One of the benefits of I²C using an open drain is that bus contention does not put the bus into a destructive state. With an open-drain output, many devices can be connected together without destructive contention. For any output on that connection, if any output pulls the line low, the line is low. This kind of connection is called a wired-AND connection. The output is the logical AND of all the outputs when tied together.

If the outputs are a push-pull type, the outputs cannot be tied together without the possibility of a destructive state. A push-pull output (often used for SPI communication) has complementary NMOS and PMOS transistors that drive the output high or low. Figure 2-4 shows a comparison between open-drain and push-pull outputs in contention.

With the open-drain connection, any device can pull the connection low at any time. The line appears low any time any device pulled the line low, but does not appear as destructive contention.

In the push-pull output, the outputs are also tied together. If two devices are active on the bus and one output is high and another output is low, this bus contention has an undetermined state, possibly settling at the mid-supply point. Additionally, one device has NMOS conducting current and another device has a PMOS conducting current. These devices source current from V_DD to GND through a very low impedance path, conducting as much current as the transistors allow. The result of this contention can be a significant amount of current, potentially damaging the devices.