FIGURE 23. The 74LS244 Quad buffers can be used as (a) simple Schmitt buffers, or as (b) three-state ganged Schmitt buffers.
FIGURE 22. Functional diagram and truth
table of the 74LS244 (or 74HC244) Octal
(dual Quad) three-state Schmitt buffer IC.
as a normal buffer by grounding its C
terminal (Figure 10), or as a three-state
buffer that drives a common bus line
by using it as shown in Figure 11.
If you need up to six simple CMOS
buffers, one of the cheapest ways to
get them is via a 4050B or 74HC4050
Hex buffer IC. Figure 12 shows the IC’s
functional diagram; each buffer can
source up to 10 mA or sink up to 40
mA of output current when the IC is
powered from a 15V supply.
If you need up to six three-state
CMOS buffers, one option is to use a
4503B Hex buffer IC. Figure 13 shows
the functional diagram and truth table
of this versatile IC, in which pin 1 acts
as a DISABLE input that controls four
of the six buffers,
and pin 15 acts as a DISABLE input
that controls the other two buffers.
Note that each buffer element acts as
a normal buffer when its DISABLE pin
is at logic 0 (low), and goes into the
high-impedance output state when its
DISABLE pin is at logic 1 (high).
Thus, this IC can be used as a
simple Hex buffer by wiring it as
shown in Figure 14 (with pins 1 and
15 grounded), or as a Hex three-state
buffer that is controlled via a single
input by wiring it as shown in Figure
15 (with pins 1 and 15 shorted together and used as a DISABLE input).
Figure 16 shows the functional
diagram of the 7407, which is a
standard TTL Hex buffer in which each
buffer has an open-collector output
that can sink up to 40 mA and can be
connected to a supply of up to 30V via
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an external current-limiting pull-up
resistor (but the actual IC must use a
5V supply). Figure 17 shows how one
of these buffers can be used as a 5V to
high-voltage (up to 30V) non-inverting
interface. Figure 18 shows how three
o.c. buffers can be made to act as a
wired AND gate by wiring all three outputs to the same pull-up resistor. The
circuit action is such that the output is
pulled low when any input is low, and
only goes high when all three inputs
are high, thus giving an AND action.
Figure 19 shows the functional
diagram and basic truth table of a
74LS365 Hex three-state buffer IC, in
which all six buffers share a common
AND-gated control line. This IC can be
used as six normal buffers by grounding its two control pins as shown in
Figure 20, or as a set of six three-state
buffers that are all switched via one
common control signal as shown in
Figure 21; AND-type three-state
control can be obtained by using both
Control terminals (pins 1 and 15).
Finally, the 74HC241, 74HC244,
and 74LS244 are ‘Octal’ three-state
Schmitt buffers in which the buffers are
split into two groups of four, with the
mode of each group controlled via a
separate input. Figure 22 shows the
functional diagram and truth table of the
74LS244 IC (the 74HC244 is similar, but
it has Schmitt-type CA and CB inputs).
The IC is really a dual Quad device,
in which buffers 1-4 are controlled via
the CA terminal, and buffers 5-8 are
controlled via the CB terminals. Each of
these Quads can be used as a set of
simple Schmitt buffers by grounding its
control terminal as shown in Figure
23(a), or as a ganged set of three-state
Schmitt buffers by using its control terminal as shown in Figure 23(b). NV
76
April 2007
