The open-drain connections are used on both SDA and SCL lines and connect to an NMOS transistor. This open-drain connection controls the I²C communication line and pulls the line low or releases the line high. The open-drain refers to the NMOS bus connection when the NMOS is turned OFF. Figure 2-2 shows the open-drain connection as the NMOS is turned on.

To set the voltage level of the SDA or SCL line, the NMOS is set on or off. When the NMOS is on, the device pulls current through the resistor to ground. This pulls the open-drain line low. Typically, the transition from high to low for I²C is a fast transition as the NMOS pulls down on SDA or SCL. The speed of the transition is determined by the NMOS drive strength and any bus capacitance on SDA or SCL.

When the NMOS turns off, the device stops pulling current, and the pullup resistor pulls the SDA or SCL line to VDD. Figure 2-3 shows an open-drain line as the NMOS is turned off. The pullup resistor pulls the line high. The transition of the open-drain line is slower because line is pulled up against the bus capacitance, and is not actively driven.

Through control of this open-drain connection, both SDA and SCL can be set high and low, enabling the I²C communication.

Because of capacitance on the I²C communication line, the SDA and SCL lines discharge with an exponential settling RC time constant depending on the size of the pullup resistor and capacitance on the I²C bus. Higher capacitance limits the speed of I²C communication, the number of devices, and the physical distance between devices on the bus. A smaller pullup resistor has a faster rise time, but requires more power for communication. A larger pullup resistor has a slower rise time leading to slower communication, but requires less power.