Interface Solutions I2C-bus. Bipolar junction transistor. Reduced models of the operation modes. Modes of Operation. Diode-Transistor Logic (DTL). Resistor-Transistor Logic (RTL) Sinking vs. Sourcing'Sink' means that the chip 'sinks' current down into itself. • 'Source' is just the reverse, the I/O pin is the actual source of the current. •