FIGURE 7. Nine popular
non-inverting buffer ICs.
FIGURE 6. Any two elements from
an inverter IC can be used to make a non-inverting buffer element.
FIGURE 9. All unwanted 74LS125
elements must be connected in one
of these ways.
it is sometimes still
useful to be able to
think in negative-logic terms, particularly
when designing gates
in which a low state
output is of special
interest. With this
point in mind, Figure 4
presents a basic set of
two-input positive and
negative logic equivalents. Thus, it can be
seen that a negative
logic AND gate action — in which the
output is low only when both inputs
are low — is directly available from a
positive logic OR gate, and so on.
FIGURE 10. Methods of using a
74LS125 element as a normal buffer.
FIGURE 8. Functional diagram and truth
table, etc., of the 74LS125 Quad three-state
buffer/bus-driver IC.
Positive Versus
Negative Logic
output symbol; the negated state is
called a not state; thus, a negated Y
output is called a not-Y output.
FIGURE 11. Methods of using a 74LS125
element as a three-state line-driving buffer.
All modern digital logic circuitry
assumes the use of the positive logic
convention, in which a logic 1 state is
high and a logic 0 state is low. In the
early days of electronic digital circuitry,
an alternative ‘negative logic’ convention — in which a logic 1 state is
low and a logic 0 state is high —
was also in common use, and
FIGURE 12. Functional diagram of the 4050B
or 74HC4050 Hex buffer IC.
Practical Buffer
IC Circuits
Digital buffer ICs have two main
purposes: to act either as simple non-inverting, current-boosting interfaces
between one part of a circuit and
another, or to act as three-state switching units that can be used to connect a
circuit’s outputs to a load, only when
required. If you ever need only a few
simple buffers, one cheap way to get
them is to make them from spare AND
or OR elements (as shown in Figure 5)
or from pairs of normal or Schmitt
inverters (as shown in Figure 6).
Figure 7 lists basic details of nine
popular, non-inverting digital buffer
ICs. When using these ICs, note that
all unused buffers must be disabled by
tying their inputs to one
of the IC’s supply lines.
In CMOS devices, the
unused inputs can be
tied directly to either
supply line, but in TTL
devices, it is best (for
lowest quiescent current consumption) to tie
all unused inputs high
via a common 10K resistor. If the unused buffer
is a three-state
type, it should
(if it has independent control) be set into
FIGURE 13.
Functional diagram
and truth table of
the 4503B Hex
buffer IC.
74
April 2007
