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Switch and Buffer

Discussion in 'Electronic Basics' started by Wong, Nov 3, 2003.

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  1. Wong

    Wong Guest

    I have to use a Bus Switch/Buffer for some uni-directional signals.
    So under this circumstances, Bus Switch/Buffer should be the same for
    me because both devices control the output using an OE pin as well.
    But I believe there should be some differences between these 2
    devices. Please let me know if you know the differences. What about if
    my signals are bi-directional ?
  2. Prasad

    Prasad Guest


    The main difference between a bus switch and a buffer is that a bus
    switch does not provide active current drive, whereas a buffer does.
    A bus switch uses a pass transistor structure, so it is like a
    conducting wire (with some finite ON resistance) when ON and an open
    switch when OFF.

    OE ---------|
    | |
    Input _____| |______ Output

    A CMOS buffer has a push-pull type output, with P-channel and
    N-channel transistors at the output. When the output is high, the
    upper P-channel is turned on, and lower N-channel is turned off. When
    the output is low, the lower N-channel is turned on, and upper
    P-channel is turned off. The outputs can be disabled using the OE

    | |
    |--- |---|
    |-O| |-O|
    | |---| | |---|
    Input -----| |----| |-----Output
    | |---| | |---|
    |--| |--|
    |---| |---|
    | |

    Consider using a buffer if you are driving a heavy load or long
    transmission line. You can use a transceiver function (SN74LV245A, for a bi-directional interface.

    If you only need to channel a signal from the input to output, then a
    bus switch would work fine. The propagation delays with bus switches
    are very minimal which is an added advantage.
  3. Wong

    Wong Guest

    Hi Prasad,
    I am a bit of confusing. If a bus switch is just like a wire when
    it's ON, then the signal should be uni-directional or bi-directional.
    But you indicated that a bus switch would work fine if it is channel a
    signal from the input to output(unidirectional).
    I am considering a bus switch for the signals since I don't want to
    control the direction flow with a control signal. If a bus switch
    works in both directions, it will be the right choice.
    Thanks Prasad for the pointer.
  4. Prasad

    Prasad Guest


    Sorry to have confused you. You are correct - bus switches can be
    used for bi-directional signals. I should've indicated "I/O1 to I/O2"
    instead of "Input to Output"

  5. Wong

    Wong Guest


    Thanks a lot for the info.
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