The RS-485 wiring standard & protocol is very well established for industrial applications, and the Devantech USB-RS485 lets you bridge this easily to your PC's USB port!
The Devantech USB-RS485 is a self powered USB to RS485 interface that bridges between a USB virtual COMM port and a RS485 network. The USB-RS485 communicates at up to 250k baud, and features automatic bus turn-around by turning on the high impedance listening mode. It uses the MAX487 RS485 driver transceiver to allow up to 128 devices on the bus.
The USB side of the interface uses the very well established FTDI chip, which has drivers available for Windows XP64, XP, 2000, ME, 98, CE.NET V4.2, Linux, Mac OS8, OS9, OSX.