I²C

The original system was created in the early 1980s as a battery control interface, but it was later used as a simple internal bus system for building prototypes using various Philips chips. It used only two bi-directional pins, clock and data, both running at +5V and pulled high with resistors. The bus operated at 100 kbps in standard mode but also include a 10 kbps low-speed mode which seemed useful in some cases (at the time). Although much slower than most bus systems, the advantages were that chips could be plugged in or removed with the systems still running, making for very rapid prototyping.

The basic bus has a seven-bit address space, allowing up to 112 nodes on one bus (16 of the 128 addresses are reserved). In 1992 the first standardized version was released, v1.0. This added a new fast mode at 400 kbps and a ten-bit addressing mode to support up to 1024 nodes. v2.0 from 1998 added high-speed mode at 3.4 Mbps, while reducing the voltage and current requirements when run in that mode (thus saving power as well as being faster). The latest v.2.1 from 2001 is a minor cleanup of 2.0.

I²C was also used as the basis for ACCESS.bus and VESA's monitor data interface.

External links:

I²C Bus / Access Bus (http://www.interfacebus.com/Design_Connector_I2C)