I have found that when I connect to a USB device that is sending data to the PC that a write to the serial port takes a few microseconds, however, if the USB is not sending data to the PC then the write to the serial port takes more time. the extra time can be changed by adjusting the port timeout. The device responds to the message and sends the requested data but it take twice the timeout setting before the write is completed. If other data is already being sent by the device the write takes microseconds to complete. Why would the write be dependent on receiving data? I am using the high resolution counter before and after the Visa write to measure the times.
