Why doesn't the CP210x manufacturing DLL work when I have the CP210x COM port opened?
The manufacturing DLL disables the UART interface on the CP210x when opened via the DLL. This is because the device does not support UART communication and PROM customization at the same time.
During runtime, with a COM port open, use the CP210xRuntime.dll to control GPIO. Users cannot customize the CP210x device when the COM port is opened.