1.1 Open-Drain for Bidirectional Communication

Open-drain refers to a type of output which can either pull the bus down to a voltage (ground, in most cases), or "release" the bus and let it be pulled up by a pull-up resistor. In the event of the bus being released by the master or a slave, the pull-up resistor (R_PU) on the line is responsible for pulling the bus voltage up to the power rail. Since no device may force a high on a line, this means that the bus will never run into a communication issue where one device may try to transmit a high, and another transmits a low, causing a short (power rail to ground). I²C requires that if a master in a multi-master environment transmits a high, but see's that the line is low (another device is pulling it down), to halt communications because another device is using the bus. Push-pull interfaces do not allow for this type of freedom, which is a benefit of I²C.

Figure 2. Basic Internal Structure of SDA/SCL Line

Figure 2 shows a simplified view of the internal structure of the slave or master device on the SDA/SCL lines, consisting of a buffer to read input data, and a pull-down FET to transmit data. A device is only able to pull the bus line low (provide short to ground) or release the bus line (high impedance to ground) and allow the pull-up resistor to raise the voltage. This is an important concept to realize when dealing with I²C devices, since no device may hold the bus high. This property is what allows bidirectional communication to take place.