I am working on a virtual COM project using USB CDC class. I have such trouble. After enumeration,PC sends the bulk IN package and the device responds NAKs. When the device wants to transmit data to the host,data will be put bulk in EP and the IN transfer is successful. But after that PC do not send Bulk IN package any longer. I don't know why the bulk IN transfer can occur only once.So long as the bulk IN tranfer succeeds once,the PC will never send bulk IN package any more. PC always can transmit data to the device via the bulk out endpoint very well. I implement the CDC class without the interrupt endpoint.Does this result in such problem?