diff --git a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/CdcAcmSerialDriver.java b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/CdcAcmSerialDriver.java
index 33eb17e..773f355 100644
--- a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/CdcAcmSerialDriver.java
+++ b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/CdcAcmSerialDriver.java
@@ -50,6 +50,7 @@ public class CdcAcmSerialDriver implements UsbSerialDriver {
 
     private final UsbDevice mDevice;
     private final UsbSerialPort mPort;
+    private UsbRequest mUsbRequest;
 
     public CdcAcmSerialDriver(UsbDevice device) {
         mDevice = device;
@@ -253,9 +254,15 @@ public class CdcAcmSerialDriver implements UsbSerialDriver {
             if (mConnection == null) {
                 throw new IOException("Already closed");
             }
+            synchronized (this) {
+                if (mUsbRequest != null)
+                    mUsbRequest.cancel();
+            }
             try {
                 mConnection.releaseInterface(mControlInterface);
                 mConnection.releaseInterface(mDataInterface);
+            } catch(Exception ignored) {}
+            try {
                 mConnection.close();
             } finally {
                 mConnection = null;
@@ -273,7 +280,11 @@ public class CdcAcmSerialDriver implements UsbSerialDriver {
                 if (!request.queue(buf, dest.length)) {
                     throw new IOException("Error queueing request.");
                 }
+                mUsbRequest = request;
                 final UsbRequest response = mConnection.requestWait();
+                synchronized (this) {
+                    mUsbRequest = null;
+                }
                 if (response == null) {
                     throw new IOException("Null response");
                 }
@@ -286,6 +297,7 @@ public class CdcAcmSerialDriver implements UsbSerialDriver {
                     return 0;
                 }
             } finally {
+                mUsbRequest = null;
                 request.close();
             }
         }
diff --git a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/Ch34xSerialDriver.java b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/Ch34xSerialDriver.java
index 31c36ed..7b36171 100644
--- a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/Ch34xSerialDriver.java
+++ b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/Ch34xSerialDriver.java
@@ -85,6 +85,7 @@ public class Ch34xSerialDriver implements UsbSerialDriver {
 
 		private UsbEndpoint mReadEndpoint;
 		private UsbEndpoint mWriteEndpoint;
+		private UsbRequest mUsbRequest;
 
 		public Ch340SerialPort(UsbDevice device, int portNumber) {
 			super(device, portNumber);
@@ -143,9 +144,15 @@ public class Ch34xSerialDriver implements UsbSerialDriver {
 			if (mConnection == null) {
 				throw new IOException("Already closed");
 			}
+			synchronized (this) {
+				if (mUsbRequest != null)
+					mUsbRequest.cancel();
+			}
 			try {
 				for (int i = 0; i < mDevice.getInterfaceCount(); i++)
 					mConnection.releaseInterface(mDevice.getInterface(i));
+			} catch(Exception ignored) {}
+			try {
 				mConnection.close();
 			} finally {
 				mConnection = null;
@@ -164,7 +171,11 @@ public class Ch34xSerialDriver implements UsbSerialDriver {
 				if (!request.queue(buf, dest.length)) {
 					throw new IOException("Error queueing request.");
 				}
+				mUsbRequest = request;
 				final UsbRequest response = mConnection.requestWait();
+				synchronized (this) {
+					mUsbRequest = null;
+				}
 				if (response == null) {
 					throw new IOException("Null response");
 				}
@@ -177,6 +188,7 @@ public class Ch34xSerialDriver implements UsbSerialDriver {
 					return 0;
 				}
 			} finally {
+				mUsbRequest = null;
 				request.close();
 			}
 		}
diff --git a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/Cp21xxSerialDriver.java b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/Cp21xxSerialDriver.java
index 67fa528..11d9823 100644
--- a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/Cp21xxSerialDriver.java
+++ b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/Cp21xxSerialDriver.java
@@ -108,6 +108,7 @@ public class Cp21xxSerialDriver implements UsbSerialDriver {
 
         private UsbEndpoint mReadEndpoint;
         private UsbEndpoint mWriteEndpoint;
+        private UsbRequest mUsbRequest;
 
         // second port of Cp2105 has limited baudRate, dataBits, stopBits, parity
         // unsupported baudrate returns error at controlTransfer(), other parameters are silently ignored
@@ -176,12 +177,18 @@ public class Cp21xxSerialDriver implements UsbSerialDriver {
             if (mConnection == null) {
                 throw new IOException("Already closed");
             }
+            synchronized (this) {
+                if(mUsbRequest != null) {
+                    mUsbRequest.cancel();
+                }
+            }
             try {
                 setConfigSingle(SILABSER_IFC_ENABLE_REQUEST_CODE, UART_DISABLE);
-            } catch (Exception ignored)
-            {}
+            } catch (Exception ignored) {}
             try {
                 mConnection.releaseInterface(mDevice.getInterface(mPortNumber));
+            } catch(Exception ignored) {}
+            try {
                 mConnection.close();
             } finally {
                 mConnection = null;
@@ -199,7 +206,11 @@ public class Cp21xxSerialDriver implements UsbSerialDriver {
                 if (!request.queue(buf, dest.length)) {
                     throw new IOException("Error queueing request.");
                 }
+                mUsbRequest = request;
                 final UsbRequest response = mConnection.requestWait();
+                synchronized (this) {
+                    mUsbRequest = null;
+                }
                 if (response == null) {
                     throw new IOException("Null response");
                 }
@@ -212,6 +223,7 @@ public class Cp21xxSerialDriver implements UsbSerialDriver {
                     return 0;
                 }
             } finally {
+                mUsbRequest = null;
                 request.close();
             }
         }
diff --git a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/FtdiSerialDriver.java b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/FtdiSerialDriver.java
index 585b84b..8dbdfd4 100644
--- a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/FtdiSerialDriver.java
+++ b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/FtdiSerialDriver.java
@@ -269,6 +269,8 @@ public class FtdiSerialDriver implements UsbSerialDriver {
             }
             try {
                 mConnection.releaseInterface(mDevice.getInterface(mPortNumber));
+            } catch(Exception ignored) {}
+            try {
                 mConnection.close();
             } finally {
                 mConnection = null;
diff --git a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/ProlificSerialDriver.java b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/ProlificSerialDriver.java
index 9830b56..d4928fc 100644
--- a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/ProlificSerialDriver.java
+++ b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/ProlificSerialDriver.java
@@ -362,6 +362,8 @@ public class ProlificSerialDriver implements UsbSerialDriver {
             } finally {
                 try {
                     mConnection.releaseInterface(mDevice.getInterface(0));
+                } catch(Exception ignored) {}
+                try {
                     mConnection.close();
                 } finally {
                     mConnection = null;