Devantech USB-RS485 USB to RS485 Interface Module
The USB-RS485 is a complete, self powered, USB to RS485 interface working at any baud rate up to 250Kbaud. It features automatic bus turn-around - any time you’re not transmitting the module is listening.
Uses the MAX487 quarter power tranceiver, so you can have up to 128 devices on the bus.
USB interface uses the FTDI chip which has drivers available for Windows XP64, XP, 2000, ME, 98, CE.NET V4.2, Linux, Mac OS8, OS9, OSX.