On 22 October 1981, Philips filed a European patent application for a communication system between integrated circuits that would later become known worldwide as the I²C bus. Forty-five years later, the basic principle is still going strong. Two lines, one for data and one for the clock, continue to connect countless chips in embedded systems, consumer electronics, cars and industrial applications. In the late 1970s, I was working at the Central Application Laboratory of Philips Elcoma in Eindhoven. One of the problems we encountered was data exchange between integrated circuits. More complex ICs often already had some form of serial communication, but their interfaces weren’t compatible. Systems designers who wanted to combine different components had to deal with multiple protocols and different electrical implementations. In 1979, Herman Schutte and I began exploring a common way for different integrated circuits within a system to exchange information. The usual requirements of consumer electronics played an important role. The solution had to use few pins, require little additional hardware and, above all, be inexpensive to implement. Various ideas and technical choices gradually came together. Eventually, we found that communication could be achieved using just two lines: a serial data line and a clock line. With that, the fundamental concept behind I²C was born. Two Wires A great deal has been written over the past decades about how I²C works technically. Much less extensively documented is its precise origin story: what problem Philips initially wanted to solve, how the first ideas emerged, what roles the various people involved played and how an internal solution eventually grew into a globally adopted interface. In “ Two wires – the story of the I²C bus ,” Ad Moelands reconstructs that history using original sources. Moelands worked at Philips in Eindhoven in the late 1970s and developed the bus together with Herman Schutte. An important source for the book is testimony the two men gave in 2002 in connection with a patent dispute between Philips and several IC manufacturers. More than four decades later, those transcripts make it possible to look back at the early years of I²C with considerable accuracy. When Moelands decided to write the book, he tried to contact Schutte and learned that his former colleague unfortunately had passed away in 2021. That makes Schutte’s 2002 testimony all the more valuable. Parts of it are included in the book, ensuring that his own recollections remain part of the story. Simplicity Of course, two connections alone don’t make a usable communication system. When multiple ICs share the same bus, each must be individually addressable. That meant we needed addressing. We also had to prevent connected ICs from electrically interfering with one another. This led to the architecture in which connected circuits can actively pull the bus lines low, while resistors pull them back high. Other questions followed. How do you indicate the beginning and end of a transmission? How does a receiver acknowledge that data has arrived? How do you accommodate components that operate at different speeds? Practical challenges like these ultimately shaped many of the features we now automatically associate with I²C. Throughout the process, simplicity remained a key criterion. No matter how elegant an interface might be from a technical standpoint, it would be less attractive for consumer electronics if it required too much chip area or too many external components. The MAB8400 test setup MAB8400 A decisive moment came with Philips’ development of the MAB8400 microcontroller. Shortly before that, the company had licensed Intel’s microcontroller designs. The intention wasn’t simply to copy those designs. Elcoma also wanted to develop its own derivatives based on them. Suddenly, the I²C interface became much more than an interesting technical solution. The serial I/O interface for I²C was one of the first major d