diff --git a/.travis.yml b/.travis.yml
index 728281e5ac..251a6212b6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -205,10 +205,11 @@ script:
   - opt_enable ULTIMAKERCONTROLLER FILAMENT_LCD_DISPLAY
   - build_marlin
   #
-  # Enable BEZIER_CURVE_SUPPORT
+  # Enable BEZIER_CURVE_SUPPORT, EXPERIMENTAL_I2CBUS, and I2C_SLAVE_ADDRESS
   #
   - restore_configs
-  - opt_enable_adv BEZIER_CURVE_SUPPORT
+  - opt_enable_adv BEZIER_CURVE_SUPPORT EXPERIMENTAL_I2CBUS
+  - opt_set_adv I2C_SLAVE_ADDRESS 1
   - build_marlin
   #
   # Enable COREXY
diff --git a/Marlin/Configuration_adv.h b/Marlin/Configuration_adv.h
index a822c83b11..2def9b3861 100644
--- a/Marlin/Configuration_adv.h
+++ b/Marlin/Configuration_adv.h
@@ -795,5 +795,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/Marlin_main.cpp b/Marlin/Marlin_main.cpp
index 30b80489d8..ac4019322a 100644
--- a/Marlin/Marlin_main.cpp
+++ b/Marlin/Marlin_main.cpp
@@ -835,6 +835,14 @@ void servo_init() {
   void enableStepperDrivers() { pinMode(STEPPER_RESET_PIN, INPUT); }  // set to input, which allows it to be pulled high by pullups
 #endif
 
+#if ENABLED(EXPERIMENTAL_I2CBUS) && I2C_SLAVE_ADDRESS > 0
+
+  void i2c_listener(int bytes) {
+    i2c.receive(bytes);        // just echo all bytes received to serial
+  }
+
+#endif
+
 /**
  * Marlin entry-point: Set up before the program loop
  *  - Set up the kill pin, filament runout, power hold
@@ -981,6 +989,10 @@ void setup() {
       for (uint8_t i = 0; i < MIXING_STEPPERS; i++)
         mixing_virtual_tool_mix[t][i] = mixing_factor[i];
   #endif
+
+  #if ENABLED(EXPERIMENTAL_I2CBUS) && I2C_SLAVE_ADDRESS > 0
+    i2c.onReceive(i2c_listener);
+  #endif
 }
 
 /**
@@ -5246,15 +5258,13 @@ inline void gcode_M121() { endstops.enable_globally(false); }
    */
   inline void gcode_M155() {
     // Set the target address
-    if (code_seen('A'))
-      i2c.address(code_value_byte());
+    if (code_seen('A')) i2c.address(code_value_byte());
 
     // Add a new byte to the buffer
-    else if (code_seen('B'))
-      i2c.addbyte(code_value_int());
+    if (code_seen('B')) i2c.addbyte(code_value_byte());
 
     // Flush the buffer to the bus
-    else if (code_seen('S')) i2c.send();
+    if (code_seen('S')) i2c.send();
 
     // Reset and rewind the buffer
     else if (code_seen('R')) i2c.reset();
@@ -5266,11 +5276,11 @@ inline void gcode_M121() { endstops.enable_globally(false); }
    * Usage: M156 A<slave device address base 10> B<number of bytes>
    */
   inline void gcode_M156() {
-    uint8_t addr = code_seen('A') ? code_value_byte() : 0;
-    int bytes    = code_seen('B') ? code_value_int() : 1;
+    if (code_seen('A')) i2c.address(code_value_byte());
 
-    if (addr && bytes > 0 && bytes <= 32) {
-      i2c.address(addr);
+    uint8_t bytes = code_seen('B') ? code_value_byte() : 1;
+
+    if (i2c.addr > 0 && bytes > 0 && bytes <= 32) {
       i2c.reqbytes(bytes);
     }
     else {
diff --git a/Marlin/example_configurations/Cartesio/Configuration_adv.h b/Marlin/example_configurations/Cartesio/Configuration_adv.h
index 3e10c3f85f..444fc7b2de 100644
--- a/Marlin/example_configurations/Cartesio/Configuration_adv.h
+++ b/Marlin/example_configurations/Cartesio/Configuration_adv.h
@@ -795,5 +795,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/Felix/Configuration_adv.h b/Marlin/example_configurations/Felix/Configuration_adv.h
index 5700735795..6f4d1139c3 100644
--- a/Marlin/example_configurations/Felix/Configuration_adv.h
+++ b/Marlin/example_configurations/Felix/Configuration_adv.h
@@ -795,5 +795,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/Hephestos/Configuration_adv.h b/Marlin/example_configurations/Hephestos/Configuration_adv.h
index b719e60727..d26e2207b0 100644
--- a/Marlin/example_configurations/Hephestos/Configuration_adv.h
+++ b/Marlin/example_configurations/Hephestos/Configuration_adv.h
@@ -795,5 +795,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/Hephestos_2/Configuration_adv.h b/Marlin/example_configurations/Hephestos_2/Configuration_adv.h
index eeb11da2d8..4b771d5d5e 100644
--- a/Marlin/example_configurations/Hephestos_2/Configuration_adv.h
+++ b/Marlin/example_configurations/Hephestos_2/Configuration_adv.h
@@ -795,5 +795,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/K8200/Configuration_adv.h b/Marlin/example_configurations/K8200/Configuration_adv.h
index dc26fd6039..92a81b22cf 100644
--- a/Marlin/example_configurations/K8200/Configuration_adv.h
+++ b/Marlin/example_configurations/K8200/Configuration_adv.h
@@ -801,5 +801,6 @@ const unsigned int dropsegments = 2; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/K8400/Configuration_adv.h b/Marlin/example_configurations/K8400/Configuration_adv.h
index 64efac9848..dde4547f4b 100644
--- a/Marlin/example_configurations/K8400/Configuration_adv.h
+++ b/Marlin/example_configurations/K8400/Configuration_adv.h
@@ -795,5 +795,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/RigidBot/Configuration_adv.h b/Marlin/example_configurations/RigidBot/Configuration_adv.h
index 2118f83d53..d077ae28e2 100644
--- a/Marlin/example_configurations/RigidBot/Configuration_adv.h
+++ b/Marlin/example_configurations/RigidBot/Configuration_adv.h
@@ -795,5 +795,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/SCARA/Configuration_adv.h b/Marlin/example_configurations/SCARA/Configuration_adv.h
index 07f1e56543..bf4d323510 100644
--- a/Marlin/example_configurations/SCARA/Configuration_adv.h
+++ b/Marlin/example_configurations/SCARA/Configuration_adv.h
@@ -795,5 +795,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/TAZ4/Configuration_adv.h b/Marlin/example_configurations/TAZ4/Configuration_adv.h
index d0d98fb9f1..1417c803d2 100644
--- a/Marlin/example_configurations/TAZ4/Configuration_adv.h
+++ b/Marlin/example_configurations/TAZ4/Configuration_adv.h
@@ -803,5 +803,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/WITBOX/Configuration_adv.h b/Marlin/example_configurations/WITBOX/Configuration_adv.h
index b719e60727..d26e2207b0 100644
--- a/Marlin/example_configurations/WITBOX/Configuration_adv.h
+++ b/Marlin/example_configurations/WITBOX/Configuration_adv.h
@@ -795,5 +795,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/delta/biv2.5/Configuration_adv.h b/Marlin/example_configurations/delta/biv2.5/Configuration_adv.h
index 9183be44b3..828391a3fd 100644
--- a/Marlin/example_configurations/delta/biv2.5/Configuration_adv.h
+++ b/Marlin/example_configurations/delta/biv2.5/Configuration_adv.h
@@ -797,5 +797,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/delta/generic/Configuration_adv.h b/Marlin/example_configurations/delta/generic/Configuration_adv.h
index eb79a30413..967a3e6b56 100644
--- a/Marlin/example_configurations/delta/generic/Configuration_adv.h
+++ b/Marlin/example_configurations/delta/generic/Configuration_adv.h
@@ -797,5 +797,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/delta/kossel_mini/Configuration_adv.h b/Marlin/example_configurations/delta/kossel_mini/Configuration_adv.h
index 7ec5948d02..64ea68fcdf 100644
--- a/Marlin/example_configurations/delta/kossel_mini/Configuration_adv.h
+++ b/Marlin/example_configurations/delta/kossel_mini/Configuration_adv.h
@@ -796,5 +796,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/delta/kossel_pro/Configuration_adv.h b/Marlin/example_configurations/delta/kossel_pro/Configuration_adv.h
index 811477921d..99fbcf08f6 100644
--- a/Marlin/example_configurations/delta/kossel_pro/Configuration_adv.h
+++ b/Marlin/example_configurations/delta/kossel_pro/Configuration_adv.h
@@ -801,5 +801,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/delta/kossel_xl/Configuration_adv.h b/Marlin/example_configurations/delta/kossel_xl/Configuration_adv.h
index 2a7399bb03..0af7481f96 100644
--- a/Marlin/example_configurations/delta/kossel_xl/Configuration_adv.h
+++ b/Marlin/example_configurations/delta/kossel_xl/Configuration_adv.h
@@ -797,5 +797,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/makibox/Configuration_adv.h b/Marlin/example_configurations/makibox/Configuration_adv.h
index 22cefd5388..7715b71a36 100644
--- a/Marlin/example_configurations/makibox/Configuration_adv.h
+++ b/Marlin/example_configurations/makibox/Configuration_adv.h
@@ -795,5 +795,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/example_configurations/tvrrug/Round2/Configuration_adv.h b/Marlin/example_configurations/tvrrug/Round2/Configuration_adv.h
index 2ececd4c3a..3cfe5860bf 100644
--- a/Marlin/example_configurations/tvrrug/Round2/Configuration_adv.h
+++ b/Marlin/example_configurations/tvrrug/Round2/Configuration_adv.h
@@ -795,5 +795,6 @@ const unsigned int dropsegments = 5; //everything with less than this number of
 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H
diff --git a/Marlin/twibus.cpp b/Marlin/twibus.cpp
index c1d6008ddd..cbefbfdfb2 100644
--- a/Marlin/twibus.cpp
+++ b/Marlin/twibus.cpp
@@ -29,25 +29,28 @@
 #include <Wire.h>
 
 TWIBus::TWIBus() {
-  Wire.begin(); // We use no address so we will join the BUS as the master
+  #if I2C_SLAVE_ADDRESS == 0
+    Wire.begin();                  // No address joins the BUS as the master
+  #else
+    Wire.begin(I2C_SLAVE_ADDRESS); // Join the bus as a slave
+  #endif
   this->reset();
 }
 
 void TWIBus::reset() {
-  this->addr = 0;
   this->buffer_s = 0;
   this->buffer[0] = 0x00;
 }
 
-void TWIBus::address(uint8_t addr) {
+void TWIBus::address(const uint8_t addr) {
   this->addr = addr;
 
   #if ENABLED(DEBUG_TWIBUS)
-    debug(PSTR("sendto"), this->addr);
+    debug(PSTR("address"), this->addr);
   #endif
 }
 
-void TWIBus::addbyte(char c) {
+void TWIBus::addbyte(const char c) {
   if (buffer_s >= sizeof(this->buffer)) return;
   this->buffer[this->buffer_s++] = c;
 
@@ -67,41 +70,37 @@ void TWIBus::send() {
   Wire.write(this->buffer, this->buffer_s);
   Wire.endTransmission();
 
-    // Reset the buffer after sending the data
+  // Reset the buffer after sending the data
   this->reset();
 }
 
-void TWIBus::reqbytes(uint8_t bytes) {
+void TWIBus::reqbytes(const uint8_t bytes) {
   if (!this->addr) return;
 
   #if ENABLED(DEBUG_TWIBUS)
     debug(PSTR("reqbytes"), bytes);
   #endif
 
+  // requestFrom() is a blocking function
   millis_t t = millis() + this->timeout;
   Wire.requestFrom(this->addr, bytes);
+  while (Wire.available() < bytes && PENDING(millis(), t)) { /*nada*/ }
 
-    // requestFrom() is a blocking function
-  while (Wire.available() < bytes) {
-    if (ELAPSED(millis(), t)) break;
-    else continue;
-  }
-
-  SERIAL_ECHO_START;
-  SERIAL_ECHOPAIR("i2c-reply: from:", this->addr);
-  SERIAL_ECHOPAIR(" bytes:", Wire.available());
-  SERIAL_ECHOPGM (" data:");
-
-    // Protect against buffer overflows if the number of received bytes
-    // is less than the number of requested bytes
-  uint8_t wba = Wire.available();
-  for (int i = 0; i < wba; i++) SERIAL_CHAR(Wire.read());
-  SERIAL_EOL;
+  this->relaydata(bytes);
 
   // Reset the buffer after sending the data
   this->reset();
 }
 
+void TWIBus::relaydata(uint8_t bytes) {
+  SERIAL_ECHO_START;
+  SERIAL_ECHOPAIR("i2c-reply: from:", this->addr);
+  SERIAL_ECHOPAIR(" bytes:", bytes);
+  SERIAL_ECHOPGM (" data:");
+  while (bytes-- && Wire.available()) SERIAL_CHAR(Wire.read());
+  SERIAL_EOL;
+}
+
 #if ENABLED(DEBUG_TWIBUS)
 
   void TWIBus::debug(const char func[], int32_t val/*=-1*/) {
diff --git a/Marlin/twibus.h b/Marlin/twibus.h
index d0d0e0ff32..d78e646e51 100644
--- a/Marlin/twibus.h
+++ b/Marlin/twibus.h
@@ -25,9 +25,13 @@
 
 #include "macros.h"
 
+#include <Wire.h>
+
 // Print debug messages with M111 S2 (Uses 236 bytes of PROGMEM)
 //#define DEBUG_TWIBUS
 
+typedef void (*twiSlaveFunc_t)(int bytes);
+
 /**
  * TWIBUS class
  *
@@ -49,79 +53,105 @@ class TWIBus {
   private:
     /**
      * @brief Timeout value in milliseconds
-     * @details For blocking operations this constant value will set the max
-     * amount of time Marlin will keep waiting for a reply. Useful is something
-     * goes wrong on the bus and the SDA/SCL lines are held up by another device.
+     * @details Maximum amount of time (ms) to wait for a reply.
+     *          Useful if something goes wrong on the bus and the
+     *          SDA/SCL lines are held up by another device.
      */
     const int timeout = 5;
 
-    /**
-     * @brief Target device address
-     * @description This stores, until the buffer is flushed, the target device
-     * address, take not we do follow Arduino 7bit addressing.
-     */
-    uint8_t addr = 0;
-
     /**
      * @brief Number of bytes on buffer
-     * @description This var holds the total number of bytes on our buffer
-     * waiting to be flushed to the bus.
+     * @description Number of bytes in the buffer waiting to be flushed to the bus.
      */
     uint8_t buffer_s = 0;
 
     /**
      * @brief Internal buffer
-     * @details This is a fixed buffer, TWI command cannot be longer than this
+     * @details A fixed buffer. TWI commands can be no longer than this.
      */
-    char buffer[30];
+    char buffer[32];
 
 
   public:
+    /**
+     * @brief Target device address
+     * @description The target device address. Persists until changed.
+     */
+    uint8_t addr = 0;
+
     /**
      * @brief Class constructor
-     * @details Initialized the TWI bus and clears the buffer
+     * @details Initialize the TWI bus and clear the buffer
      */
     TWIBus();
 
     /**
      * @brief Reset the buffer
-     * @details Brings the internal buffer to a known-empty state
+     * @details Set the buffer to a known-empty state
      */
     void reset();
 
     /**
      * @brief Send the buffer data to the bus
-     * @details Flushed the buffer into the bus targeting the cached slave device
-     * address.
+     * @details Flush the buffer to the bus at the target address.
      */
     void send();
 
     /**
      * @brief Add one byte to the buffer
-     * @details Adds the byte to the buffer in a sequential way, if buffer is full
-     * the request is silently ignored.
+     * @details Add a byte to the end of the buffer.
+     *          Silently fails if the buffer is full.
      *
      * @param c a data byte
      */
-    void addbyte(char c);
+    void addbyte(const char c);
 
     /**
-     * @brief Sets the target slave address
-     * @details The target slave address is stored so it can be later used when
-     * the complete packet needs to be sent over the bus.
+     * @brief Set the target slave address
+     * @details The target slave address for sending the full packet.
      *
      * @param addr 7-bit integer address
      */
-    void address(uint8_t addr);
+    void address(const uint8_t addr);
 
     /**
-     * @brief Request data from slave device
-     * @details Requests data from a slave device, when the data is received it will
-     * be relayed to the serial line using a parser-friendly formatting.
+     * @brief Request data from the slave device
+     * @details Request a number of bytes from a slave device.
+     *          This implementation simply sends the data to serial
+     *          in a parser-friendly format.
      *
      * @param bytes the number of bytes to request
      */
-    void reqbytes(uint8_t bytes);
+    void reqbytes(const uint8_t bytes);
+
+    /**
+     * @brief Relay data from the slave device to serial
+     * @details Relay a number of bytes from the bus to
+     *          serial in a parser-friendly format.
+     *
+     * @param bytes the number of bytes to request
+     */
+    void relaydata(uint8_t bytes);
+
+    #if I2C_SLAVE_ADDRESS > 0
+
+      /**
+       * @brief Receive bytes (passively)
+       * @details Receive bytes sent to our slave address.
+       *          and simply echo them to serial.
+       */
+      inline void receive(uint8_t bytes) { relaydata(bytes); }
+
+      /**
+       * @brief Register a slave handler
+       * @details Set a handler to receive data from the bus,
+       *          so we can act as a slave.
+       *
+       * @param handler A function to handle receiving bytes
+       */
+      inline void onReceive(const twiSlaveFunc_t handler) { Wire.onReceive(handler); }
+
+    #endif
 
     #if ENABLED(DEBUG_TWIBUS)