WillR:
http://www.nxp.com/acrobat_download2/literature/9398/39340011.pdfhttp://ics.nxp.com/products/i2chubs/
Long Distance I2C or SMBus
The P82B715, P82B96, and PCA9600 are specifically designed to extend the reach of the I2C or SMBus over long distances, in the range of kilometers, depending on the maximum bus speed. This allows the designer to use I2C to interconnect equipment cabinets or for large area systems such as warehouse pick and place systems.
The 9600 works great for longer distances with little electromagnetic interference. Differential signaling is used in RS485 to transmit a single over two complementary signals sent over two paired wires. The 9600 chip breaks the I2C clock and data signals into inbound and outbound data lines. The Am26LS32 and AM26LS31 can be used to then turn each of the directional signals into a differential pair. To do this, I have used the i2C breakout board (http://sjtbits.com/i2c-to-differential-breakout-board/) by sjtbits.com. If believe they have the schematics on their site if you are interested in the wiring details.