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Logic Gates

Discussion in 'Electronic Basics' started by Bart, Dec 12, 2005.

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  1. Bart

    Bart Guest

    I look at all the symbols for the different logic gates, NAND, NOR, XNOR,
    etc. and on some of them they have a little circle on the output. I look at
    datasheets for different IC's and I see the same little circle on some of
    the pin designations. My electronics books show these but I can't find what
    it means.
    Any help is appreciated,
  2. Richard

    Richard Guest

    First, if your book doesn't cover this material, you need a different book.
    You'll see these circles on inputs and outputs in various schematic

    Try a Google search for 'logic gate symbols'.

    The first hit I got was, and there
    were several others.

    Richard Seriani
  3. A small circle on either input or output means an inversion takes
    place, there. So an OR gate with a ball on the output means that the
    output is low when any input is high, instead of high when any input
    is high. It just takes the output from an OR gate and inverts it.
    The result is called a NOR (not OR). It would have been more logical
    if it had been called a ORN (OR not) but that is harder to pronounce,
    I guess.
  4. flank

    flank Guest

    An excellent resource when working with a digital IC is the device's
    datasheet. There are many places to find the datasheets, but one in
    particular is You will need to do a search for
    whichever part you are looking. The sheets are often many pages,
    containing quite a bit more information than you might need. But the
    first page (sometimes the second) shows the truth table for the device.
    The truth table is simply a table indicating all the possible inputs
    to the chip, and the resulting output. The datasheet also has the
    pinout diagram, which is always nice to have.

  5. Bart

    Bart Guest

    Thanks a bunch, makes sense now! Oh, if the output is high I assume I
    can source an led to turn on, and if an output is low, I can sink an led to
    come on?

  6. No.
  7. Bart

    Bart Guest

  8. "Rats" spelled backwards is "Star."
  9. Source, yes. Usually.

    Sink, no. Unless it's an open collector.
  10. Jasen Betts

    Jasen Betts Guest

    yes, depending in the device some devices can't source much or can't sink
  11. Usually, yes.
    A typical logic gate is capable of sourcing or sinking enough current
    to drive a typical LED.
    The output voltage may be out of spec when drawing this much current,
    but that doesn't matter if you are just driving a LED and not another
    logic gate as well.
    Some gates have open-collector outputs which can only sink current.

    Dave :)
  12. flank

    flank Guest

    Not all chips supply/sink the same amount of current. For the most
    part, ICs cannot supply or sink much current at all. An led needs
    around 5-15 mA. This is stretching it for some ICs. A 7404 (hex
    inverter gate) can sink a pretty fair amount a current. I typically
    wire my LEDs such that the high pin is at 5V, and the low pin is tied
    to the output of an inververter gate through a 330 ohm resistor. Then,
    when I send a high signal to the input of the hex gate, the output goes
    low and turns of the LED.
    Also, it is a good idea to isolate the signal used to drive your LEDs
    away from the logic signals. An LED can draw enough current from a
    gate to cause the gate to malfunction.

  13. Rich Grise

    Rich Grise Guest

    No, it's because it's not an "Or Not". An "Or Not" has only one input
    inverted. (X = A + /B) ;-)

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