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what is meant by high impedence output?

Discussion in 'Electronic Basics' started by [email protected], Mar 20, 2007.

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  1. Guest

    I'm doing a project and need to write the output vector of an IC. One
    particular 74 series IC is 74LS240 which is tri state buffer(http://
    rabbit.eng.miami.edu/info/datasheets/74LS240.pdf). However, in the
    function table, one of the pin output is high impedence. Does it means
    that the output if measured by a voltmeter is low or 0V?
     
  2. It means that the output is nearly disconnected in that case. The
    function table lists a series of input combinations and in some of
    then the output (I'm not looking at the data sheet, but just speaking
    generally here) will be 'tristated.' That means the output is left
    floating to a large degree. This mode allows other chips which may
    also have active outputs that are tri-statable to be combined
    externally.

    In the case of a tri-statable buffer, they are preparing it for the
    case where there may be multiple such buffers sharing the same bus and
    where only one of them, at most, will have their outputs enabled and
    active and all the rest will be tri-stated. No two of them (or more)
    should be active, though. Bad news, then. The tri-stating just
    "lifts the pins off the bus," so to speak. If this is the only such
    buffer whose output can be on the bus, you might be able to just stay
    away from tri-stating it and let it stay active all the time.

    Does that make sense?

    Jon
     

  3. Learn how to post links so they don't end up split onto several lines:

    <<http://rabbit.eng.miami.edu/info/datasheets/74LS240.pdf>


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  4. Jamie

    Jamie Guest

    it's like disconnecting it from the circuit. this allows you to have
    another device on the same line. for example an IO Input or Output!
    so you could have input logic connected to the output logic at the
    same time. when the output logic is not in the Z state, it will output
    and the connected input device will simply see it reflected back, when
    the output device is in hi-Z, this line can now be used for an input
    and the input logic which is still there will see the value from else where.
    etc.. i think you get it.
     
  5. Randy

    Randy Guest

    a high impedence circuit has has less current but more voltage, a low
    impedance path will allow more current but drop less voltage
     
  6. Robin

    Robin Guest

    A "high impedance output" is one that is a "weak ouput"; it cannot
    impart much "effect" on anything so if you measure it with your
    voltmeter then, it is possible that it may not be able to drive the
    needle off its stop so it might read 0V but it might not either e.g.
    "static electricity" also has a "high impedance".

    The opposite of this, a "low impedance output" is a "strong output";
    it can impart much "effect" e.g. a car battery.

    So without knowing anything about electronics, you could correctly
    guess that a watch battery will have a higher impedance output than a
    flashlight battery will have a higher impedance output than a car
    battery.

    The highest sort of output impedance would be... nothing at all.

    When a battery is new, its "output impedance" is low. When it gets old
    its "output impedance" gets higher.

    If you temporarily short-out a dry-cell, its output impedance will
    temporarily go high (as gasses build up inside) and then it will
    recover its original lower output impedance (as the internal gasses
    are absorbed).

    The slangy term "output impedance" is more of an "output resistance"
    in these cases.

    Without a clear understanding of "ouput/input impedance" you won't get
    anywhere so you are right to ask - more so than you realise. See
    Thevenin's (tay-venins) Theorem.

    Robin
     
  7. Me

    Me Guest

    No it means that if you push it it don't budge. If the pin goes high to 4.8
    volts it will only deliver a microamp. to use it you need a follower circuit
    that gives a big current for a small voltage change.A curent amplifier to
    make the 1 microamp input into a 100 milliamp output. called a cathode
    follower. or emitter follower, a darlington pair such as a 2n918 pushing a
    bcy45 would deliver the goods as a double emmitter folower.

    I made the viking possible and pushed apollo so they imprisoed me and
    chopped me brain out.

    go forth and help the brits with their mental problems. They need their
    brains removed, not workers of course, we do the work you get the money....

    go forth and multiply.
     
  8. NRen2k5

    NRen2k5 Guest

    Depending on the newsreader, even doing that, the link may still end up
    split. Best thing to do is use a service like TinyURL when posting long
    links to Usenet.
     


  9. Some people won't click on those TinyURL, or similar links because
    they have no idea what the real URL is. It could easily be to a site
    that will try to install spyware, or worse on your computer.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  10. That may be true in some cases, but in this case, the OP is talking
    about a TTL logic part with a "tri-state" output. With such parts,
    when the output is "tir-stated", or placed in a high-impedance
    condition, the output of the internal circuit is effectively
    disconnected from the output pin. The chip does not drive the output
    either high or low. this type of output allows the outputs of several
    chips to be connected to a common bus, provided that only one chip has
    its output enabled (not tri-stated) at a time.


    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
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