Index: /trunk/BNC/src/serial/win_qextserialport.cpp
===================================================================
--- /trunk/BNC/src/serial/win_qextserialport.cpp	(revision 10938)
+++ /trunk/BNC/src/serial/win_qextserialport.cpp	(revision 10939)
@@ -151,5 +151,7 @@
     if (!isOpen()) {
         /*open the port*/
-        Win_Handle=CreateFileA(port.toLatin1(), GENERIC_READ|GENERIC_WRITE,
+        /* the \\.\ prefix is required for COM10 and above, see Microsoft KB115831 */
+        QString devName = "\\\\.\\" + port;
+        Win_Handle=CreateFileA(devName.toLatin1(), GENERIC_READ|GENERIC_WRITE,
                               FILE_SHARE_READ|FILE_SHARE_WRITE, NULL, OPEN_EXISTING, 0, NULL);
         if (Win_Handle!=INVALID_HANDLE_VALUE) {