Play Meter

By Paul Crankshaw
of Electronic Amusement Service
Learning logic circuits
INPUT
OliTPUf
0
I
1
0
INVERTERS
Figure 1
All your digital equipment , from
Pong to Sea Wolf II to the electronic
pin games, are constructed from a
few basic circuits. The microproces-
sor games have other more complex
circuits but still use many of the basic
circuits. Once you learn the basic
logic circuits you will be able to
understand and determine the oper-
ation or detect the non -operation of
the digital game .
A logic circuit performs some
specific logic functions on binary
data. Binary data is information in
the form of O's and 1's. A logic 0 is a
lo or ground and a logic 1 is a hi or
+ 5 v. level. Ajgic data ~be
termed as A or A. (or 1H or 1H) ; a
signa! name may mean anything , but
single letters are easier for examples .
Data A is lliLnerally an originating
data signal ; A~ pronounced A or A
NOT, is its complement.
Data A can be a 1 or a 0 or pulsing
data . Data that has been ANDED
~S§ther is represented as A • B or
• . Data that has been ORED is
represented as A+ B or A + S'.
Signal names with a • between them
is the result of ANDing , and signal
names with a + between them is the
result of ORing . A truth table is a
chart that shows all possible input
states and the resulting outputs of a
logic circuit.
The simplest logic circuit is the
inverter or NOT circuit. The output
of an inverter is always the opposite
or the complement of its input. If the
inJllit is A the output isA, if the input
isAthe output is A.
An inverter has one input and one
output that is its complement . The
circuit symbol itself is the triangle
while the circle on the output
designates the inverter (Figure One) .
The same symbol (the triangle)
without the circle would be a buffer
where the output would be the same
as the input.
The AND gate (Figure 2A) is a
multiple input device that has an
active H1 or 1 output when all the
inputs are Hi. The truth table shows
that any LO or 0 on any input will
result in a LO out, and only a H 1 on
all inputs will give a 1 out. The same
is true for an AND gate with more
than the two inputs shown . The
NAND gate (Figure 2B) output is
exactly opposite that of the AND
gate . Both inputs must be a 1 for an
active LO out. Simply it would be an
AND gate whose output goes
through an inverter.
Where the AND gate can be
represented mechanically by two
switches in series, the OR gate can be
represented by two parallel switches .
The OR gate device will have an
active Hi output when anyone of its
inputs is an active Hl. Only when
both A and B inputs are a LO will the
output be a low . The more common
NOR device again operates exactly
opposite the OR gate as can be seen
by comparing the truth tables of
Figures 2C and 2D .
AND GATE
A
=iCY-
OR GATE
i[Y-
c
NAND GATE
u~
B
I
LO
LO
HI
HI
2
LO
HI
LO
3
LO
LO
LO
HI
HI
I
2
3
LO
LO
HI
HI
LO
HI
LO
HI
HI
HI
HI
I
LO
LO
HI
HI
2
LO
3
HI
HI
HI
HI
NOR GATE
LO
LO
HI
HI
D
EXCLUSIVE OR GATE
E
~
F'lgure 2
LO
LO
HI
HI
LO
HI
LO
LO
2
3
LO
HI
LO
HI
HI
2
3
LO
HI
LO
HI
HI
HI
LO
LO
LO
LO
LO