When I connect a single CP210x device to my PC, I can communicate with it.  When I connect two or more devices, I can no longer communicate with any of them.  What am I doing wrong?

In order for the operating system to identify each CP210x that is simultaneously connected to a PC, each CP210x must have a unique combination of Vendor ID (VID), Product ID (PID), and product serial string.  In most applications, each CP210x device is given the same VID and PID with unique serial number values. 

To write unique identifying information to non-volatile CP210x storage space, follow the instructions in the 'Customizing CP210x Drivers to recognize new VID/PID' Knowledge Base article in the Related Articles links.

Note that CP210x devices are assigned a VID/PID of 0x10C4/0xEA60 by default.  The default VCP drivers have been written to look for all devices connected to the bus with this default VID/PID combination.  If a different VID/PID combination is used for a CP210x, a corresponding custom driver must also be generated to work with the VID/PID in the devices. 

To use the Silicon Laboratories VID and request a unique PID, follow the instructions found in the 'Obtaining a Unique PID' Knowledge Base article in the Related Articles links.