diff --git a/config.h.dist b/config.h.dist
index 6b434f9..98f9fc0 100644
--- a/config.h.dist
+++ b/config.h.dist
@@ -183,6 +183,11 @@
 
 #define	STEPPER_ENABLE_PIN		DIO9
 
+/*
+	If your endstops are inverted, change this to a zero
+*/
+#define ENDSTOP_OK_VALUE 1
+
 /*
  *	list of PWM-able pins and corresponding timers
  *	timer1 is used for step timing so don't use OC1A/OC1B
@@ -226,6 +231,14 @@
 #define	FAN_PIN								DIO5
 #define	FAN_PWM								OCR0B
 
+/*
+	Define this to disable PID and use bang-bang instead.
+	If you do this, you must also comment out "HEATER_PWM" above.
+	Otherwise it won't work correctly. Using bang-bang takes MUCH less
+	space.
+*/
+//#define BANG_BANG
+
 // --------------------------------------------------------------------------
 // you shouldn't need to edit anything below this line
 
diff --git a/gcode_process.c b/gcode_process.c
index add0d34..74e54d1 100644
--- a/gcode_process.c
+++ b/gcode_process.c
@@ -264,6 +264,7 @@ void process_gcode_command() {
 				break;
 
 				#ifdef	HEATER_PIN
+				#ifndef BANG_BANG
 				// M130- heater P factor
 			case 130:
 				if (next_target.seen_S)
@@ -288,6 +289,7 @@ void process_gcode_command() {
 			case 134:
 				heater_save_settings();
 				break;
+				#endif  /* BANG_BANG */
 				#endif	/* HEATER_PIN */
 				
 				// M190- power on
diff --git a/heater.c b/heater.c
index 819896a..c4bd7ce 100644
--- a/heater.c
+++ b/heater.c
@@ -1,6 +1,7 @@
 #include	"heater.h"
 
 #ifdef HEATER_PIN
+#ifndef BANG_BANG
 
 #ifndef SIMULATION
 	#include	<avr/eeprom.h>
@@ -105,5 +106,12 @@ void heater_tick(int16_t current_temp, int16_t target_temp) {
 		disable_heater();
 	#endif
 }
-
-#endif	/* HEATER_PIN */
\ No newline at end of file
+#else /* BANG_BANG */
+void heater_tick(int16_t current_temp, int16_t target_temp) {
+	if ( current_temp < target_temp )
+		enable_heater();
+	else
+		disable_heater();
+}
+#endif /* BANG_BANG */
+#endif	/* HEATER_PIN */
diff --git a/mendel.c b/mendel.c
index cd54e8b..0520f58 100644
--- a/mendel.c
+++ b/mendel.c
@@ -36,15 +36,54 @@ void io_init(void) {
 	// setup I/O pins
 	WRITE(X_STEP_PIN, 0);	SET_OUTPUT(X_STEP_PIN);
 	WRITE(X_DIR_PIN,  0);	SET_OUTPUT(X_DIR_PIN);
-	WRITE(X_MIN_PIN,  1);	SET_INPUT(X_MIN_PIN);
+	#ifdef X_MIN_PIN
+		#if ENDSTOP_OK_VALUE == 0
+			WRITE(X_MIN_PIN,  0);	SET_INPUT(X_MIN_PIN);
+		#else
+			WRITE(X_MIN_PIN,  1);	SET_INPUT(X_MIN_PIN);
+		#endif
+	#endif
+	#ifdef X_MAX_PIN
+		#if ENDSTOP_OK_VALUE == 0
+			WRITE(X_MAX_PIN,  0);	SET_INPUT(X_MAX_PIN);
+		#else
+			WRITE(X_MAX_PIN,  1);	SET_INPUT(X_MAX_PIN);
+		#endif
+	#endif
 
 	WRITE(Y_STEP_PIN, 0);	SET_OUTPUT(Y_STEP_PIN);
 	WRITE(Y_DIR_PIN,  0);	SET_OUTPUT(Y_DIR_PIN);
-	WRITE(Y_MIN_PIN,  1);	SET_INPUT(Y_MIN_PIN);
+	#ifdef Y_MIN_PIN
+		#if ENDSTOP_OK_VALUE == 0
+			WRITE(Y_MIN_PIN,  0);	SET_INPUT(Y_MIN_PIN);
+		#else
+			WRITE(Y_MIN_PIN,  1);	SET_INPUT(Y_MIN_PIN);
+		#endif
+	#endif
+	#ifdef Y_MAX_PIN
+		#if ENDSTOP_OK_VALUE == 0
+			WRITE(Y_MAX_PIN,  0);	SET_INPUT(Y_MAX_PIN);
+		#else
+			WRITE(Y_MAX_PIN,  1);	SET_INPUT(Y_MAX_PIN);
+		#endif
+	#endif
 
 	WRITE(Z_STEP_PIN, 0);	SET_OUTPUT(Z_STEP_PIN);
 	WRITE(Z_DIR_PIN,  0);	SET_OUTPUT(Z_DIR_PIN);
-	WRITE(Z_MIN_PIN,  1);	SET_INPUT(Z_MIN_PIN);
+	#ifdef Z_MIN_PIN
+		#if ENDSTOP_OK_VALUE == 0
+			WRITE(Z_MIN_PIN,  0);	SET_INPUT(Z_MIN_PIN);
+		#else
+			WRITE(Z_MIN_PIN,  1);	SET_INPUT(Z_MIN_PIN);
+		#endif
+	#endif
+	#ifdef Z_MAX_PIN
+		#if ENDSTOP_OK_VALUE == 0
+			WRITE(Z_MAX_PIN,  0);	SET_INPUT(Z_MAX_PIN);
+		#else
+			WRITE(Z_MAX_PIN,  1);	SET_INPUT(Z_MAX_PIN);
+		#endif
+	#endif
 
 	WRITE(E_STEP_PIN, 0);	SET_OUTPUT(E_STEP_PIN);
 	WRITE(E_DIR_PIN,  0);	SET_OUTPUT(E_DIR_PIN);
@@ -95,7 +134,11 @@ void init(void) {
 	clock_setup();
 
 	// read PID settings from EEPROM
+	#ifdef HEATER_PIN
+	#ifndef BANG_BANG
 	heater_init();
+	#endif
+	#endif
 
 	// set up default feedrate
 	current_position.F = startpoint.F = next_target.target.F = SEARCH_FEEDRATE_Z;