1
EC262 Topic 2: Introduction to Combinational Systems
Much of a computer’s hardware is comprised of digital logic circuits.
Digital logic circuits are built from just a
handful of primitive elements, called logic gates, combined in various ways.
In a digital logic circuit, only two values may be present.
The values may be
5
−
and + 5 volts.
Or the values
may be 0.5 and 3.5 volts.
Or the values may be… you get the picture.
To allow consideration of all of these
possibilities, we will say that digital logic circuits allow the presence of two logical values: 0 and 1.
So, signals in a digital logic circuit take on the values of 0 or 1.
Logic gates are devices which compute
functions of these binary signals.
The Three Basic Logic Gates
AND Gate
.
Consider the circuit below which has 2 switches in series:
Switch S1
Switch S2
Light
When will the light turn on?
We can represent the light's operation using a table:
S1
S2
Light
open open
off
open shut
off
shut open
off
shut
shut
on
Change the words
open
and
off
to
0
and the words
shut
and
on
to
1
and the table becomes
S1
S2
Light
This is the
truth table
for the AND operation.
0
0
0
0
1
0
1
0
0
1
1
1
The simplest AND gate gas two inputs and one output.
If both inputs are
1
then the output is
1
.
Otherwise,
the output is
0
.
The symbol for the AND gate is:
We represent the AND operation by using either the multiplication symbol (i.e., “
∙
“) or by writing the
inputs together.
Thus, for the AND gate above, we would write the output
c
as
c
a b
=
or as
c
=
ab
.
This would be pronounced: “
C
=
a
and
b
.”

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