Page 103 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes changed. However, in standard decoder type circuits the address input will determine which
single data output will have the same value as the data input with all other data outputs having
the value of logic “0”.
The implementation of the Boolean expression above using individual logic gates would require
the use of six individual gates consisting of AND and NOT gates as shown:
Figure 6.12: 4 Channel Demultiplexer using Logic Gates

The symbol used in logic diagrams to identify a demultiplexer is as follows:
Figure 6.13: Demultiplexer Symbol

Standard Demultiplexer IC packages available are the TTL 74LS138 1 to 8-output demultiplexer, the
TTL 74LS139 Dual 1-to-4 output demultiplexer or the CMOS CD4514 1-to-16 output demultiplexer.
Another type of demultiplexer is the 24-pin, 74LS154 which is a 4-bit to 16-line demultiplexer/
decoder. Here the individual output positions are selected using a 4-bit binary coded input. Like
multiplexers, demultiplexers can also be cascaded together to form higher order demultiplexers.
Unlike multiplexers which convert data from a single data line to multiple lines and demultiplexers
which convert multiple lines to a single data line, there are devices available which convert data
to and from multiple lines and in the next tutorial about combinational logic devices, we will
look at Encoders which convert multiple input lines into multiple output lines, converting the
data from one form to another.

Draw the circuit diagram and truth table of 2-to-1 multiplexer.

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