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Logic gates usually come
packaged as integrated
circuits which have type
numbers such as 7400 or
4001.
They belong to
semiconductor families
such as TTL (transistor,
transistor logic) or
CMOS (complementary
metal oxide
semiconductor). The
names describe their
internal construction.
They are DIGITAL devices
not ANALOGUE.
A thermometer is an
analogue device because
it can record an
infinite number of
values such 100 degrees,
0.1 degrees or 34.354
degrees etc.
Other analogue devices
are a car speedo and a
Hi Fi amplifier (which
can handle lots of
different frequencies
and loudnesses.)
A digital device or
system uses only two
values. These can be
expressed in several
ways.
high or low
true or false
5 volts or zero
volts
on or off
1 or 0
etc.
A light switch and a rat
trap are digital
devices.
Most gates usually have
two or more inputs and
one output.
The state of the output
(high or low) depends
upon the combination of
the input states.
In the case of the gate
shown, the output will
only be high if both
inputs are high. If
either one input or both
inputs are low then the
output will be low
These characteristics
can be shown using a
TRUTH TABLE. In the
following example 1
indicates a high and 0
indicates a low.
Note that Z is only a 1
when A AND B are both
at 1.
There is a form of
mathematics associated
with logic gates called
BOOLEAN ALGEBRA.
It was invented a few
hundred years ago by Mr
Boole, before the days
of electronics. He used
it to solve problems in
logic.
For example
Some cats are black AND
black items cannot be
seen against a black
wall.Therefore it is
TRUE that some cats
cannot be seen against a
black wall.
Here is a Boolean
expression for the gate
shown. A . B = Z
Read this as IF A AND B
ARE HIGH THEN Z IS HIGH.
(The . is read as AND).
The most frequently used
gates are AND, OR, NAND,
NOR, NOT and EXOR.
An integrated circuit
containing 4 AND gates
each with 2 inputs is
called a QUAD 2 INPUT
AND ic. An ic with 6 NOT
gates is called a HEX
INVERTER ic.
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