- Some types of digital outputs can only work in one direction, they
can either source or sink current but not both. Maybe an example
would help, your water faucet can source a flow of water by turning
the handle but it will not suck water back into the faucet. That is
an example of a current source. A pull down resistor adds a small
drain in the bottom of our example.
In the world of electronics, is it more common to have an output that
can sink current but cannot source any current. They are usually
called open collector or open drain outputs. If you have a high
output level on an open drain device, you will not see a change in
the output voltage. A high level means that it has stopped sinking
current but since it cannot source current the voltage remains low. A
resistor connected from a voltage source to the output allows the
output go to a high voltage level as soon as the output turns off.
This is called a pull up resistor.
--- In Electronics_101@y..., "dNixon9" <mk5852@h...> wrote:
> Could someone explain to a struggling novice what is meant by "pull-
> up" and "pull-down" resistors and what their functions are?