PULLOFF 5.0 // mm
+#endif
+
+#ifdef DEFAULTS_BOBSCNC_E4
+  // Grbl settings for Bob's CNC E4 Machine
+  // https://www.bobscnc.com/products/e4-cnc-router
+  #define DEFAULT_X_STEPS_PER_MM 80.0
+  #define DEFAULT_Y_STEPS_PER_MM 80.0
+  #define DEFAULT_Z_STEPS_PER_MM 2267.717
+  #define DEFAULT_X_MAX_RATE 10000.0 // mm/min
+  #define DEFAULT_Y_MAX_RATE 10000.0 // mm/min
+  #define DEFAULT_Z_MAX_RATE 500.0 // mm/min
+  #define DEFAULT_X_ACCELERATION (500.0*60*60) // 10*60*60 mm/min^2 = 10 mm/sec^2
+  #define DEFAULT_Y_ACCELERATION (500.0*60*60) // 10*60*60 mm/min^2 = 10 mm/sec^2
+  #define DEFAULT_Z_ACCELERATION (300.0*60*60) // 10*60*60 mm/min^2 = 10 mm/sec^2
+  #define DEFAULT_X_MAX_TRAVEL 610.0 // mm NOTE: Must be a positive value.
+  #define DEFAULT_Y_MAX_TRAVEL 610.0 // mm NOTE: Must be a positive value.
+  #define DEFAULT_Z_MAX_TRAVEL 85.0 // mm NOTE: Must be a positive value.
+  #define DEFAULT_SPINDLE_RPM_MAX 1000.0 // rpm
+  #define DEFAULT_SPINDLE_RPM_MIN 0.0 // rpm
+  #define DEFAULT_STEP_PULSE_MICROSECONDS 5
+  #define DEFAULT_STEPPING_INVERT_MASK 0
+  #define DEFAULT_DIRECTION_INVERT_MASK 0
+  #define DEFAULT_STEPPER_IDLE_LOCK_TIME 25 // msec (0-254, 255 keeps steppers enabled)
+  #define DEFAULT_STATUS_REPORT_MASK 1 // MPos enabled
+  #define DEFAULT_JUNCTION_DEVIATION 0.01 // mm
+  #define DEFAULT_ARC_TOLERANCE 0.002 // mm
+  #define DEFAULT_REPORT_INCHES 1 // true
+  #define DEFAULT_INVERT_ST_ENABLE 0 // false
+  #define DEFAULT_INVERT_LIMIT_PINS 1 // true
+  #define DEFAULT_SOFT_LIMIT_ENABLE 1 // true
+  #define DEFAULT_HARD_LIMIT_ENABLE 0  // false
+  #define DEFAULT_INVERT_PROBE_PIN 0 // false
+  #define DEFAULT_LASER_MODE 0 // false
+  #define DEFAULT_HOMING_ENABLE 1  // true
+  #define DEFAULT_HOMING_DIR_MASK 3 // move xy -dir, z dir
+  #define DEFAULT_HOMING_FEED_RATE 500.0 // mm/min
+  #define DEFAULT_HOMING_SEEK_RATE 4000.0 // mm/min
+  #define DEFAULT_HOMING_DEBOUNCE_DELAY 250 // msec (0-65k)
+  #define DEFAULT_HOMING_PULLOFF 5.0 // mm
+#endif
+
 #ifdef DEFAULTS_ZEN_TOOLWORKS_7x7
   // Description: Zen Toolworks 7x7 mill with three Shinano SST43D2121 65oz-in NEMA 17 stepper motors.
   // Leadscrew is different from some ZTW kits, where most are 1.25mm/rev rather than 8.0mm/rev here.
diff --git a/grbl/grbl.h b/grbl/grbl.h
index 6a842100..89884a2d 100644
--- a/grbl/grbl.h
+++ b/grbl/grbl.h
@@ -22,8 +22,8 @@
 #define grbl_h
 
 // Grbl versioning system
-#define GRBL_VERSION "1.1g"
-#define GRBL_VERSION_BUILD "20181112"
+#define GRBL_VERSION "1.1h"
+#define GRBL_VERSION_BUILD "20190724"
 
 // Define standard libraries used by Grbl.
 #include <avr/io.h>
@@ -111,6 +111,30 @@
   #error "Override refresh must be greater than zero."
 #endif
 
+#if defined(ENABLE_DUAL_AXIS)
+  #if !((DUAL_AXIS_SELECT == X_AXIS) || (DUAL_AXIS_SELECT == Y_AXIS))
+    #error "Dual axis currently supports X or Y axes only."
+  #endif
+  #if defined(DUAL_AXIS_CONFIG_CNC_SHIELD_CLONE) && defined(VARIABLE_SPINDLE)
+    #error "VARIABLE_SPINDLE not supported with DUAL_AXIS_CNC_SHIELD_CLONE."
+  #endif
+  #if defined(DUAL_AXIS_CONFIG_CNC_SHIELD_CLONE) && defined(DUAL_AXIS_CONFIG_PROTONEER_V3_51)
+    #error "More than one dual axis configuration found. Select one."
+  #endif
+  #if !defined(DUAL_AXIS_CONFIG_CNC_SHIELD_CLONE) && !defined(DUAL_AXIS_CONFIG_PROTONEER_V3_51)
+    #error "No supported dual axis configuration found. Select one."
+  #endif
+  #if defined(COREXY)
+    #error "CORE XY not supported with dual axis feature."
+  #endif
+  #if defined(USE_SPINDLE_DIR_AS_ENABLE_PIN)
+    #error "USE_SPINDLE_DIR_AS_ENABLE_PIN not supported with dual axis feature."
+  #endif
+  #if defined(ENABLE_M7)
+    #error "ENABLE_M7 not supported with dual axis feature."
+  #endif
+#endif
+
 // ---------------------------------------------------------------------------------------
 
 #endif
diff --git a/grbl/limits.c b/grbl/limits.c
index 3cc2556b..1348c147 100644
--- a/grbl/limits.c
+++ b/grbl/limits.c
@@ -30,6 +30,14 @@
   #define HOMING_AXIS_LOCATE_SCALAR  5.0 // Must be > 1 to ensure limit switch is cleared.
 #endif
 
+#ifdef ENABLE_DUAL_AXIS
+  // Flags for dual axis async limit trigger check.
+  #define DUAL_AXIS_CHECK_DISABLE     0  // Must be zero
+  #define DUAL_AXIS_CHECK_ENABLE      bit(0)
+  #define DUAL_AXIS_CHECK_TRIGGER_1   bit(1)
+  #define DUAL_AXIS_CHECK_TRIGGER_2   bit(2)
+#endif
+
 void limits_init()
 {
   LIMIT_DDR &= ~(LIMIT_MASK); // Set as input pins
@@ -79,6 +87,9 @@ uint8_t limits_get_state()
     for (idx=0; idx<N_AXIS; idx++) {
       if (pin & get_limit_pin_mask(idx)) { limit_state |= (1 << idx); }
     }
+    #ifdef ENABLE_DUAL_AXIS
+      if (pin & (1<<DUAL_LIMIT_BIT)) { limit_state |= (1 << N_AXIS); }
+    #endif
   }
   return(limit_state);
 }
@@ -159,6 +170,20 @@ void limits_go_home(uint8_t cycle_mask)
   // Initialize variables used for homing computations.
   uint8_t n_cycle = (2*N_HOMING_LOCATE_CYCLE+1);
   uint8_t step_pin[N_AXIS];
+  #ifdef ENABLE_DUAL_AXIS
+    uint8_t step_pin_dual;
+    uint8_t dual_axis_async_check;
+    int32_t dual_trigger_position;
+    #if (DUAL_AXIS_SELECT == X_AXIS)
+      float fail_distance = (-DUAL_AXIS_HOMING_FAIL_AXIS_LENGTH_PERCENT/100.0)*settings.max_travel[Y_AXIS];
+    #else
+      float fail_distance = (-DUAL_AXIS_HOMING_FAIL_AXIS_LENGTH_PERCENT/100.0)*settings.max_travel[X_AXIS];
+    #endif
+    fail_distance = min(fail_distance, DUAL_AXIS_HOMING_FAIL_DISTANCE_MAX);
+    fail_distance = max(fail_distance, DUAL_AXIS_HOMING_FAIL_DISTANCE_MIN);
+    int32_t dual_fail_distance = trunc(fail_distance*settings.steps_per_mm[DUAL_AXIS_SELECT]);
+    // int32_t dual_fail_distance = trunc((DUAL_AXIS_HOMING_TRIGGER_FAIL_DISTANCE)*settings.steps_per_mm[DUAL_AXIS_SELECT]);
+  #endif
   float target[N_AXIS];
   float max_travel = 0.0;
   uint8_t idx;
@@ -175,6 +200,9 @@ void limits_go_home(uint8_t cycle_mask)
       max_travel = max(max_travel,(-HOMING_AXIS_SEARCH_SCALAR)*settings.max_travel[idx]);
     }
   }
+  #ifdef ENABLE_DUAL_AXIS
+    step_pin_dual = (1<<DUAL_STEP_BIT);
+  #endif
 
   // Set search mode with approach at seek rate to quickly engage the specified cycle_mask limit switches.
   bool approach = true;
@@ -187,6 +215,11 @@ void limits_go_home(uint8_t cycle_mask)
 
     // Initialize and declare variables needed for homing routine.
     axislock = 0;
+    #ifdef ENABLE_DUAL_AXIS
+      sys.homing_axis_lock_dual = 0;
+      dual_trigger_position = 0;
+      dual_axis_async_check = DUAL_AXIS_CHECK_DISABLE;
+    #endif
     n_active_axis = 0;
     for (idx=0; idx<N_AXIS; idx++) {
       // Set target location for active axes and setup computation for homing rate.
@@ -217,6 +250,9 @@ void limits_go_home(uint8_t cycle_mask)
         }
         // Apply axislock to the step port pins active in this cycle.
         axislock |= step_pin[idx];
+        #ifdef ENABLE_DUAL_AXIS
+          if (idx == DUAL_AXIS_SELECT) { sys.homing_axis_lock_dual = step_pin_dual; }
+        #endif
       }
 
     }
@@ -242,11 +278,41 @@ void limits_go_home(uint8_t cycle_mask)
                 else { axislock &= ~(step_pin[A_MOTOR]|step_pin[B_MOTOR]); }
               #else
                 axislock &= ~(step_pin[idx]);
+                #ifdef ENABLE_DUAL_AXIS
+                  if (idx == DUAL_AXIS_SELECT) { dual_axis_async_check |= DUAL_AXIS_CHECK_TRIGGER_1; }
+                #endif
               #endif
             }
           }
         }
         sys.homing_axis_lock = axislock;
+        #ifdef ENABLE_DUAL_AXIS
+          if (sys.homing_axis_lock_dual) { // NOTE: Only true when homing dual axis.
+            if (limit_state & (1 << N_AXIS)) { 
+              sys.homing_axis_lock_dual = 0;
+              dual_axis_async_check |= DUAL_AXIS_CHECK_TRIGGER_2;
+            }
+          }
+          
+          // When first dual axis limit triggers, record position and begin checking distance until other limit triggers. Bail upon failure.
+          if (dual_axis_async_check) {
+            if (dual_axis_async_check & DUAL_AXIS_CHECK_ENABLE) {
+              if (( dual_axis_async_check &  (DUAL_AXIS_CHECK_TRIGGER_1 | DUAL_AXIS_CHECK_TRIGGER_2)) == (DUAL_AXIS_CHECK_TRIGGER_1 | DUAL_AXIS_CHECK_TRIGGER_2)) {
+                dual_axis_async_check = DUAL_AXIS_CHECK_DISABLE;
+              } else {
+                if (abs(dual_trigger_position - sys_position[DUAL_AXIS_SELECT]) > dual_fail_distance) {
+                  system_set_exec_alarm(EXEC_ALARM_HOMING_FAIL_DUAL_APPROACH);
+                  mc_reset();
+                  protocol_execute_realtime();
+                  return;
+                }
+              }
+            } else {
+              dual_axis_async_check |= DUAL_AXIS_CHECK_ENABLE;
+              dual_trigger_position = sys_position[DUAL_AXIS_SELECT];
+            }
+          }
+        #endif
       }
 
       st_prep_buffer(); // Check and prep segment buffer. NOTE: Should take no longer than 200us.
@@ -273,7 +339,11 @@ void limits_go_home(uint8_t cycle_mask)
         }
       }
 
-    } while (STEP_MASK & axislock);
+    #ifdef ENABLE_DUAL_AXIS
+      } while ((STEP_MASK & axislock) || (sys.homing_axis_lock_dual));
+    #else
+      } while (STEP_MASK & axislock);
+    #endif
 
     st_reset(); // Immediately force kill steppers and reset step segment buffer.
     delay_ms(settings.homing_debounce_delay); // Delay to allow transient dynamics to dissipate.
diff --git a/grbl/planner.c b/grbl/planner.c
index 4839bbf4..49ff7229 100644
--- a/grbl/planner.c
+++ b/grbl/planner.c
@@ -456,7 +456,7 @@ uint8_t plan_buffer_line(float *target, plan_line_data_t *pl_data)
     float nominal_speed = plan_compute_profile_nominal_speed(block);
     plan_compute_profile_parameters(block, nominal_speed, pl.previous_nominal_speed);
     pl.previous_nominal_speed = nominal_speed;
-
+    
     // Update previous path unit_vector and planner position.
     memcpy(pl.previous_unit_vec, unit_vec, sizeof(unit_vec)); // pl.previous_unit_vec[] = unit_vec[]
     memcpy(pl.position, target_steps, sizeof(target_steps)); // pl.position[] = target_steps[]
diff --git a/grbl/report.c b/grbl/report.c
index 3e78b471..666608a3 100644
--- a/grbl/report.c
+++ b/grbl/report.c
@@ -436,6 +436,9 @@ void report_build_info(char *line)
   #ifndef FORCE_BUFFER_SYNC_DURING_WCO_CHANGE // NOTE: Shown when disabled.
     serial_write('W');
   #endif
+  #ifdef ENABLE_DUAL_AXIS
+    serial_write('2');
+  #endif
   // NOTE: Compiled values, like override increments/max/min values, may be added at some point later.
   serial_write(',');
   print_uint8_base10(BLOCK_BUFFER_SIZE-1);
@@ -569,9 +572,21 @@ void report_realtime_status()
       printPgmString(PSTR("|Pn:"));
       if (prb_pin_state) { serial_write('P'); }
       if (lim_pin_state) {
-        if (bit_istrue(lim_pin_state,bit(X_AXIS))) { serial_write('X'); }
-        if (bit_istrue(lim_pin_state,bit(Y_AXIS))) { serial_write('Y'); }
-        if (bit_istrue(lim_pin_state,bit(Z_AXIS))) { serial_write('Z'); }
+        #ifdef ENABLE_DUAL_AXIS
+          #if (DUAL_AXIS_SELECT == X_AXIS)
+            if (bit_istrue(lim_pin_state,(bit(X_AXIS)|bit(N_AXIS)))) { serial_write('X'); }
+            if (bit_istrue(lim_pin_state,bit(Y_AXIS))) { serial_write('Y'); }
+          #endif
+          #if (DUAL_AXIS_SELECT == Y_AXIS)
+            if (bit_istrue(lim_pin_state,bit(X_AXIS))) { serial_write('X'); 
+            if (bit_istrue(lim_pin_state,(bit(Y_AXIS)|bit(N_AXIS)))) { serial_write('Y'); }
+          #endif
+          if (bit_istrue(lim_pin_state,bit(Z_AXIS))) { serial_write('Z'); }
+        #else
+          if (bit_istrue(lim_pin_state,bit(X_AXIS))) { serial_write('X'); }
+          if (bit_istrue(lim_pin_state,bit(Y_AXIS))) { serial_write('Y'); }
+          if (bit_istrue(lim_pin_state,bit(Z_AXIS))) { serial_write('Z'); }
+        #endif
       }
       if (ctrl_pin_state) {
         #ifdef ENABLE_SAFETY_DOOR_INPUT_PIN
diff --git a/grbl/spindle_control.c b/grbl/spindle_control.c
index 61cb90b8..550b7520 100644
--- a/grbl/spindle_control.c
+++ b/grbl/spindle_control.c
@@ -30,7 +30,6 @@
 void spindle_init()
 {
   #ifdef VARIABLE_SPINDLE
-
     // Configure variable spindle PWM and enable pin, if requried. On the Uno, PWM and enable are
     // combined unless configured otherwise.
     SPINDLE_PWM_DDR |= (1<<SPINDLE_PWM_BIT); // Configure as PWM output pin.
@@ -39,17 +38,16 @@ void spindle_init()
     #ifdef USE_SPINDLE_DIR_AS_ENABLE_PIN
       SPINDLE_ENABLE_DDR |= (1<<SPINDLE_ENABLE_BIT); // Configure as output pin.
     #else
-      SPINDLE_DIRECTION_DDR |= (1<<SPINDLE_DIRECTION_BIT); // Configure as output pin.
+      #ifndef ENABLE_DUAL_AXIS
+        SPINDLE_DIRECTION_DDR |= (1<<SPINDLE_DIRECTION_BIT); // Configure as output pin.
+      #endif
     #endif
-
     pwm_gradient = SPINDLE_PWM_RANGE/(settings.rpm_max-settings.rpm_min);
-
   #else
-
-    // Configure no variable spindle and only enable pin.
     SPINDLE_ENABLE_DDR |= (1<<SPINDLE_ENABLE_BIT); // Configure as output pin.
-    SPINDLE_DIRECTION_DDR |= (1<<SPINDLE_DIRECTION_BIT); // Configure as output pin.
-
+    #ifndef ENABLE_DUAL_AXIS
+      SPINDLE_DIRECTION_DDR |= (1<<SPINDLE_DIRECTION_BIT); // Configure as output pin.
+    #endif
   #endif
 
   spindle_stop();
@@ -58,31 +56,39 @@ void spindle_init()
 
 uint8_t spindle_get_state()
 {
-	#ifdef VARIABLE_SPINDLE
+  #ifdef VARIABLE_SPINDLE
     #ifdef USE_SPINDLE_DIR_AS_ENABLE_PIN
-		  // No spindle direction output pin. 
-			#ifdef INVERT_SPINDLE_ENABLE_PIN
-			  if (bit_isfalse(SPINDLE_ENABLE_PORT,(1<<SPINDLE_ENABLE_BIT))) { return(SPINDLE_STATE_CW); }
-	    #else
-	 			if (bit_istrue(SPINDLE_ENABLE_PORT,(1<<SPINDLE_ENABLE_BIT))) { return(SPINDLE_STATE_CW); }
-	    #endif
+      // No spindle direction output pin. 
+      #ifdef INVERT_SPINDLE_ENABLE_PIN
+        if (bit_isfalse(SPINDLE_ENABLE_PORT,(1<<SPINDLE_ENABLE_BIT))) { return(SPINDLE_STATE_CW); }
+      #else
+        if (bit_istrue(SPINDLE_ENABLE_PORT,(1<<SPINDLE_ENABLE_BIT))) { return(SPINDLE_STATE_CW); }
+      #endif
     #else
       if (SPINDLE_TCCRA_REGISTER & (1<<SPINDLE_COMB_BIT)) { // Check if PWM is enabled.
-        if (SPINDLE_DIRECTION_PORT & (1<<SPINDLE_DIRECTION_BIT)) { return(SPINDLE_STATE_CCW); }
-        else { return(SPINDLE_STATE_CW); }
+        #ifdef ENABLE_DUAL_AXIS
+          return(SPINDLE_STATE_CW);
+        #else
+          if (SPINDLE_DIRECTION_PORT & (1<<SPINDLE_DIRECTION_BIT)) { return(SPINDLE_STATE_CCW); }
+          else { return(SPINDLE_STATE_CW); }
+        #endif
       }
     #endif
-	#else
-		#ifdef INVERT_SPINDLE_ENABLE_PIN
-		  if (bit_isfalse(SPINDLE_ENABLE_PORT,(1<<SPINDLE_ENABLE_BIT))) { 
-		#else
-		  if (bit_istrue(SPINDLE_ENABLE_PORT,(1<<SPINDLE_ENABLE_BIT))) {
-		#endif
-      if (SPINDLE_DIRECTION_PORT & (1<<SPINDLE_DIRECTION_BIT)) { return(SPINDLE_STATE_CCW); }
-      else { return(SPINDLE_STATE_CW); }
+  #else
+    #ifdef INVERT_SPINDLE_ENABLE_PIN
+      if (bit_isfalse(SPINDLE_ENABLE_PORT,(1<<SPINDLE_ENABLE_BIT))) { 
+    #else
+      if (bit_istrue(SPINDLE_ENABLE_PORT,(1<<SPINDLE_ENABLE_BIT))) {
+    #endif
+      #ifdef ENABLE_DUAL_AXIS    
+        return(SPINDLE_STATE_CW);
+      #else
+        if (SPINDLE_DIRECTION_PORT & (1<<SPINDLE_DIRECTION_BIT)) { return(SPINDLE_STATE_CCW); }
+        else { return(SPINDLE_STATE_CW); }
+      #endif
     }
-	#endif
-	return(SPINDLE_STATE_DISABLE);
+  #endif
+  return(SPINDLE_STATE_DISABLE);
 }
 
 
@@ -223,6 +229,7 @@ void spindle_stop()
 #endif
 {
   if (sys.abort) { return; } // Block during abort.
+
   if (state == SPINDLE_DISABLE) { // Halt or set spindle direction and rpm.
   
     #ifdef VARIABLE_SPINDLE
@@ -231,8 +238,8 @@ void spindle_stop()
     spindle_stop();
   
   } else {
-  
-    #ifndef USE_SPINDLE_DIR_AS_ENABLE_PIN
+    
+    #if !defined(USE_SPINDLE_DIR_AS_ENABLE_PIN) && !defined(ENABLE_DUAL_AXIS)
       if (state == SPINDLE_ENABLE_CW) {
         SPINDLE_DIRECTION_PORT &= ~(1<<SPINDLE_DIRECTION_BIT);
       } else {
diff --git a/grbl/stepper.c b/grbl/stepper.c
index 3bfccd49..8e0b151d 100644
--- a/grbl/stepper.c
+++ b/grbl/stepper.c
@@ -67,6 +67,9 @@ typedef struct {
   uint32_t steps[N_AXIS];
   uint32_t step_event_count;
   uint8_t direction_bits;
+  #ifdef ENABLE_DUAL_AXIS
+    uint8_t direction_bits_dual;
+  #endif
   #ifdef VARIABLE_SPINDLE
     uint8_t is_pwm_rate_adjusted; // Tracks motions that require constant laser power/rate
   #endif
@@ -106,6 +109,10 @@ typedef struct {
   uint8_t step_pulse_time;  // Step pulse reset time after step rise
   uint8_t step_outbits;         // The next stepping-bits to be output
   uint8_t dir_outbits;
+  #ifdef ENABLE_DUAL_AXIS
+    uint8_t step_outbits_dual;
+    uint8_t dir_outbits_dual;
+  #endif
   #ifdef ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING
     uint32_t steps[N_AXIS];
   #endif
@@ -125,6 +132,10 @@ static uint8_t segment_next_head;
 // Step and direction port invert masks.
 static uint8_t step_port_invert_mask;
 static uint8_t dir_port_invert_mask;
+#ifdef ENABLE_DUAL_AXIS
+  static uint8_t step_port_invert_mask_dual;
+  static uint8_t dir_port_invert_mask_dual;
+#endif
 
 // Used to avoid ISR nesting of the "Stepper Driver Interrupt". Should never occur though.
 static volatile uint8_t busy;
@@ -311,12 +322,21 @@ ISR(TIMER1_COMPA_vect)
 
   // Set the direction pins a couple of nanoseconds before we step the steppers
   DIRECTION_PORT = (DIRECTION_PORT & ~DIRECTION_MASK) | (st.dir_outbits & DIRECTION_MASK);
+  #ifdef ENABLE_DUAL_AXIS
+    DIRECTION_PORT_DUAL = (DIRECTION_PORT_DUAL & ~DIRECTION_MASK_DUAL) | (st.dir_outbits_dual & DIRECTION_MASK_DUAL);
+  #endif
 
   // Then pulse the stepping pins
   #ifdef STEP_PULSE_DELAY
     st.step_bits = (STEP_PORT & ~STEP_MASK) | st.step_outbits; // Store out_bits to prevent overwriting.
+    #ifdef ENABLE_DUAL_AXIS
+      st.step_bits_dual = (STEP_PORT_DUAL & ~STEP_MASK_DUAL) | st.step_outbits_dual;
+    #endif
   #else  // Normal operation
     STEP_PORT = (STEP_PORT & ~STEP_MASK) | st.step_outbits;
+    #ifdef ENABLE_DUAL_AXIS
+      STEP_PORT_DUAL = (STEP_PORT_DUAL & ~STEP_MASK_DUAL) | st.step_outbits_dual;
+    #endif
   #endif
 
   // Enable step pulse reset timer so that The Stepper Port Reset Interrupt can reset the signal after
@@ -353,6 +373,9 @@ ISR(TIMER1_COMPA_vect)
         st.counter_x = st.counter_y = st.counter_z = (st.exec_block->step_event_count >> 1);
       }
       st.dir_outbits = st.exec_block->direction_bits ^ dir_port_invert_mask;
+      #ifdef ENABLE_DUAL_AXIS
+        st.dir_outbits_dual = st.exec_block->direction_bits_dual ^ dir_port_invert_mask_dual;
+      #endif
 
       #ifdef ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING
         // With AMASS enabled, adjust Bresenham axis increment counters according to AMASS level.
@@ -384,6 +407,9 @@ ISR(TIMER1_COMPA_vect)
 
   // Reset step out bits.
   st.step_outbits = 0;
+  #ifdef ENABLE_DUAL_AXIS
+    st.step_outbits_dual = 0;
+  #endif
 
   // Execute step displacement profile by Bresenham line algorithm
   #ifdef ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING
@@ -393,6 +419,9 @@ ISR(TIMER1_COMPA_vect)
   #endif
   if (st.counter_x > st.exec_block->step_event_count) {
     st.step_outbits |= (1<<X_STEP_BIT);
+    #if defined(ENABLE_DUAL_AXIS) && (DUAL_AXIS_SELECT == X_AXIS)
+      st.step_outbits_dual = (1<<DUAL_STEP_BIT);
+    #endif
     st.counter_x -= st.exec_block->step_event_count;
     if (st.exec_block->direction_bits & (1<<X_DIRECTION_BIT)) { sys_position[X_AXIS]--; }
     else { sys_position[X_AXIS]++; }
@@ -404,6 +433,9 @@ ISR(TIMER1_COMPA_vect)
   #endif
   if (st.counter_y > st.exec_block->step_event_count) {
     st.step_outbits |= (1<<Y_STEP_BIT);
+    #if defined(ENABLE_DUAL_AXIS) && (DUAL_AXIS_SELECT == Y_AXIS)
+      st.step_outbits_dual = (1<<DUAL_STEP_BIT);
+    #endif
     st.counter_y -= st.exec_block->step_event_count;
     if (st.exec_block->direction_bits & (1<<Y_DIRECTION_BIT)) { sys_position[Y_AXIS]--; }
     else { sys_position[Y_AXIS]++; }
@@ -421,7 +453,12 @@ ISR(TIMER1_COMPA_vect)
   }
 
   // During a homing cycle, lock out and prevent desired axes from moving.
-  if (sys.state == STATE_HOMING) { st.step_outbits &= sys.homing_axis_lock; }
+  if (sys.state == STATE_HOMING) { 
+    st.step_outbits &= sys.homing_axis_lock;
+    #ifdef ENABLE_DUAL_AXIS
+      st.step_outbits_dual &= sys.homing_axis_lock_dual;
+    #endif
+  }
 
   st.step_count--; // Decrement step events count
   if (st.step_count == 0) {
@@ -431,6 +468,9 @@ ISR(TIMER1_COMPA_vect)
   }
 
   st.step_outbits ^= step_port_invert_mask;  // Apply step port invert mask
+  #ifdef ENABLE_DUAL_AXIS
+    st.step_outbits_dual ^= step_port_invert_mask_dual;
+  #endif
   busy = false;
 }
 
@@ -450,6 +490,9 @@ ISR(TIMER0_OVF_vect)
 {
   // Reset stepping pins (leave the direction pins)
   STEP_PORT = (STEP_PORT & ~STEP_MASK) | (step_port_invert_mask & STEP_MASK);
+  #ifdef ENABLE_DUAL_AXIS
+    STEP_PORT_DUAL = (STEP_PORT_DUAL & ~STEP_MASK_DUAL) | (step_port_invert_mask_dual & STEP_MASK_DUAL);
+  #endif
   TCCR0B = 0; // Disable Timer0 to prevent re-entering this interrupt when it's not needed.
 }
 #ifdef STEP_PULSE_DELAY
@@ -461,6 +504,9 @@ ISR(TIMER0_OVF_vect)
   ISR(TIMER0_COMPA_vect)
   {
     STEP_PORT = st.step_bits; // Begin step pulse.
+    #ifdef ENABLE_DUAL_AXIS
+      STEP_PORT_DUAL = st.step_bits_dual;
+    #endif
   }
 #endif
 
@@ -475,6 +521,13 @@ void st_generate_step_dir_invert_masks()
     if (bit_istrue(settings.step_invert_mask,bit(idx))) { step_port_invert_mask |= get_step_pin_mask(idx); }
     if (bit_istrue(settings.dir_invert_mask,bit(idx))) { dir_port_invert_mask |= get_direction_pin_mask(idx); }
   }
+  #ifdef ENABLE_DUAL_AXIS
+    step_port_invert_mask_dual = 0;
+    dir_port_invert_mask_dual = 0;
+    // NOTE: Dual axis invert uses the N_AXIS bit to set step and direction invert pins.    
+    if (bit_istrue(settings.step_invert_mask,bit(N_AXIS))) { step_port_invert_mask_dual = (1<<DUAL_STEP_BIT); }
+    if (bit_istrue(settings.dir_invert_mask,bit(N_AXIS))) { dir_port_invert_mask_dual = (1<<DUAL_DIRECTION_BIT); }
+  #endif
 }
 
 
@@ -500,6 +553,12 @@ void st_reset()
   // Initialize step and direction port pins.
   STEP_PORT = (STEP_PORT & ~STEP_MASK) | step_port_invert_mask;
   DIRECTION_PORT = (DIRECTION_PORT & ~DIRECTION_MASK) | dir_port_invert_mask;
+  
+  #ifdef ENABLE_DUAL_AXIS
+    st.dir_outbits_dual = dir_port_invert_mask_dual;
+    STEP_PORT_DUAL = (STEP_PORT_DUAL & ~STEP_MASK_DUAL) | step_port_invert_mask_dual;
+    DIRECTION_PORT_DUAL = (DIRECTION_PORT_DUAL & ~DIRECTION_MASK_DUAL) | dir_port_invert_mask_dual;
+  #endif
 }
 
 
@@ -510,6 +569,11 @@ void stepper_init()
   STEP_DDR |= STEP_MASK;
   STEPPERS_DISABLE_DDR |= 1<<STEPPERS_DISABLE_BIT;
   DIRECTION_DDR |= DIRECTION_MASK;
+  
+  #ifdef ENABLE_DUAL_AXIS
+    STEP_DDR_DUAL |= STEP_MASK_DUAL;
+    DIRECTION_DDR_DUAL |= DIRECTION_MASK_DUAL;
+  #endif
 
   // Configure Timer 1: Stepper Driver Interrupt
   TCCR1B &= ~(1<<WGM13); // waveform generation = 0100 = CTC
@@ -636,6 +700,15 @@ void st_prep_buffer()
         // segment buffer finishes the prepped block, but the stepper ISR is still executing it.
         st_prep_block = &st_block_buffer[prep.st_block_index];
         st_prep_block->direction_bits = pl_block->direction_bits;
+        #ifdef ENABLE_DUAL_AXIS
+          #if (DUAL_AXIS_SELECT == X_AXIS)
+            if (st_prep_block->direction_bits & (1<<X_DIRECTION_BIT)) { 
+          #elif (DUAL_AXIS_SELECT == Y_AXIS)
+            if (st_prep_block->direction_bits & (1<<Y_DIRECTION_BIT)) { 
+          #endif
+            st_prep_block->direction_bits_dual = (1<<DUAL_DIRECTION_BIT); 
+          }  else { st_prep_block->direction_bits_dual = 0; }
+        #endif
         uint8_t idx;
         #ifndef ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING
           for (idx=0; idx<N_AXIS; idx++) { st_prep_block->steps[idx] = (pl_block->steps[idx] << 1); }
diff --git a/grbl/system.h b/grbl/system.h
index 4d4f6128..cfc92738 100644
--- a/grbl/system.h
+++ b/grbl/system.h
@@ -38,15 +38,16 @@
 #define EXEC_SLEEP          bit(7) // bitmask 10000000
 
 // Alarm executor codes. Valid values (1-255). Zero is reserved.
-#define EXEC_ALARM_HARD_LIMIT           1
-#define EXEC_ALARM_SOFT_LIMIT           2
-#define EXEC_ALARM_ABORT_CYCLE          3
-#define EXEC_ALARM_PROBE_FAIL_INITIAL   4
-#define EXEC_ALARM_PROBE_FAIL_CONTACT   5
-#define EXEC_ALARM_HOMING_FAIL_RESET    6
-#define EXEC_ALARM_HOMING_FAIL_DOOR     7
-#define EXEC_ALARM_HOMING_FAIL_PULLOFF  8
-#define EXEC_ALARM_HOMING_FAIL_APPROACH 9
+#define EXEC_ALARM_HARD_LIMIT                 1
+#define EXEC_ALARM_SOFT_LIMIT                 2
+#define EXEC_ALARM_ABORT_CYCLE                3
+#define EXEC_ALARM_PROBE_FAIL_INITIAL         4
+#define EXEC_ALARM_PROBE_FAIL_CONTACT         5
+#define EXEC_ALARM_HOMING_FAIL_RESET          6
+#define EXEC_ALARM_HOMING_FAIL_DOOR           7
+#define EXEC_ALARM_HOMING_FAIL_PULLOFF        8
+#define EXEC_ALARM_HOMING_FAIL_APPROACH       9
+#define EXEC_ALARM_HOMING_FAIL_DUAL_APPROACH  10
 
 // Override bit maps. Realtime bitflags to control feed, rapid, spindle, and coolant overrides.
 // Spindle/coolant and feed/rapids are separated into two controlling flag variables.
@@ -131,6 +132,9 @@ typedef struct {
   uint8_t step_control;        // Governs the step segment generator depending on system state.
   uint8_t probe_succeeded;     // Tracks if last probing cycle was successful.
   uint8_t homing_axis_lock;    // Locks axes when limits engage. Used as an axis motion mask in the stepper ISR.
+  #ifdef ENABLE_DUAL_AXIS
+    uint8_t homing_axis_lock_dual;
+  #endif
   uint8_t f_override;          // Feed rate override value in percent
   uint8_t r_override;          // Rapids override value in percent
   uint8_t spindle_speed_ovr;   // Spindle speed value in percent