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.
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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.
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Power up a 7400 chip.
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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.
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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.