We have used the virtual com port driver with an STM32F1 chip for 3 to 4 years without problems. Recently some Windows 7 desktop PCs are not communicating properly. The USB chip will be detected and the com port will be set up but the port will only send and receive one command and then stop communicating. It remains in the device manager and is enumerated but it will not communicate. Powering off the chip and restarting will cause the same behavior. This has happened with several windows 7 PCs and with several chips so it is not a hardware failure problem. On some of the PCs with problems, the virtual com port will work on the front USB ports but not on the rear ports. On other PCs no USB ports will work. Has anyone seen this behavior or have any suggestions? It does not appear to depend on the PC vendor i.e. we have seen it with a couple of PC manufacturers.