A pull up is a resistor that tends to pull a line towards a positive level as in +v. A pull down attempts to pull a line towards ground level. The choice of a pull up or pull down is often selected based on many factors. Some are, open collector outputs, or if line may go tri-state, or the driver needs help sourcing or sinking current, these are but a few reasons.
hi, could you elaborate on this 'open collector outputs'? i'm confused with sourcing and sinking current and never able to comprehend the application and the interface. hope to get some advice. Thanks