question about disconnecting output on a gate

Discussion in 'Homework Help' started by robotguy123, Oct 26, 2014.

  1. robotguy123

    Thread Starter New Member

    Oct 26, 2014
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    hello,

    if i have a TTL and gate and i disconnect one of the inputs, i understand that it floats to a 2v level and gets read in a high state.

    but what happens if , for example, an output of that gate is disconnected? does it remain the same , or does it also float to a high state?

    for example, if an AND gate is reading 2 high inputs and the output is disconnected, does it output a high output as it should , or something else?

    thanks :)
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    TTL totem pole outputs don't float. They have low output impedance. The logic output will be dictated by the logic function of the device and the loading effects of any load connected to the outputs.

    Thus for 2-input AND gate with high inputs, the output should be high, Vcc - 0.5V approx. However this can change depending on the amount of current drawn from the output. Read the spec sheets of the specific device. It will tell you how much current you can source from the output.
     
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Output depends on input. If nothing is connected to output, it will just sit there and be whater the input tells it to be.

    Example.
    Let say you have a NOT gate.
    If you feed Low to the input, the output is High. You have nothing connected to the output. The output is still High. You can attach a voltmeter and see that output is High.
    If you feed High to the input, the output is Low. You have nothing connected to the output. The output is still Low. You can verify it with a voltmeter.
     
  4. robotguy123

    Thread Starter New Member

    Oct 26, 2014
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    [​IMG]
    assuming these are all and gates, if pin 1 and 2 were reading high and pin 5 was not connected, what would pin 7 be reading for an input??
     
  5. MrChips

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    Oct 2, 2009
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    When an input is not connected, consider the logic to be indeterminate. Hence always connect unused inputs unless you are certain the function has no effect. Always connect all unused CMOS inputs otherwise the penalty can be high power consumption.
     
  6. robotguy123

    Thread Starter New Member

    Oct 26, 2014
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    well its for a troubleshooting assignment, so i just wanted to clear it up.. but telling by that picture, would pin 7 be reading high or low? if pin 5 was not connected and pin 1 and 2 were high
     
  7. MrChips

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    Oct 2, 2009
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    If pin 5 is not connected then it doesn't matter what is on pins 1 and 2.

    As you said in your opening post, unconnected TTL inputs tend to float to 2V which would be considered a logic high.
     
  8. WBahn

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    Mar 31, 2012
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    You need to recognize that if Pin 5 is unconnected, then that means it is unconnected from Pin 7 and therefore has absolutely no bearing on what Pin 7 is or is not. For TTL, Pin 7 is simply an unconnected input that should be pulled high internally (though, as already stated, this is a poor idea to do intentionally).
     
  9. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Pin 7 would be floating. Therefore the state of Pin 7 is not predictable.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    Remember that he specified TTL. While it is poor design to leave a TTL input floating, the behavior IS predictable (provided there isn't too much noise, which there usually isn't). This is NOT the case for most CMOS families.
     
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