USB to RS-232 Convertor
This is the commercial "USB to RS-232 Convertor" schematic. The heart of the circuit is a PL-2303 IC produced by Prolific Technology Inc. It provides all the necessary functions and signals to use it easy in the USB projects. Also you can download its free Driver to reduce programming complexity.
Source:
Other circuits
USB power control switch
USB interface board
USB RS232 converter
USB to Serial Interface
USB Data acquisition by PIC18F4550
USB to 5V SEPIC DC-DC Converter with Short-Circuit Protection
Control stepping motor via USB interface by PIC18F4550