Unformatted text preview: 42 Dual Gating Functions, Symbolic Notation and Practical Gate Applications Now that we have seen the same circuit can perform
different or dual logic functions, we must pro—
vide a means for circuit analysis. We would
show the 'AND function with truth table as in
Fig. 42a and the OR function with truth table
in Fig. 4—2b. "MW Y (#1 n I
I .
3)— lb) Fig. 42 Note that the integrated circuit identified by the
7408 number and the truth tables are identical.
The circuit function of Fig. 42a is the AND
function symbol and the absence of circles at
inputs and outputs indicate that when A and B are
HI then Y is HI. The circuit function of Fig. 42b
is the OR function, the symbol is the OR func
tion symbol and the presence of circles at the
inputs and outputs indicate that when A or B or
both are LO then Y is L0. The dual symbols for each gate are given in Table
41. Again, note that the truth tables are identical
for the dual pairs. The logic functions differ as
indicated by the logic symbols and the input!
output states. Note that an input A shown in the
symbol without a circle is indicated A for the
circuit function and the truth table shows A
must be 1. Similarly, input A shown in the symbol
with a circle is indicated as A for the circuit func
tion and the truth table shows A must be 0. This
also follows for B and Y in the table. Symbolic
notation and gate applications will become more
clear as we discuss practical decision making
circuits. Additional Reading See the bibliography in the back of this manual
for additional reading material on logic gates,
dual functions and complementary inputs. Yis‘lwhenAorBis‘I Y=A+B Yis‘lwhenAandBareO A Y=A§ V=A YislwhenAISO Y=A NOTE: A, B AND Y ARE (LO) WHERE INDICATED BY CIRCLES
TO OBTAIN THE INDICATED SYMBOLIC FUNCTION: Table 41 Equipment and Materials
The Model ASl8145 Digital Logic Trainer with:  Hex Inverters (7404)  2input NAND Gates (7 400)
 4input NAND Gates (7420)
2input AND Gate (7408)  2input OR Gates (7432)  Switches  LED Displays Niacin—4min».
I Objective A. Determine the logic states and gating
functions required for a MAJORITY VOTE sys
tem and verify that the proper logic decisions
are made. Preparatory Information. The circuit shown in
Fig. 43 may be used to determine if a majority
of voters has voted “FOR” or “AGAINST” a
particular issue. For simplicity, we will only con
cern ourselves with three voters (A, B, C) and any ...
View
Full Document
 Fall '05
 Myer,B

Click to edit the document details