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Basic Questions over the function of Logic Gates

  1. Jul 17, 2010 #1
    Hello, this is not homework. I am working on a personal project and had I think a fairly simple set of questions. They really are all the same, so answering one as true probably means the others are true as well, but if not please point out where and how I'm wrong.

    I have a very basic idea of how logic gates may work but I need to know if I am correct:

    For an And gate:
    100px-AND_ANSI.svg.png
    is the basic meaning that in order for the one right wire to output energy, both left wires must input energy?

    For an Or gate:
    100px-OR_ANSI.svg.png
    to output, one of the left must input?

    For a Not gate:
    100px-NOT_ANSI.svg.png
    input from the left is 'stopped' and there is no output on the right? (and so the circuit may continue to an And gate with one that does have output and the And gate would then not output energy either?)

    For a Nand gate:
    100px-NAND_ANSI.svg.png
    Only if there is energy input from both on the left, there will be no energy output on the right?

    For a Nor gate:
    100px-NOR_ANSI.svg.png
    Only if there is no energy input on the left, there will be energy output on the right?

    For a Xor gate:
    100px-XOR_ANSI.svg.png
    Only if the input on the left differs from each other (one inputs, the other doesn't), there will be energy output on the right?

    For a Xnor gate:
    100px-XNOR_ANSI.svg.png
    Only if the input on the left is the same for each (either both no energy, or both energy), there will be energy output on the right?

    I only need to know if I have the right idea, it doesn't matter weather 'energy' is not the right word, or if I need to have a source. Though I do need to know if what controls the circuit is that 'buttons' act as switches between going through no gate and going through a Not gate.
     
  2. jcsd
  3. Jul 17, 2010 #2
    Hi
    The subject heading for your query should be "Boolean Truth Tables", or "Logical Truth Tables".
    Rather than talking about energy, these tables are written in terms of Truth and Falsehood. A decent explanation can be found at http://www.bbc.co.uk/dna/h2g2/A1325251, and another take on the same subject is at http://archive.chipcenter.com/circuitcellar/august99/c89cd2.htm [Broken]
    Hope this is what you wanted.
    I remember showing my fellow techs a circuit designed using those logic symbols 25 years ago, and nobody had seen them.
     
    Last edited by a moderator: May 4, 2017
  4. Jul 17, 2010 #3
    Hi
    The subject heading for your query should be "Boolean Truth Tables", or "Logical Truth Tables".
    Rather than talking about energy, these tables are written in terms of Truth and Falsehood. A decent explanation can be found at http://www.bbc.co.uk/dna/h2g2/A1325251, and another take on the same subject is at http://archive.chipcenter.com/circuitcellar/august99/c89cd2.htm [Broken]
    Hope this is what you wanted.

    ...and you are about on the right track.

    BTW I remember showing my fellow techs a circuit designed using the logic symbols you have used 25 years ago, and nobody would admit they had seen them. Those symbols are now outdated!
     
    Last edited by a moderator: May 4, 2017
  5. Jul 18, 2010 #4
    Don't worry the bureaucrats have been trying to change the symbols for years, but useful symbols like these remain alive and well. Each generation of bureacrats invents a new set of symbols that gets lost in the mists of....

    However on a more important note you have a fundamental misconception about gates.

    The output energy source is not via the gate inputs as your post implies.

    The gate inputs are control terminals. The input and output have their own power sources. These may be from a common battery or supply but they are still considered separate.

    The implications of this are seen most clearly in the not or negation function. Thus a double Nand or Nor gate will give an output for no input. This can only happen if the output derives from a separate supply.

    In general the energy levels of the inputs are much lower than the energy levels of the outputs, or there would be little point in the circuitry.

    We normally talk about voltage rather than energy. The inputs and outputs are defined in terms of specific voltage (or in some rare cases current) levels, not specific energy levels.

    Hope this helps.
     
  6. Jul 18, 2010 #5
    Thanks, well I found a simulator site and it has cleared things up:
    http://richardbowles.tripod.com/dig_elec/tools/sim/sim.htm [Broken]
    it's kind of fun to play with.

    My only other question has also been answered above I guess, and that is: if this a widely known symbol system or is there other new symbols that are more well known?
     
    Last edited by a moderator: May 4, 2017
  7. Jul 18, 2010 #6
    Also, one other question: If a SINGLE destination (a light) was made and a BRANCHING TREE of gates (2 per previous 1 wire) went out and at the end there were a switch per wire, would you be able to turn the light on with only 2 switches? I have a feeling this is the case but am unsure.
     
  8. Jul 18, 2010 #7
    You seem to be asking whether lots of inputs can be logically combined to give a single output.
    The answer to that question is "Yes"'.
    Ripple-through logic is an important method of machine control, though it has largely been superceded by microprocessors.
     
  9. Jul 18, 2010 #8
    Given that I'm still using the CMOS logic data book I got from Nat'lSemi in 1980....

    Err...when did "they" (try to) change the logic gate symbols?
     
  10. Jul 18, 2010 #9
    Err...1960, 1970, 1980, 1990, 2000

    I think most decades have had their moments.
     
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