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7901A
LOGIC SYMBOLOGY.
A-1.
INTRODUCTION.
A-2.
This appendix covers basic logic information and
symbology as used in this manual and related manuals.
A-3.
LOGIC STATES.
A-4.
The logic signals are always in one of two possible
states, a "1" or a "0." These two states are also referred to
as high (H) or low (L). The high and low states reflect the
relative voltage levels of the signals; the high state is always
relatively more positive than the low state. Note that both
states may have actual voltage values that are positive, or
both may be absolutely negative; the significance is in the
relative levels of the two states. In the text of the manuals,
logic states are normally described as "high" or "low."
A-5.
The "not" bar associated with signal names is used
to indicate whether the "active" state of the signal is high or
low. For example, if the presence of data on a signal line is
represented by a low signal, the signal name for the line
might be "not" Data 1; if a signal clears the output register
when the signal is low, the signal might be described as
"not" Clear Output Register (COR). The "not" bar must
be considered an integral part of the signal name; this means
that there are high states for "not" signals and low states
for "not" signals, just as there are high and low states for
signals without the "not" bar.
A-6.
lOGIC SYMBOLOGY.
A-7.
Three basic symbol shapes distinguish the major
classes of logic circuits depicted in this manual. These are
gates, regenerative switching elements, and .amplifiers. Each
symbol and a brief explanation of its operation is given in
the following paragraphs.
A-B.
In addition to the basic symbols, a general multi-
purpose symbol is used wherever a standardized logic sym-
bol does not exist. A brief explanation of this multipurpose
symbol is included.
A-g.
INVERSION.
A-10.
Logic inversion is indicated by an inversion dot at
the input or output of a logic symbol. When this dot appears
at the input of a logic symbol, the input will be effective
when the input signal is low. When the dot appears at the
output of a logic symbol the output will be of the opposite
state to what would be delivered if the dot were not present.
A-11.
GATES.
A-12.
A gate is a circuit that produces a binary output
when certain input conditions are met. The gate symbol has
input lines connecting to one side of the symbol, and out-
put lines connecting to the other side, as shown in figure
A-l. Since the inputs and outputs are easily identifiable,
the symbol can be shown left-facing, right-facing, or facing
up or down.
A-13.
There are four basic types of gates: "and," "or,"
"nand," and "nor," each named for the logic function that
it performs. Each of these gates is described in the following
paragraphs. In addition, a brief explanation of an "expander"
gate is given following the descriptions of the basic logic gates.
INPUTS
OUTPUT
INPUTS
OUTPUT
7900-105A
Figure A-l. Gate Symbols
A-14.
"AND" GATE.
A-15.
The "and" gate shown in figure A-2 performs a
logical "and" function.
It
will produce a high output only
when all of the input lines are high. Input A and input Band
input C must be high for a high output to be generated.
:=0
D
A
B
C
D
L
L
L
L
L
L
H
L
L
H
L
L
L
H
H
L
H
L
L
L
H
L
H
L
H
H
L
L
H
H
H
H
7900-106
Figure A-2. Three-Input "And" Gate Logic Symbol
and Truth Table
A-1
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