Implementation of Decoder using Logic Gates.docx -...

Implementation of Decoder using
Logic Gates
Objectives:
• To implement 2 X 4 Decoder.
• To implement 3 X 8 Decoder.
• Circuit equipments:
• IC 74LS139
• IC 74LS138
Back ground theory:
Decoder:
The name “Decoder” means to translate or decode coded
information from one format into another, so a binary decoder
transforms “n” binary input signals into an equivalent code using
2 n outputs.
Binary Decoders are another type of digital logic device that has
inputs of 2-bit, 3-bit or 4-bit codes depending upon the number of
data input lines, so a decoder that has a set of two or more bits
will be defined as having an n -bit code, and therefore it will be
possible to represent 2 n possible values. Thus, a decoder
generally decodes a binary value into a non-binary one by setting
exactly one of its n outputs to logic “1”.
If a binary decoder receives n inputs (usually grouped as a single
Binary or Boolean number) it activates one and only one of
its 2 n outputs based on that input with all other outputs
deactivated.

So for example, an inverter ( NOT-gate ) can be classed as a 1-
to-2 binary decoder as 1-input and 2-outputs (2
1
) is possible
because with an input A it can produce two outputs A and A (not-
A) as shown.
Then we can say that a standard combinational logic decoder is
an n-to-m decoder, where m ≤ 2
n
, and whose output, Q is
dependent only on its present input states. In other words, a
binary decoder looks at its current inputs, determines which
binary code or binary number is present at its inputs and selects
the appropriate output that corresponds to that binary input.