Digital
Logic Laboratory Exercises (2) - Gate Properties
Here
are some more things you can do to learn about the properties of gates.
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Observe the values
of a zero and a one.
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Learn how the input
of a gate acts.
Do the following
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Power up a 7400 chip.
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Connect a NAND gate
as an inverter. (You did that last week.)
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Using a variable
power supply increase
the input changing it slowly from zero volts to five volts. Note
where the output of the gate changes state.
-
Use an LED indicator
to show the output.
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Now slowly decrease
the input back to zero volts. Note again where the output of the
gate changes state.
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Does the gate change
state at the same point going up and going down?
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What range of voltage
inputs are truly zero and truly one?
Now, check what happens if you leave an input not connected.
-
Power up a 7400 chip.
-
Leaving one input
of a NAND gate "floating" - not connected to anything - put in a zero and
a one at the other input of the gate.
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Does the floating
input act consistently like a zero or a one?
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What happens to the
output when both inputs are floating?
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Note: A floating
input has nothing connected to it. If you had an LED circuit connected
to this input to show the signal state, disconnect that as well when you
want a floating input.
Finally, check the input of the gate.
-
Power up a 7400 chip.
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At a floating input,
measure
the voltage that appears at the input.
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Is the voltage zero?
If not, what is it.
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If the voltage isn't
zero, do the following.
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Measure the voltage.
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Put a resistor across
the input, and get a plot of voltage vs. current drawn.
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Determine how much
current can flow when the input is shorted to ground.
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Comment on your results.
Click here for a form to use.