Here is the first RS-485 monitor or adapter I built in small quanity. I started with a RS-485 board from Modtronix, and used their prototype daughter board to add a USB connector (note: required two simple cuts to get USB to work).
My firmware for the PIC18F2550 adapts USB serial to RS-485 and vice versa (bidirectional). It bridges the gap between the PC and 9-bit RS-485 protocols.
Also, here is a breadboarded version from scratch that uses identical firmware. The RS-485 A/B wires are not show, would leave the LTC1487 chip at pins 6/7. A USB B socket breakout board is used to help prototype the circuit.