Digital Electronics: Principles, Devices and Applications

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 ...