diff --git a/IODevice.h b/IODevice.h
index fe1d3e6..5d6abc3 100644
--- a/IODevice.h
+++ b/IODevice.h
@@ -120,7 +120,7 @@ public:
   }
 
   // User-friendly function for configuring a servo pin.
-  inline static bool configureServo(VPIN vpin, uint16_t activePosition, uint16_t inactivePosition, uint8_t profile, uint16_t duration, uint8_t initialState=0) {
+  inline static bool configureServo(VPIN vpin, uint16_t activePosition, uint16_t inactivePosition, uint8_t profile=0, uint16_t duration=0, uint8_t initialState=0) {
     int params[] = {(int)activePosition, (int)inactivePosition, profile, (int)duration, initialState};
     return IODevice::configure(vpin, CONFIGURE_SERVO, 5, params);
   }
@@ -203,7 +203,7 @@ protected:
 
   // Method to perform updates on an ongoing basis (optionally implemented within device class)
   virtual void _loop(unsigned long currentMicros) {
-    (void)currentMicros; // Suppress compiler warning.
+    delayUntil(currentMicros + 0x7fffffff); // Largest time in the future!  Effectively disable _loop calls.
   };
 
   // Method for displaying info on DIAG output (optionally implemented within device class)
diff --git a/IO_GPIOBase.h b/IO_GPIOBase.h
index 9b1bee5..4269a70 100644
--- a/IO_GPIOBase.h
+++ b/IO_GPIOBase.h
@@ -53,6 +53,7 @@ protected:
   T _portOutputState;
   T _portMode;
   T _portPullup;
+  T _portInUse;
   // Interval between refreshes of each input port
   static const int _portTickTime = 4000;
 
@@ -100,7 +101,8 @@ void GPIOBase<T>::_begin() {
 #endif
     _portMode = 0;  // default to input mode
     _portPullup = -1; // default to pullup enabled
-    _portInputState = -1; 
+    _portInputState = -1;
+    _portInUse = 0;
     _setupDevice();
     _deviceState = DEVSTATE_NORMAL;
   } else {
@@ -126,11 +128,15 @@ bool GPIOBase<T>::_configure(VPIN vpin, ConfigTypeEnum configType, int paramCoun
     _portPullup |= mask;
   else
     _portPullup &= ~mask;
+  // Mark that port has been accessed
+  _portInUse |= mask;
+  // Set input mode
+  _portMode &= ~mask;
 
   // Call subclass's virtual function to write to device
+  _writePortModes();
   _writePullups();
-  // Re-read port following change
-  _readGpioPort();
+  // Port change will be notified on next loop entry.
 
   return true;
 }
@@ -149,6 +155,8 @@ void GPIOBase<T>::_loop(unsigned long currentMicros) {
         I2CManager.getErrorMessage(status));
     }
     _processCompletion(status);
+  // Set unused pin and write mode pin value to 1
+    _portInputState |= ~_portInUse | _portMode;
 
     // Scan for changes in input states and invoke callback (if present)
     T differences = lastPortStates ^ _portInputState;
@@ -199,8 +207,9 @@ void GPIOBase<T>::_write(VPIN vpin, int value) {
   DIAG(F("%S I2C:x%x Write Pin:%d Val:%d"), _deviceName, _I2CAddress, pin, value);
   #endif
 
-  // Set port mode output
+  // Set port mode output if currently not output mode
   if (!(_portMode & mask)) {
+    _portInUse |= mask;
     _portMode |= mask;
     _writePortModes();
   }
@@ -220,12 +229,16 @@ int GPIOBase<T>::_read(VPIN vpin) {
   int pin = vpin - _firstVpin;
   T mask = 1 << pin;
 
-  // Set port mode to input
-  if (_portMode & mask) {
+  // Set port mode to input if currently output or first use
+  if ((_portMode | ~_portInUse) & mask) {
     _portMode &= ~mask;
+    _portInUse |= mask;
+    _writePullups();
     _writePortModes();
     // Port won't have been read yet, so read it now.
     _readGpioPort();
+  // Set unused pin and write mode pin value to 1
+    _portInputState |= ~_portInUse | _portMode;
     #ifdef DIAG_IO
     DIAG(F("%S I2C:x%x PortStates:%x"), _deviceName, _I2CAddress, _portInputState);
     #endif
diff --git a/IO_MCP23008.h b/IO_MCP23008.h
index 3557b49..18ff12f 100644
--- a/IO_MCP23008.h
+++ b/IO_MCP23008.h
@@ -42,14 +42,18 @@ private:
     I2CManager.write(_I2CAddress, 2, REG_GPIO, _portOutputState);
   }
   void _writePullups() override {
-    I2CManager.write(_I2CAddress, 2, REG_GPPU, _portPullup);  
+    // Set pullups only for in-use pins.  This prevents pullup being set for a pin that
+    //  is intended for use as an output but hasn't been written to yet.
+    I2CManager.write(_I2CAddress, 2, REG_GPPU, _portPullup & _portInUse);  
   }
   void _writePortModes() override {
-    // Each bit is 1 for an input, 0 for an output, i.e. inverted.
-    I2CManager.write(_I2CAddress, 2, REG_IODIR, ~_portMode);
-    // Enable interrupt-on-change for pins that are inputs (_portMode=0)
+    // Write 0 to IODIR for in-use pins that are outputs, 1 for others.
+    uint8_t temp = ~(_portMode & _portInUse);
+    I2CManager.write(_I2CAddress, 2, REG_IODIR, temp);
+    // Enable interrupt-on-change for in-use pins that are inputs (_portMode=0)
+    temp = ~_portMode & _portInUse;
     I2CManager.write(_I2CAddress, 2, REG_INTCON, 0x00);
-    I2CManager.write(_I2CAddress, 2, REG_GPINTEN, ~_portMode);
+    I2CManager.write(_I2CAddress, 2, REG_GPINTEN, temp);
   }
   void _readGpioPort(bool immediate) override {
     if (immediate) {
diff --git a/IO_MCP23017.h b/IO_MCP23017.h
index d7c27ce..930b051 100644
--- a/IO_MCP23017.h
+++ b/IO_MCP23017.h
@@ -48,14 +48,19 @@ private:
     I2CManager.write(_I2CAddress, 3, REG_GPIOA, _portOutputState, _portOutputState>>8);
   }
   void _writePullups() override {
-    I2CManager.write(_I2CAddress, 3, REG_GPPUA, _portPullup, _portPullup>>8);  
+    // Set pullups only for in-use pins.  This prevents pullup being set for a pin that
+    //  is intended for use as an output but hasn't been written to yet.
+    uint16_t temp = _portPullup & _portInUse;
+    I2CManager.write(_I2CAddress, 3, REG_GPPUA, temp, temp>>8);  
   }
   void _writePortModes() override {
-    // Write 1 to IODIR for pins that are inputs, 0 for outputs (i.e. _portMode inverted)
-    I2CManager.write(_I2CAddress, 3, REG_IODIRA, ~_portMode, (~_portMode)>>8);
-    // Enable interrupt for those pins which are inputs (_portMode=0)
+    // Write 0 to IODIR for in-use pins that are outputs, 1 for others.
+    uint16_t temp = ~(_portMode & _portInUse);
+    I2CManager.write(_I2CAddress, 3, REG_IODIRA, temp, temp>>8);
+    // Enable interrupt for in-use pins which are inputs (_portMode=0)
+    temp = ~_portMode & _portInUse;
     I2CManager.write(_I2CAddress, 3, REG_INTCONA, 0x00, 0x00);
-    I2CManager.write(_I2CAddress, 3, REG_GPINTENA, ~_portMode, (~_portMode)>>8);
+    I2CManager.write(_I2CAddress, 3, REG_GPINTENA, temp, temp>>8);
   }
   void _readGpioPort(bool immediate) override {
     if (immediate) {
diff --git a/IO_PCF8574.h b/IO_PCF8574.h
index 2a8d363..dea5e2c 100644
--- a/IO_PCF8574.h
+++ b/IO_PCF8574.h
@@ -17,6 +17,24 @@
  *  along with CommandStation.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/* 
+ * The PCF8574 is a simple device; it only has one register.  The device 
+ * input/output mode and pullup are configured through this, and the 
+ * output state is written and the input state read through it too.
+ * 
+ * This is accomplished by having a weak resistor in series with the output,
+ * and a read-back of the other end of the resistor.  As an output, the 
+ * pin state is set to 1 or 0, and the output voltage goes to +5V or 0V
+ * (through the weak resistor).
+ * 
+ * In order to use the pin as an input, the output is written as
+ * a '1' in order to pull up the resistor.  Therefore the input will be
+ * 1 unless the pin is pulled down externally, in which case it will be 0.
+ * 
+ * As a consequence of this approach, it is not possible to use the device for
+ * inputs without pullups.
+ */
+
 #ifndef IO_PCF8574_H
 #define IO_PCF8574_H
 
@@ -70,7 +88,7 @@ private:
     if (status == I2C_STATUS_OK) 
       _portInputState = ((uint16_t)inputBuffer[0]) & 0xff;
     else  
-      _portInputState = 0xff;
+      _portInputState = 0xff; 
   }
 
   // Set up device ports