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When is high not high.

Discussion in 'Electronic Basics' started by Chris W, Feb 19, 2005.

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  1. Chris W

    Chris W Guest

    I have my inventory of stuff I got on that ebay auction so I decided to
    build something and see if it would work. I think I learned something,
    but I'm not sure exactly what and was hoping some one could tell me. I
    took my MH7442, a 10 LED bar thing-a-majig, a set of dip switches and a
    resistor. Wired it up to display the LED 0 through 9, based on the
    Binary input from the dip switch. At first it didn't work. After some
    experimenting, I found what I thought was a high going into th BCD input
    of the MH7442 was not what it wanted. I had the switch set up so a low
    was the absence of a voltage source, what it really wanted, was that a
    high was the absence of a ground connection. My question is, is there
    some way I could have known this? Is there something in the data sheet
    that would tell me that?

    Chris W

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  2. For bipolar TTL logic chips, the inputs are effectively emitters of
    NPN transistors. Therefore, if the input is not connected, the chip
    will see it as a high. You must ground the input for the chip to see
    it as a low.

    For CMOS parts, an input is the insulated gate on a FET, and, if left
    unconnected, will float randomly between high and low. If the input
    stays in the "maybe" state (half way between high and low), the chip
    is likely to draw excessive current. Therefore, with CMOS parts, all
    inputs, whether you use them or not, MUST be connected to either
    ground or the positive supply, either directly of through a resistor.

    Peter Bennett, VE7CEI
    peterbb4 (at)
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  3. Yes. If this is the same as a 74HC42, then the line over the Ys means
    they are 'active low'. That means that an active output is 0 rather than 1.

    Thus, if you input a number b0010, then the /Y1 output will be 0, the
    rest will 1.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  4. Andrew Holme

    Andrew Holme Guest

    The 7442, without letters between the 74 and the 42, belongs to the original
    TTL family. What you have observed is characteristic of TTL inputs. LS
    devices behave similarly. This might not be mentioned explicitly on the
    7442 datasheet because it is a feature of the whole family, not just that
    particular device. A good book, such as "The Art of Electronics" by
    Horowitz and Hill, would explain all.
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