Despite their obvious downsides, serial resistances can be useful sometimes when analyzing I2C bus traffic to track down I2C-related problems. As stated above, serial resistances Rs lead to different voltage levels during the low phase, depending on which device currently pulls SDA or SCL low. Thus, it is possible to recognize which I2C device is currently active by analyzing the low level on the I2C lines.The picture below shows a complete master receiver transfer between an I2C master and a slave device. Due to serial resistors, the low level of the slave device is lower than the one of the master device. This applies for both lines, SDA and SCL. Thus, it is possible to see easily that the slave device stretches the SCL line shortly during the acknowledge clock cycle (marked with “!”).
A complete master receiver transfer between an I2C master and I2C slave device with Rs = 200 Ω, measured at the slave device.