In the circuit shown in the figure, inputs A and B are each connected through an ideal diode to the output point Y, and a resistor connects Y to the ground. An input is either $0\ \text{V}$ (logic 0) or $5\ \text{V}$ (logic 1). This circuit works as

Fig. 1: Inputs A and B each pass through a diode pointing towards a common point, which is the output Y. A resistor R joins Y to the ground.