What are the diffence between active and passive pull ups supplied at the output of logic devices?

Question

and what are some advantgaes and disadvantages of them.

Answer 1

A passive pull up is simply a resistor on an output. If nothing is pulling the output to ground, you'll get a high. However, the resistor's value must be high enough to not interfere with the active output devices and that limits the amount of current you can draw. For a practical example, many devices can sink (ground) enough current to light an LED, but can't source (+ voltage) enough to light an LED which is why many circuits turn on an LED by grounding it. Lower resistance values also can hurt power consumption.

An active pull up is a transistor and can be turned on when needed (e.g., CMOS or domino logic). This is good for power consumption and for supplying current through the pull up. However, it takes more space on the die (or another active device if you are not on an IC).