A multiplexer is a combinational circuit having many inputs which allows for the outputting a single data input at a time by using selection input lines. A multiplexer has 2ⁿ data input lines , n select input lines and a single output.

Discussion by analogy : A Television remote control used to access 4 channels

Teacher describes a remote control with 2 buttons that can be toggled on and off to switch between 4 channels

A 4 to 1 multiplexer

The truth table is shown below:

s1	s2	Output at F
0	0	x­1
0	1	x2
1	0	x­3
1	1	x­4

Note that the data from any input line can be either a 0 or 1 instantaneously depending what the input at a the time. For example, let us consider the situation where over the next 4 cycles of execution on a cpu that

• dataline x_­1 produces 0011 on each cycle respectively,
• dataline x_­2 produces 1100 on each cycle respectively; and
  
• during cycles 1 and 2, s₁ and s₂ are 0 and 0 respectively
• during cycles 3 and 4, s₁ and s₂ are 0 and 1 respectively

The resulting output at f over the four cycles are 0000.

That is, two bits of data from x_­1, then two bits of data from x_­2.

In sequence , we selected x₁ for 2 consecutive cycles, then x₂ for another 2 consecutive cycles.

Special Note on Spelling: MultiplexEr or MultiplexOr ?

Using the Oxford English Dictionary, the original spelling is multiplexer, as evidenced by it’s widespread use. However it’s use in quotation is as early as 1961, the spelling as multiplexor appeared in quotation in 1957.

Despite this, It’s generally accepted that the original spelling is multiplexer, and multiplexor should be considered a variation on the.

See:

MultiplexEr: https://www.oed.com/search/advanced/Quotations?textTermText0=multiplexer&textTermOpt0=QuotText&quotDateFirstUse=all&page=1&sortOption=DateOldFirst

MultiplexOr: https://www.oed.com/search/advanced/Quotations?textTermText0=multiplexor&textTermOpt0=QuotText&quotDateFirstUse=all&page=1&sortOption=DateOldFirst

Attribution to media used in this post

User:Mdd4696, Public domain, via Wikimedia Commons

Updates to this post

2023/11/25-

Added Special Note on Spelling: MultiplexEr or MultiplexOr ?

Changed Main spellings from multiplexor to multiplexer in note and URL

© 2021  Vedesh Kungebeharry. All rights reserved.