diff --git a/README.md b/README.md
index 2bee0992..2ed86b9f 100644
--- a/README.md
+++ b/README.md
@@ -29,7 +29,7 @@ allprojects {
 Add library to dependencies
 ```gradle
 dependencies {
-    implementation 'com.github.mik3y:usb-serial-for-android:3.3.0'
+    implementation 'com.github.mik3y:usb-serial-for-android:3.3.1'
 }
 ```
 
diff --git a/usbSerialForAndroid/src/androidTest/java/com/hoho/android/usbserial/DeviceTest.java b/usbSerialForAndroid/src/androidTest/java/com/hoho/android/usbserial/DeviceTest.java
index 630ae4f6..37e6f60e 100644
--- a/usbSerialForAndroid/src/androidTest/java/com/hoho/android/usbserial/DeviceTest.java
+++ b/usbSerialForAndroid/src/androidTest/java/com/hoho/android/usbserial/DeviceTest.java
@@ -818,7 +818,7 @@ public void writeTimeout() throws Exception {
         purgeWriteBuffer(purgeTimeout);
 
         // determine write buffer size
-        int writePacketSize = ((CommonUsbSerialPort)usb.serialPort).getWriteEndpoint().getMaxPacketSize();
+        int writePacketSize = usb.serialPort.getWriteEndpoint().getMaxPacketSize();
         byte[] pbuf = new byte[writePacketSize];
         int writePackets = 0;
         try {
@@ -1321,7 +1321,9 @@ public void purgeHwBuffers() throws Exception {
     @Test
     public void IoManager() throws Exception {
         SerialInputOutputManager.DEBUG = true;
-        usb.ioManager = new SerialInputOutputManager(null);
+        usb.open(EnumSet.of(UsbWrapper.OpenCloseFlags.NO_IOMANAGER_THREAD));
+        assertNull(usb.ioManager);
+        usb.ioManager = new SerialInputOutputManager(usb.serialPort);
         assertNull(usb.ioManager.getListener());
         usb.ioManager.setListener(usb);
         assertEquals(usb, usb.ioManager.getListener());
@@ -1335,7 +1337,7 @@ public void IoManager() throws Exception {
         usb.ioManager.setWriteTimeout(11);
         assertEquals(11, usb.ioManager.getWriteTimeout());
 
-        assertEquals(4096, usb.ioManager.getReadBufferSize());
+        assertEquals(usb.serialPort.getReadEndpoint().getMaxPacketSize(), usb.ioManager.getReadBufferSize());
         usb.ioManager.setReadBufferSize(12);
         assertEquals(12, usb.ioManager.getReadBufferSize());
         assertEquals(4096, usb.ioManager.getWriteBufferSize());
@@ -1346,6 +1348,7 @@ public void IoManager() throws Exception {
         usb.ioManager.setWriteBufferSize(usb.ioManager.getWriteBufferSize());
         usb.ioManager.setReadTimeout(usb.ioManager.getReadTimeout());
         usb.ioManager.setWriteTimeout(usb.ioManager.getWriteTimeout());
+        usb.close();
 
         usb.open(EnumSet.of(UsbWrapper.OpenCloseFlags.NO_IOMANAGER_START)); // creates new IoManager
         usb.setParameters(19200, 8, 1, UsbSerialPort.PARITY_NONE);
@@ -2007,8 +2010,8 @@ public void commonMethods() throws Exception {
         usb.open();
         assertTrue(usb.serialPort.isOpen());
 
-        assertEquals(((CommonUsbSerialPort)usb.serialPort).getWriteEndpoint().getMaxPacketSize(),
-                     ((CommonUsbSerialPort)usb.serialPort).getReadEndpoint().getMaxPacketSize());
+        assertEquals(usb.serialPort.getWriteEndpoint().getMaxPacketSize(),
+                     usb.serialPort.getReadEndpoint().getMaxPacketSize());
         s = usb.serialPort.getSerial();
         // with target sdk 29 can throw SecurityException before USB permission dialog is confirmed
         // not all devices implement serial numbers. some observed values are:
diff --git a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/CommonUsbSerialPort.java b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/CommonUsbSerialPort.java
index 62ba500e..354712db 100644
--- a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/CommonUsbSerialPort.java
+++ b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/CommonUsbSerialPort.java
@@ -66,16 +66,10 @@ public int getPortNumber() {
         return mPortNumber;
     }
 
-    /**
-     * Returns the write endpoint.
-     * @return write endpoint
-     */
+    @Override
     public UsbEndpoint getWriteEndpoint() { return mWriteEndpoint; }
 
-    /**
-     * Returns the read endpoint.
-     * @return read endpoint
-     */
+    @Override
     public UsbEndpoint getReadEndpoint() { return mReadEndpoint; }
 
     /**
diff --git a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/UsbSerialPort.java b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/UsbSerialPort.java
index e18a769f..3ed94706 100644
--- a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/UsbSerialPort.java
+++ b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/UsbSerialPort.java
@@ -8,6 +8,7 @@
 
 import android.hardware.usb.UsbDevice;
 import android.hardware.usb.UsbDeviceConnection;
+import android.hardware.usb.UsbEndpoint;
 import android.hardware.usb.UsbManager;
 
 import androidx.annotation.IntDef;
@@ -74,6 +75,18 @@ enum ControlLine { RTS, CTS,  DTR, DSR,  CD, RI }
      */
     int getPortNumber();
 
+    /**
+     * Returns the write endpoint.
+     * @return write endpoint
+     */
+    UsbEndpoint getWriteEndpoint();
+
+    /**
+     * Returns the read endpoint.
+     * @return read endpoint
+     */
+    UsbEndpoint getReadEndpoint();
+
     /**
      * The serial number of the underlying UsbDeviceConnection, or {@code null}.
      *
diff --git a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/util/SerialInputOutputManager.java b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/util/SerialInputOutputManager.java
index ed7f0aea..42d74dc7 100644
--- a/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/util/SerialInputOutputManager.java
+++ b/usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/util/SerialInputOutputManager.java
@@ -34,7 +34,7 @@ public class SerialInputOutputManager implements Runnable {
     private final Object mReadBufferLock = new Object();
     private final Object mWriteBufferLock = new Object();
 
-    private ByteBuffer mReadBuffer = ByteBuffer.allocate(BUFSIZ);
+    private ByteBuffer mReadBuffer; // default size = getReadEndpoint().getMaxPacketSize()
     private ByteBuffer mWriteBuffer = ByteBuffer.allocate(BUFSIZ);
 
     public enum State {
@@ -62,11 +62,13 @@ public interface Listener {
 
     public SerialInputOutputManager(UsbSerialPort serialPort) {
         mSerialPort = serialPort;
+        mReadBuffer = ByteBuffer.allocate(serialPort.getReadEndpoint().getMaxPacketSize());
     }
 
     public SerialInputOutputManager(UsbSerialPort serialPort, Listener listener) {
         mSerialPort = serialPort;
         mListener = listener;
+        mReadBuffer = ByteBuffer.allocate(serialPort.getReadEndpoint().getMaxPacketSize());
     }
 
     public synchronized void setListener(Listener listener) {