Buffer,, Gate, and Logic
by Ray Marston
PART 4: NOR Gate, EX-OR, and EX-NOR Gate Circuits
This month, Ray Marston describes practical digital NOR gate,
EX-OR gate, and EX-NOR gate logic ICs.
FIGURE 1. A simple NOR gate can be made
by feeding the output of a diode OR gate
through a transistor or IC inverter stage.
FIGURE 2. Ten popular NOR gate ICs.
FIGURE 4. Functional diagram of the 4001B
or 4001UB Quad two-input NOR gate IC.
68 July 2007
The first three installments of
this five-part series explained
modern TTL and CMOS logic
gate basics and gave practical
descriptions of some of the most
popular digital buffer, inverter,
AND gate, NAND gate, and OR
gate digital ICs that are available.
This month, we expand on this
theme and describe a variety
of popular NOR gate, EX-OR
gate, and EX-NOR gate ICs that
are available from either your
local supplier or from specialist
Digital IC Circuits
The output of a digital NOR
gate goes low when any of its
inputs (A or B, etc.) go high.
One easy way to make a
NOR gate is to combine a
basic diode OR gate with a
transistor or IC inverter
stage, as shown in the three-input NOR gate circuits of
Figures 1(a) and 1(b). NOR
gates of this type are
reasonably fast and cost-effective and can easily be
expanded to accept any
desired number of inputs by
simply adding one new
diode for each new input.
Figure 2 lists basic
details of the 10 most popular NOR gate ICs; of these,
the 74LS02, 74HC02,
4001B, and the unbuffered
4011B (see Figures 3 and 4)
are all Quad two-input
types. The 4025B and the
74LS27 and 74HC27 (see
Figures 5 and 6) are Triple
three-input types, and the
FIGURE 3. Functional diagram of the 74LS02
or 74HC02 Quad two-input NOR gate IC.
FIGURE 5. Functional diagram of the 4025B
Triple three-input NOR gate IC.