Question : Problem: What is a PULL-UP resistor?

Hi,

I am a software engineer trying to learn hardware. When reading
about the I2C protocol for PIC programming, I came across the following (see 2nd
sentence before the end of the paragraph):

"An open-drain or open-collector pin has output drivers that can
only pull the signal line to ground. They cannot drive it high. This
has the advantage that more than one device connected to a signal
line may pull it low. If this were not the case, one device
attempting to pull the line low while another tried to pull it high
would result in a short circuit, with disastrous results. Interrupt
lines are typically open-collector. All open-collector signals need
a PULL-UP RESISTOR and are active low. The idle state (when no
device is asserting) is to be pulled high by the resistor."

I understand most of what is being said, but don't understand why
the resistor is specifically named as such. What other types of
resistors are there?

Regards,

Todd
Alameda, CA 94501

Answer : Problem: What is a PULL-UP resistor?

Hi toddsab,
    That is simple.. With positive logic, +5 V is HIGH state and 0 V is LOW state. The resistor will keep the voltage at that point at +5 V (If there is no resistor, HIGH and LOW will get shorted). Since it pulls the voltage to +5 V, it is called PULL UP resistor.

Bye
----
Harish