Hi all ! I did a device using the STM32f103, managed by a GUI from windows PC. I did my protocol and so on. STM32, after any command, answers with an ACK. Then the PC sends the following command and so on. Now, doing a test about the communication speed, I noted that when the PC receives an ACK, waits about 15 ms, and then sends the following message. I did the GUI using VB. Than I tried to use the C++. the results is the same. Anytime Windows introduces this wait. Is this a problem due by Windows or the VCP_V1.3.1 ?? There is possibility to send commands faster, with no waits ? I am looking for the faster USB communications for STM32F103 ! Thanks for all !!