4002B and 74LS260 (see
Figures 7 and 8) are Dual
four-input and five-input
types, respectively. The
4078B (Figure 9) is an eight-input NOR gate IC.
When using NOR gate
ICs, each unwanted gate
should be disabled by
shorting all of its inputs
together and tying them to
one of the IC’s supply lines.
In CMOS ICs, the shorted
inputs can be wired directly
to either supply line, but in
TTL ICs the inputs must
(to give minimum quiescent
current consumption with
good stability) be tied
directly to the ground rail, as
shown in Figure 10.
Sometimes when using
NOR gate ICs, you may not
want to use all of a gate’s
input terminals. In this case,
the unwanted inputs are
best disabled by shorting
them directly to ground, as
shown in the examples of
Figure 11. A NOR gate can
be made to act as a simple FIGURE 10. Ways of disabling TTL NOR gates.
inverter by either shorting all of its
inputs together or by grounding all
but one of its inputs; note, however,
that the fan-in of a TTL NOR gate is
directly proportional to the number of
inputs used, so the first method is thus
(theoretically) the best, since it offers
the lowest fan-in value, as shown in
NOR gates are fairly versatile
devices. The effective number of inputs
of a NOR gate can be increased by
applying the extra inputs via a diode or
IC OR gate, as shown in Figure 13.
Figure 14 shows various ways of using
two-input elements to make two-input The output of an
or three-input OR gates or a three- EX-OR gate goes high
input NOR gate. Note that a NOR gate
can be converted into an AND gate by
simply inverting all of
its inputs. Figure 15
shows how three
two-input NOR gates
can be used to make a
single AND gate.
FIGURE 6. Functional diagram of the 74LS27
or 74HC27 Triple three-input NOR gate IC.
FIGURE 7. Functional diagram of the 4002B
Dual four-input NOR gate IC.
FIGURE 8. Functional diagram of the 74LS260
Dual five-input NOR gate IC.
FIGURE 9. Functional diagram of the 4078B
Dual eight-input NOR gate IC.
FIGURE 11. Basic ways of disabling
unwanted TTL NOR gate inputs.
FIGURE 12. Ways of using a TTL NOR gate as a simple inverter.
FIGURE 13. Ways of increasing the effective number of
inputs of a NOR gate.
Ways of using
gates to make
various OR and
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