Combinational Logic with MSI and LSI

Logic Devices

Decoder

• A decoder is a combinational circuit that converts binary information from n inputs to unique output lines.

If input=2, output=4If input=3, output=8

2-to-4 Decoder

3-to-8 Decoder

Implementation of a full adder with a decoder

Encoder

• An encoder has input lines and n output lines.

4-to-2 Encoder

8-to-3 Encoder

Multiplexers

• A multiplexer is a combinational circuit that selects binary information from one of many input lines and directs it to a single output line.

• The selection of a particular input line is controlled by a set of selection lines. Normally. there are input lines and II selection lines

Multiplexers

2 to 1 line multiplexer 4 to 1 line multiplexer

Demultiplexers

• It has one input line, n selection lines and output lines.

ROM

• ROM= Read Only Memory• It is a device that includes both decoder and

OR gates.• It consists of n input lines and m output lines.

Types of ROMs

1. PROM= Programmable read only memorywhen ordered, PROM units contain all 0’s or all 1’s.

2. EPROM= Erasable PROMwhen an EPROM is placed under a special uv light for a given period of time, the short-wave radiation discharges the internal gates that serve as contacts. After erase, the ROM returned to its initial state.

3. EAROM= Electrically alterable ROM This ROM can be erased with electrical signalsinstead of uv light.

Programmable Logic Array(PLA)

• A programmable logic array (PLA) is a kind of programmable logic device used to implement combinational logic circuits.

• The PLA has a set of programmable AND gate planes, which link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output.

Block diagram of PLA

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