CO

UNIT-I

2

Multiplexers:• A multiplexer selects information from an input

line and directs the information to an output line

• A typical multiplexer has– n selection control inputs (Sn - 1, … S0)

– 2n information data inputs (I2n

- 1, … I0)– one output Y

3

Multiplexers (continued)• A multiplexer can be designed to have

– n selection inputs– m information inputs with m < 2n

Design of a 2/1 Mux

• 2/1 mux Block Diagram• Truth Table

D1

D0

S

OS D1 D0 O

0 0 0 0

0 0 1 1

0 1 0 0

0 1 1 1

1 0 0 0

1 0 1 0

1 1 0 1

1 1 1 1

A Single Bit 4-to-1 Line Multiplexer

4-to-1-Line Multiplexer

I0

I1

I2

I3

I0

I1

I2

I3

Uses of Multiplexers

– Used in data communications for several computers to communicate over 1 line

– Used in radio to select one channel from many– Used to route data within a computer– Used for function generation

Demultiplexer

• Takes one input• Out to one of 2n possible outputs

1-to-4-Line Demultiplexer

Encoder

• Encoder is the opposite of decoder• 2n inputs

– or less – 10 inputs in “Decimal to BCD” encoder: I0, I1, I2, I3, …, I9

• n outputs– 4 output lines “Decimal to BCD”encoder

Truth Table: 8-to-3 Binary Encoder

Decoders

• Typically n inputs and 2n outputs • The output line corresponding to the input

code is high

1-to-2 Line Decoder

11

• 2-to-4-Line Decoder

Note that the 2-4-line made up of 2 1-to-2- line decoders and 4 AND gates.

Decoder Examples

A1

0

0

1

1

A0

0

1

0

1

D0

1

0

0

0

D1

0

1

0

0

D2

0

0

1

0

D3

0

0

0

1

(a)

D0 A1 A0

D1 A1 A0

D2 A1 A0

D3 A1 A0

(b)

A1

A0

=

=

=

=

2-to-4 Decoder with Enable

Uses for Decoders

• Binary number might serve to select some operation– Computer operation codes are encoded

• Decoder lines might select add operation, or subtract operation, or multiply operation, etc.

• Memory address lines