FIGURE 15. NOR gates
used to make a two-input
FIGURE 16. Functional diagram of the 74LS86
or 74HC86 Quad EX-OR gate IC.
FIGURE 17. Functional diagram of the 4070B
Quad EX-OR gate IC.
FIGURE 20. An EX-NOR
gate made from two TTL
FIGURE 18. Two ways of disabling unwanted
TTL EX-OR gates (see text).
FIGURE 19. Ways of using TTL EX-OR gates
as (a) buffers or (b) inverters.
FIGURE 21. Functional diagram of the 4077B
Quad EX-NOR gate IC.
FIGURE 22. Functional diagram of
the 74HC266 Quad EX-NOR gate IC with
open-drain (o.d.) outputs.
FIGURE 23. Four-bit True/Complement generator. FIGURE 24. Four-bit logic state comparator.
only when its two inputs are at different
logic levels. The most widely used
EX-OR gate ICs are the 74LS86 TTL and
the 74HC86 and 4070B CMOS Quad
70 July 2007
types (see Figures 16 and 17). If one or
more of a CMOS EX-ORIC’s gates are
unwanted, it can be disabled by simply
grounding both inputs. In the case of a
TTL EX-OR IC, unwanted
gates are best disabled by
grounding one input terminal
and tying the other high via a
1K resistor, as shown in
Figure 18, as this results in
minimum quiescent current-consumption. Alternatively, if
current drain is not important, both inputs can simply
be tied to ground as shown.
EX-OR gates are quite
versatile. An EX-OR gate can
be made to act as a non-inverting buffer by simply
grounding its unused input,
as shown in Figure 19(a), or
as an inverting buffer by tying
the unused input high (via na
1K resistor in TTL types), as
shown in Figure 19(b). Thus,
two EX-OR gates can be used
to make a single EX-NOR
gate by connecting them as
shown in Figure 20, where
the right-hand gate is used to
invert the output of the left-hand gate. Note that Quad
EX-NOR CMOS ICs are also
available in dedicated IC
forms as the 4077B and the
74HC266 (see Figures 21
and 22), but that the latter
IC’s gates have open-drain outputs.
Figures 23 and 24 show two other
useful EX-OR gate applications. In
Figure 23, four EX-OR gates are fed