Unformatted text preview: Performance Objectives A. Identify a BCD to Decimal DECODER,
demonstrate that a combination of logic
gates can be used to decode BCD to deci
mal numbers and verify the DECODER
operation. B. Analyze some trouble symptoms of a de
coder circuit and signal trace the circuit to
verify erroneous outputs. Basic Concepts 1. A decoder circuit is used to recognize the
various combinations of an input word
and provide an output for each com
bination. 2. If an input word contains N “bits” then
the decoder can have 2” outputs. 3. A combination of gates can be used to
construct a decoder circuit. 4. A BCD to Decimal decoder is used to pro—
vide decimal information from a binary coded input. Introductory Information A decoder can be thought of in the opposite
sense as an encoder. You should recall from the
encoder exercise that a decimal number was coded
so that digital operation could be performed using
binary numbers. Now we will see that binary in—
formation which was processed in a coded form
by digital circuits can be recognized or decoded
by a decoder circuit. The resulting outputs will
represent the original input code. A typical ex
ample of this is a BCD to Decimal decoder. Table
61 shows the decimal equivalents of a four
“bit” binary word. In addition it shows the states
of the four inputs ABCD to a decoder required
to represent decimal numbers 0 through 9. Notice that the table shows a four bit binary word
can produce up to 16 distinct outputs. A general
rule for decoders is that when the number of inputs
is equal to a number N, the decoder will produce
2” outputs. For Table 61, N equals 4 and there
will be 24 or 16 outputs. In this Laboratory Exer
cise, however, we need only produce one digit of
a decimal number or the decimal symbols 0
through 9. Laboratory Exercise 6
THE DECODER OUTPUT
DECIMAL NUMBER DIGITAL LOGIC
STATE
4 2 I mmewlmwmlmlmwwmw UUUUUOUUOIUIOIUICIUIUIUIUW
monomololoioooomoloiolo
>>>>l>>l>>lﬁ>>l>>l
LDCONIOEU‘obOJNHO >D>3>Il>l mmwlw Table 61 Fig. 61 shows a typical application for a BCD to
Decimal decoder. The functions shown will pro
vide a two digit numeric readout using Nixie ® tubes
as display devices. The requirement is to provide
asignal to energize the proper number,zero through
nine of the Nixie tube, when a certain number of
input pulses are counted in Binary Coded Decimal
form. We will discuss the counter and readout de
vices in later exercises and concentrate on the
decoder requirements. That is, a four bit BCD
word exists as inputs to the decoder and outputs
zero through nine are required for Nixie tube
drivers. The circuitry for a second digit is shown
in block form, is identical to the first digit and
accepts a pulse following each zero through nine
sequence of the first digit. Simultaneously, a reset
pulse within the counter will initiate the BCD 61 ...
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 Fall '05
 Myer,B

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