DLD-1.docx - EGEE245L - L06: ENCODER DECODERS USING 74155...

EGEE245L - L06: ENCODER AND
DECODERS USING 74155 CHIP
Introduction
Decoder is a combinational circuit that has ‘n’ input lines and maximum of 2n output lines. One
of these outputs will be active High based on the combination of inputs present, when the
decoder is enabled. That means decoder detects a particular code. A demultiplexer is the opposite
of a multiplexer. There is one data input, whose value appears on one of the data outputs,
depending on the value of the control inputs. A demultiplexer (or demux) is a device that takes a
single input line and routes it to one of several digital output lines. A demultiplexer of 2n outputs
has n select lines, which are used to select which output line to send the input. A demultiplexer is
also called a data distributor.
Lab Tasks (Simulations)
I. Review 2-TO-4 Decoder ( Using Standard Logic Gates )
In the 2 to 4 line decoder, there is a total of three inputs, i.e., A
0
, and A
1
and E and four outputs,
i.e., Y
0
, Y
1
, Y
2
, and Y
3
. For each combination of inputs, when the enable 'E' is set to 1, one of
these four outputs will be 1.

II. Two 2-TO-4 Decoder Using 74LS155 Chip: ( Show Datasheet

III. Single 3-To-8 Line Decoder ( Just a Different Configuration of the Chip )
The 3 to 8 line decoder is also known as Binary to Octal Decoder . In a 3 to 8 line decoder, there
is a total of eight outputs, i.e., Y
0
, Y
1
, Y
2
, Y
3
, Y
4
, Y
5
, Y
6
, and Y
7
and three outputs, i.e., A
0
, A1,
and A
2
. This circuit has an enable input 'E'. Just like 2 to 4 line decoder, when enable 'E' is set to
1, one of these four outputs will be 1.

IV. 1-TO-4 De-Multiplexer ( Using Standard Logic Gates )

V. 1-TO-4 De-Multiplexer Using the 74LS155
Conclusion
A multiplexer and a decoder can be used together to allow sharing of a data transmission line by
a number of signals. Decoders are simply a collection of logic gates which are arranged in a
specific way so as to breakdown any combination of inputs to a set of terms that are all set to '0'
apart from one term. A demultiplexer is used to connect a single source to multiple destinations.