USB serial bridge chips connect to a PC using its USB port, but appear as a virtual COM port to the application running on the PC. A custom driver, supplied by the chip vendor, is required to do this COM-USB transformation. A Windows application that talks to a serial (COM) port (e.g., HyperTerminal) can be used to talk to an USB-connected device using this method. The advantage of this approach is that no enumeration firmware or host driver is required. The disadvantages are in performance, flexibility, and support:
Performance: Because the bridge approach emulates a serial port, the maximum achievable bandwidth is about 1 Mbps, well below the USB signaling rate of 12 Mbps.
Flexibility: The serial-USB bridge chips are hard-wired to emulate serial-port devices. They are not capable of implementing standard Windows class devices (like HID) or custom device types.
Support: Designing a product using one of these chips will require the companion driver to be installed by your customer. Because it is a custom driver, it is not guaranteed to work with future versions of the operating system. If you choose this approach, try to make sure that the vendor is committed to supporting the driver for the lifetime of your product.