Pull Up Resistors

Discussion in 'General Electronics Chat' started by nanobyte, Oct 21, 2005.

  1. nanobyte

    Thread Starter AAC Fanatic!

    May 26, 2004
    What are pull up resistors and what are they used for? What is the difference between active and passive pull up resistors? Why can't an active pull up resistor be used in wired-collector logic for Diode Transistor Logic (DTL) and Transistor Transistor Logic (TTL).
  2. pebe

    AAC Fanatic!

    Oct 11, 2004
    Pull up resistors are used when the output of some device is an NPN transistor (or its equivalent FET) and is simply a switch. A switch won't do any good unless it has a load - the current through the load then gives a voltage drop across it. The voltage at the transistor/load junction will then give a logical '0' or '1'.

    The resistor is called a 'pull up' because when the device is switched off, the collector (drain) voltage is pulled up to V+.

    You cannot have an 'active' pull up resistor, because by definition a resistor is a passive device.
  3. beenthere

    Retired Moderator

    Apr 20, 2004

    You don't see pull-up resistors used in common-collector logic because the bus drivers are so much better than in the past. It was quite common in TTL computers to have all open collector logic on the data bus with a pull-up resistor on each line.

    Those were also the days when I remember an engineer friend having to hand select TTL chips that would clock at 40 mHz without gate-racing.