1. Multiplexers and De-Multiplexers
©Paul Godin
Updated Sept 2009

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

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

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

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

6. 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

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

8. Multiplexers

9. 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

10. 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.

11. 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

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

13. A 4-to-1 Multiplexer with EnableS0 S1 Z B A C D
En’
What is the Boolean Expression for this circuit?
Build a function table for this circuit

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

15. Multi-Channel MUX in a Digital Alarm ClockV
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.

16. A 7-Segment Display for Multiplexingc 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.

17. 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.

18. 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

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

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

21. 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

22. 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

23. 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

24. A multi-channel De-MUXb1
Channel 1 A c1 B d1 C a2 D b2
Channel 2 c2 d2 S

25. 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

26. 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.

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

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

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

30. End
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