8
Multiplexers and Demultiplexers
In the previous chapter, we described at length those combinational logic circuits that can be used
to perform arithmetic and related operations. This chapter takes a comprehensive look at yet another
class of building blocks used to design more complex combinational circuits, and covers building
blocks such as multiplexers and demultiplexers and other derived devices such as encoders and
decoders. Particular emphasis is given to the operational basics and use of these devices to design more
complex combinational circuits. Application-relevant information in terms of the list of commonly used
integrated circuits available in this category, along with their functional description is given towards
the end of the chapter. The text has been adequately illustrated with the help of a large number of
solved examples.
8.1 Multiplexer
A multiplexer or MUX, also called a data selector, is a combinational circuit with more than one
input line, one output line and more than one selection line. There are some multiplexer ICs that
provide complementary outputs. Also, multiplexers in IC form almost invariably have an ENABLE
or STROBE input, which needs to be active for the multiplexer to be able to perform its intended
function. A multiplexer selects binary information present on any one of the input lines, depending
upon the logic status of the selection inputs, and routes it to the output line. If there are n selection lines,
then the number of maximum possible input lines is 2
n and the multiplexer is referred to as a 2
n -to-1
multiplexer or 2
n
× 1 multiplexer. Figures 8.1(a) and (b) respectively show the circuit representation
and truth table of a basic 4-to-1 multiplexer.
To familiarize readers with the practical multiplexer devices available in IC form, Figs 8.2 and 8.3
respectively show the circuit representation and function table of 8-to-1 and 16-to-1 multiplexers. The
8-to-1 multiplexer of Fig. 8.2 is IC type number 74151 of the TTL family. It has an active LOW
ENABLE input and provides complementary outputs. Figure 8.3 refers to IC type number 74150 of
the TTL family. It is a 16-to-1 multiplexer with active LOW ENABLE input and active LOW output.
Digital Electronics: Principles, Devices and Applications Anil K. Maini
© 2007 John Wiley & Sons, Ltd. ISBN: 978-0-470-03214-5
Multiplexers and Demultiplexers
In the previous chapter, we described at length those combinational logic circuits that can be used
to perform arithmetic and related operations. This chapter takes a comprehensive look at yet another
class of building blocks used to design more complex combinational circuits, and covers building
blocks such as multiplexers and demultiplexers and other derived devices such as encoders and
decoders. Particular emphasis is given to the operational basics and use of these devices to design more
complex combinational circuits. Application-relevant information in terms of the list of commonly used
integrated circuits available in this category, along with their functional description is given towards
the end of the chapter. The text has been adequately illustrated with the help of a large number of
solved examples.
8.1 Multiplexer
A multiplexer or MUX, also called a data selector, is a combinational circuit with more than one
input line, one output line and more than one selection line. There are some multiplexer ICs that
provide complementary outputs. Also, multiplexers in IC form almost invariably have an ENABLE
or STROBE input, which needs to be active for the multiplexer to be able to perform its intended
function. A multiplexer selects binary information present on any one of the input lines, depending
upon the logic status of the selection inputs, and routes it to the output line. If there are n selection lines,
then the number of maximum possible input lines is 2
n and the multiplexer is referred to as a 2
n -to-1
multiplexer or 2
n
× 1 multiplexer. Figures 8.1(a) and (b) respectively show the circuit representation
and truth table of a basic 4-to-1 multiplexer.
To familiarize readers with the practical multiplexer devices available in IC form, Figs 8.2 and 8.3
respectively show the circuit representation and function table of 8-to-1 and 16-to-1 multiplexers. The
8-to-1 multiplexer of Fig. 8.2 is IC type number 74151 of the TTL family. It has an active LOW
ENABLE input and provides complementary outputs. Figure 8.3 refers to IC type number 74150 of
the TTL family. It is a 16-to-1 multiplexer with active LOW ENABLE input and active LOW output.
Digital Electronics: Principles, Devices and Applications Anil K. Maini
© 2007 John Wiley & Sons, Ltd. ISBN: 978-0-470-03214-5