Synopsis:
I was having errors when certain applications (e.g. PuTTY) tried to open COM ports in Windows VM for CP210x-based USB-to-UART devices.
Error messages such as "Unable to open connection to COM4. Unable to open serial port" would appear, even though Windows Device Manager indicated ports were working correctly.
The solution I found is to change the USB compatibility setting of the VM from USB 3.0 to USB 2.0.
Details:
VMware Workstation Pro 15 on a Windows 10 host.
Need to use some USB-to-UART serial devices from within Windows 10 and Windows 8.1 VMs.
The tools used (PuTTY, RealTerm, ESP-IDF) will access these as COM ports (e.g. COM5).
When USB compatibility of the VM is the default of 3.0 AND the serial devices are Silicon Labs CP210x parts, the applications get an error when trying to open the COM port, even though Windows shows the port in Device Manager as working normally.
FTDI-based USB-to-UART serial devices don't show any issue.
Example Steps if USB compat set to 3.0:
1. Install Silicon Labs CP210x virtual com port (VCP) drivers
2. Attach CP210x USB-to-UART bridge device, e.g. AdaFruit CP2104 Friend or Sparkfun ESP32 Thing Plus.
3. See in Windows Device Manager, Ports, that it shows as driver working and an assigned COM port number, e.g. COM4, COM5.
4. Launch a program such as PuTTY (or RealTerm, ESP-IDF, etc.) which tries to open the COM port. It will fail and an error message occurs, such as
"Unable to open connection to COM4. Unable to open serial port"
