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Logic Symbology
A-l6.
"OR" GATE.
A-l7.
The "or" gate performs a logical "or" function.
It
produces a high output when one
or
more inputs are high.
The truth table in figure A-3 shows the various states of a
three-input "or" gate.
A
L
L
L
L
H
H
H
H
79OO-107A
~=[)-o
B
C
0
L
L
L
L
H
H
H
L
H
H
H
H
L
L
H
L
H
H
H
L
H
H
H
H
Figure A-3. Three-Input "Or" Gate Logic
Symbol and Truth Table
A-l8.
"NAND" GATE.
A-l9.
The "nand" gate is similar to the "and" gate de-
scribed previously, except that its output is inverted. The
gate generates a low output when all inputs are high. The
various states of a three-input "nand" gate are shown in the
truth table in figure A-4.
7900-108
A-20.
A
D
B
0
C
A
B
C
0
L
L
L
H
L
L
H
H
L
H
L
H
L
H
H
H
H
L
L
H
H
L
H
H
H
H
L
H
H
H
H
L
Figure A-4. Three-Input "Nand" Gate Logic
Symbol and Truth Table
"NOR" GATE.
A-21.
The "nor" gate is identical to the "or" gate de-
scribed previously, except that its output is inverted. The
gate generates a low output when one or more inputs are
high. The various states of a three-input "nor" gate are
shown in the truth table in figure A-5.
A-2
A
L
L
L
L
H
H
H
H
7900-109A
A
D-0
B
C
B
C
0
L
L
H
L
H
L
H
L
L
H
H
L
L
L
L
L
H
L
H
L
L
H
H
L
Figure A-5. Three-Input "Nor" Gate Logic
Symbol and Truth Table
A-22.
"EXCLUSIVE OR" GATE.
7901A
A-23.
The "exclusive or" gate is a variation of the basic
"or" gate.
It
has two or more input signals. The output is
high when only one input is high. The truth table in figure
A-6 shows the functioning of a three-input exclusive "or"
gate.
A
~D-0
B
c
A
B
C
0
L
L
L
L
L
L
H
H
L
H
L
H
L
H
H
L
H
L
L
H
H
L
H
L
H
H
L
L
H
H
H
L
7900-1 lOA
Figure A-6. Three-Input "Exclusive Or" Gate Logic
Symbol and Truth Table
A-24.
EXPANDER GATE.
A-25.
The expander gate provides a means for increasing
the number of inputs to a gate. Figure A-7 shows a simplified
method of illustrating this type of gate, and figure A-8 shows
the actual connections between the gates involved. The X
and
X
outputs of the expander gate are not logical opposites,
but they do carry a voltage differential with respect to each
other. When one or more inputs to the expander gate are
low, there is a voltage difference of a few volts between X
and
X.
When all inputs to the expander gate are high, the
voltage difference decreases; the two outputs of the ex-
pander then act as a high input to the expanded gate. The
actual output-voltage differential of the expander gate de-
pends on the type used.
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