A digital computer uses binary number system for its operation. In the binary system there are only 2 digits, 0 and 1. The manipulation of binary information is done by logic circuit known as logic gates. The important logical operations which are frequently performed in the design of a digital system are: (1) AND (2) OR (3) NOT (4) NAND (5) NOR and EXCLUSIVE OR.

An electronic circuit which performs a logical operation is called a logic gate.

Digital multiplexer

A digital multiplexer has N inputs & only one output. By applying control signals any one input can be made, available at the output terminal. It‟s called data sector. 

Combinational & Sequential Circuits

There are two types of logic circuits, Combinational & Sequential. A combinational circuit is one in which the state of the O/P at any instant is entirely determined by the states of the I/P‟s at that time. Combinational circuits are those logic circuits whose operation can be completely described by a truth table.

A sequential circuit consists of a combinational logic & storage elements. The O/P of a sequential circuit is not only a function of the present I/P‟s but also of past I/P‟s. The state of the storage elements depends upon this preceding I/Ps and the preceding states of the elements.

Flip Flops

A device is said to be bi-stable if it has 2 stable elements. A flip-flop is a bi-stable device. It has 2 stable stales. Its O/P remains either high or low. The high stable state is called SET. The other stable state is called RESET. It can store binary bit, rather 0 or 1. Thus, it has storing capability, i.e., memory.

Types of Flip Flops 

The following types of flip-flops are listed below :

S-R Flip-Flop : A SR flip flop can be realized by connecting 2 NOR

 J-K Flip-Flops : In an S-R flip flop, the state of the O/P is unpredictable when S-R =1. A J–K Flip flop allows inputs J=K = 1. In this situation, the state of the O/P is changed. The complement of the previous state is available at the output terminal.

D-Flip Flops : An S-R flip flop has 2 inputs S& R. To store 1, a high S and low R an required. To store 0, a high R & low S are needed. Thus, 2 signals are to be generated to drive a S-R flip flop.

T-Flip Flop : A T-flip flop acts a toggle switch. Toggle means to switch over to the opposite state.

Shift Registers

In a shift register, the flip-flops are connected in series. The O/P of each flip-flop is connected to the I/P of the adjacent flip flop. The contents of a shift register can be shifted within the registers without changing the order of the bits. Data are shifted one position left or right at a time when 1 clock pulse is applied. Shift register are used for the temporary storage of data. They are suited for processing serial data; converting serial data to parallel data & vice versa. Then are 4 types of shift registers.

(ii) Serial In - Serial Out
(iii) Serial In - Parallel Out
(iv) Parallel In - Serial Out
(v) Parallel In - Parallel Out


A decoder is similar to demultiplexer without any data input. Most digital system require decoding of data. Decoding is necessary in applications such as data demultiplexing digital display, digital to analog converters etc. A decoder is a logic circuit that converts an n-bit binary input code (data) in 2^n O/P lines, such that each O/P lines will be activated for only one of the possible combinations of I/Ps.
In a decoder, the number of O/P is greater than the number of I/Ps.
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