diff --git a/IODevice.cpp b/IODevice.cpp
index d320c09..c6af137 100644
--- a/IODevice.cpp
+++ b/IODevice.cpp
@@ -61,10 +61,14 @@ void IODevice::begin() {
   // Allocates 32 pins 100-131
   PCA9685::create(100, 16, 0x40);
   PCA9685::create(116, 16, 0x41);
+  PCA9685::create(132, 16, 0x41);  // should fail
+  PCA9685::create(118, 4, 0x42);  // should fail
+  
   // Predefine two MCP23017 module 0x20/0x21
   // Allocates 32 pins 164-195
   MCP23017::create(164, 16, 0x20);
   MCP23017::create(180, 16, 0x21);
+MCP23017::create(196, 16, 0x40);  // should fail
 
   // Call the begin() methods of each configured device in turn
   for (IODevice *dev=_firstDevice; dev!=NULL; dev = dev->_nextDevice) {
@@ -275,16 +279,36 @@ IODevice *IODevice::findDevice(VPIN vpin) {
 
 // Private helper function to check for vpin overlap. Run during setup only.
 // returns true if pins DONT overlap with existing device
-bool IODevice::checkNoOverlap(VPIN firstPin, uint8_t nPins) { 
-  for (VPIN testPin=firstPin; testPin< (firstPin+nPins); testPin++)
-  if (findDevice(testPin)) {
-    DIAG(F("WARNING HAL Pin %d overlap, re-definition of pins  %d to %d ignored."),
-            testPin, firstPin, firstPin+nPins);
-    return false; 
+bool IODevice::checkNoOverlap(VPIN firstPin, uint8_t nPins, uint8_t i2cAddress) {
+  DIAG(F("Check no overlap %d %d 0x%x"), firstPin,nPins,i2cAddress);
+  VPIN lastPin=firstPin+nPins-1;
+  for (IODevice *dev = _firstDevice; dev != 0; dev = dev->_nextDevice) {
+    
+    // check for pin range overlaps (verbose but compiler will fix that)  
+    VPIN firstDevPin=dev->_firstVpin;
+    VPIN lastDevPin=firstDevPin+dev->_nPins-1;
+    bool noOverlap= firstPin>lastDevPin || lastPin<firstDevPin;
+    if (!noOverlap) {
+        DIAG(F("WARNING HAL Overlap definition of pins  %d to %d ignored."),
+            firstPin, lastPin);
+        return false;
+    } 
+  
+    // Check for overlapping I2C address
+    if (dev->_matchI2CAddress(i2cAddress)) {
+      DIAG(F("WARNING HAL Overlap. i2c Addr 0x%x ignored."),i2cAddress);
+      return false;
+    } 
   }
-  return true;
+  return true;  // no overlaps... OK to go on with constructor
 }
   
+ bool IODevice::_matchI2CAddress(uint8_t i2cAddress) { 
+   // Overridden for I2c devices. 
+   (void) i2cAddress;
+   return false;
+  }
+
 //==================================================================================================================
 // Static data
 //------------------------------------------------------------------------------------------------------------------
diff --git a/IODevice.h b/IODevice.h
index a688ba3..77325c2 100644
--- a/IODevice.h
+++ b/IODevice.h
@@ -228,8 +228,11 @@ protected:
   //  pin low if an input changes state.
   int16_t _gpioInterruptPin = -1;
 
+  // non-i2c hal drivers return false, i2c drivers override this in IO_GPIOBase
+  virtual bool _matchI2CAddress(uint8_t i2cAddress);
+    
   // Method to check if pins will overlap before creating new device. 
-  static bool checkNoOverlap(VPIN firstPin, uint8_t nPins=1);
+  static bool checkNoOverlap(VPIN firstPin, uint8_t nPins=1, uint8_t i2cAddress=0);
 
   // Static support function for subclass creation
   static void addDevice(IODevice *newDevice);
@@ -242,7 +245,7 @@ private:
   bool owns(VPIN vpin);
   // Method to find device handling Vpin
   static IODevice *findDevice(VPIN vpin);
-  
+  uint8_t _I2CAddress;
   IODevice *_nextDevice = 0;
   unsigned long _nextEntryTime;
   static IODevice *_firstDevice;
@@ -284,7 +287,9 @@ private:
   void updatePosition(uint8_t pin);
   void writeDevice(uint8_t pin, int value);
   void _display() override;
-
+  virtual bool _matchI2CAddress(uint8_t i2caddress) override {
+    return i2caddress && i2caddress==_I2CAddress;
+  }
   uint8_t _I2CAddress; // 0x40-0x43 possible
 
   struct ServoData {
diff --git a/IO_AnalogueInputs.h b/IO_AnalogueInputs.h
index 5849501..1af351d 100644
--- a/IO_AnalogueInputs.h
+++ b/IO_AnalogueInputs.h
@@ -60,7 +60,7 @@
 class ADS111x: public IODevice { 
 public:
   static void create(VPIN firstVpin, int nPins, uint8_t i2cAddress) {
-    if (checkNoOverlap(firstVpin,nPins)) new ADS111x(firstVpin, nPins, i2cAddress);
+    if (checkNoOverlap(firstVpin,nPins,i2cAddress)) new ADS111x(firstVpin, nPins, i2cAddress);
   }
 private:
   ADS111x(VPIN firstVpin, int nPins, uint8_t i2cAddress) {
diff --git a/IO_GPIOBase.h b/IO_GPIOBase.h
index 3bc75af..d2a1339 100644
--- a/IO_GPIOBase.h
+++ b/IO_GPIOBase.h
@@ -45,9 +45,13 @@ protected:
   int _read(VPIN vpin) override;
   void _display() override;
   void _loop(unsigned long currentMicros) override;
+  virtual bool _matchI2CAddress(uint8_t i2cAddress) override {
+    DIAG(F("MatchI2c %x %x"), i2cAddress, _I2CAddress);
+    return (i2cAddress && i2cAddress==_I2CAddress);
+  }
 
   // Data fields
-  uint8_t _I2CAddress; 
+  uint8_t _I2CAddress;
   // Allocate enough space for all input pins
   T _portInputState; 
   T _portOutputState;
diff --git a/IO_MCP23008.h b/IO_MCP23008.h
index 916ebbe..aa4679b 100644
--- a/IO_MCP23008.h
+++ b/IO_MCP23008.h
@@ -25,7 +25,7 @@
 class MCP23008 : public GPIOBase<uint8_t> {
 public:
   static void create(VPIN firstVpin, uint8_t nPins, uint8_t I2CAddress, int interruptPin=-1) {
-    if (checkNoOverlap(firstVpin, nPins)) new MCP23008(firstVpin, nPins, I2CAddress, interruptPin);
+    if (checkNoOverlap(firstVpin, nPins,I2CAddress)) new MCP23008(firstVpin, nPins, I2CAddress, interruptPin);
   }
 
 private:
diff --git a/IO_MCP23017.h b/IO_MCP23017.h
index e6ff5be..65769f6 100644
--- a/IO_MCP23017.h
+++ b/IO_MCP23017.h
@@ -31,7 +31,7 @@
 class MCP23017 : public GPIOBase<uint16_t> {
 public:
   static void create(VPIN vpin, int nPins, uint8_t I2CAddress, int interruptPin=-1) {
-    if (checkNoOverlap(vpin, nPins)) new MCP23017(vpin, min(nPins,16), I2CAddress, interruptPin);
+    if (checkNoOverlap(vpin, nPins, I2CAddress)) new MCP23017(vpin, min(nPins,16), I2CAddress, interruptPin);
   }
 
 private:  
diff --git a/IO_PCA9685.cpp b/IO_PCA9685.cpp
index ae6d3dd..3d7c347 100644
--- a/IO_PCA9685.cpp
+++ b/IO_PCA9685.cpp
@@ -39,7 +39,7 @@ static void writeRegister(byte address, byte reg, byte value);
 
 // Create device driver instance.
 void PCA9685::create(VPIN firstVpin, int nPins, uint8_t I2CAddress) {
-  if (checkNoOverlap(firstVpin, nPins)) new PCA9685(firstVpin, nPins, I2CAddress);
+  if (checkNoOverlap(firstVpin, nPins,I2CAddress)) new PCA9685(firstVpin, nPins, I2CAddress);
 }
 
 // Configure a port on the PCA9685.
diff --git a/IO_PCF8574.h b/IO_PCF8574.h
index d3f98c8..44eccad 100644
--- a/IO_PCF8574.h
+++ b/IO_PCF8574.h
@@ -43,7 +43,7 @@
 class PCF8574 : public GPIOBase<uint8_t> {
 public:
   static void create(VPIN firstVpin, uint8_t nPins, uint8_t I2CAddress, int interruptPin=-1) {
-    if (checkNoOverlap(firstVpin, nPins)) new PCF8574(firstVpin, nPins, I2CAddress, interruptPin);
+    if (checkNoOverlap(firstVpin, nPins,I2CAddress)) new PCF8574(firstVpin, nPins, I2CAddress, interruptPin);
   }
 
 private:
diff --git a/IO_VL53L0X.h b/IO_VL53L0X.h
index 47204d5..9d51345 100644
--- a/IO_VL53L0X.h
+++ b/IO_VL53L0X.h
@@ -130,7 +130,7 @@ private:
 
   public:
   static void create(VPIN firstVpin, int nPins, uint8_t i2cAddress, uint16_t onThreshold, uint16_t offThreshold, VPIN xshutPin = VPIN_NONE) {
-     if (checkNoOverlap(firstVpin, nPins)) new VL53L0X(firstVpin, nPins, i2cAddress, onThreshold, offThreshold, xshutPin);
+     if (checkNoOverlap(firstVpin, nPins,i2cAddress)) new VL53L0X(firstVpin, nPins, i2cAddress, onThreshold, offThreshold, xshutPin);
   }
 
 protected: