Multiplexers and De-Multiplexers ©Paul Godin Updated Sept 2009

Definitions Multiplexer: A device that selectively forwards a single source of data from multiple sources. Often called a MUX. Input Signals Output signal Selection

Definitions De-Multiplexer: A device that forwards an input signal to a selected output. Often called a DEMUX. Input Signal Output signal Selection

Multiplexer – Selected input to single output A B C D S0 S1 Y A B C D Output Rotary selector switch Mechanical Equivalent Logic Diagram

De-multiplexer – Single input to selected output A A Y Input B B C C D D Rotary selector switch S0 S1 Mechanical Equivalent Logic Diagram

MUX and DEMUX Combination A mechanical combination MUX and DEMUX is often used when evaluating stereo equipment in a store. Unit #1 Unit #2 Unit #3 Unit #4 Speakers #1 Speakers #2 Speakers #3 Speakers #4 Selector (MUX) (DEMUX) Direction of data

MUX and DEMUX Combination A combination MUX and DEMUX is used in data communications.

Multiplexers

Examples of Multiplexers Examples of selectively forwarding an input signal to an output includes: Selection switch on a home stereo, where one selects between several sources to be sent to the amplifier and speakers. A multi-line telephone where one line is selected. A video surveillance monitor that switches between cameras. Multiplexers are used extensively in data and voice communication networks

Multiplexers Digital applications for multiplexers include external and internal applications (steering gates). Multiplexers are often used in conjunction with other digital circuits such as decoders, counters and other multi-function devices.

A Simple Multiplexer (2-input) Z B A If S = 0, value A is forwarded If S = 1, value B is forwarded Boolean Expression: Z = S’A + SB

A 4-to-1 Multiplexer S0 S1 Z B C D A

A 4-to-1 Multiplexer with Enable S0 S1 Z B A C D En’ What is the Boolean Expression for this circuit? Build a function table for this circuit

A multi-channel MUX Channel 1 a b c d Channel 2 En S A1 B1 C1 D1 A2 B2

Multi-Channel MUX in a Digital Alarm Clock V Decoder Decoder Decoder AM PM Alarm / Real Time Decoder Digital Clock Real Time 13 bits Digital Clock Alarm Time 13 bits The MUX permits the sharing of the displays and the decoders.

A 7-Segment Display for Multiplexing c b a d e f g Common: Digit 1 Common: Digit 2 c b a d e f g Common: Digit 1 Common: Digit 2 To display a digit enable the common for that digit only.

Multi-Channel MUX in a Shared Display Oscillator Select Decoder 10’s counter 1’s counter Oscillator With a high enough frequency, the digits appear to both be lit at the same time.

4-to-1 MUX in a Basic Frequency Selector Binary Value S0 S1 ÷2 250Hz ÷2 500Hz Output ÷4 1kHz ÷1000 4kHz The MUX selects the output frequency. 4MHz Crystal

Questions What is the difference between a multiplexer and a decoder?

De-Multiplexers De-Multiplexers are often used in conjunction with other digital circuits such as decoders, counters and other multi-function devices.

Selects whether input “A” goes to “X” or “Y” A simple De-MUX X A Y Selects whether input “A” goes to “X” or “Y” S

A Simple De-Multiplexer (2-output) If S = 1, value Z is forwarded to A If S = 0, value Z is forwarded to B A A S B B Z Boolean Expression: A = SZ Boolean Expression: B = S’Z

Build a function table for this circuit A 1-to-4 De-Multiplexer S0 S1 Z B C D A Build a function table for this circuit

A multi-channel De-MUX b1 Channel 1 A c1 B d1 C a2 D b2 Channel 2 c2 d2 S

Address De-Multiplexer Microprocessors often multiplex their address bus with their data bus. This is to save on pins and physical size. At times, address information is on the bus and at other times data is on the bus. μP

Address De-Multiplexer AD Bus Memory ADDRESS DATA Control The Control output from the microprocessor indicates whether the bus is in Address mode or in Data mode.

Questions What is the difference between a de-multiplexer and an encoder?

From Texas Instruments, Specification sheet for SN74LS194 Questions Identify the following as MUX or De-MUX From Texas Instruments, Specification sheet for SN74LS194

From Texas Instruments, Specification sheet for CD74AC139 Questions Identify the following as MUX or De-MUX From Texas Instruments, Specification sheet for CD74AC139

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