Page 66 - Computer science 868 Class 12
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02
COMPUTER HARDWARE
Learning Objectives
2.1 Fundamental Gates or Basic Logic Gates 2.2 Universal Gates
2.3 Half Adder and Full Adder 2.4 Encoder
2.5 Decoder 2.6 Multiplexer
A logic gate is a digital circuit that depicts a logical relationship between the input and output voltages of the circuit.
They are the building blocks of a digital circuit.
A physical gate acts as a barrier and controls entry into a building. Similarly, logic gates are used to control the flow of
information based on logical relations. A logic gate can accept one or more inputs but always produces single output.
They produce signal 1 as output when the input logic requirement is satisfied, 0 otherwise.
Logic gates require a power supply. In both input and output 0 volts represents off(0) and 5 volts represent on(1). Each
logic gate is depicted with a specific graphical symbol.
2.1 FUNDAMENTAL GATES OR BASIC LOGIC GATES
The three basic logical operations namely conjunction (and), disjunction (or) and negation (not) are represented by
their corresponding logic gates defined below. These gates are also called fundamental gates.
2.1.1 NOT Gate
It requires a single input and is also called a unary gate. Not gate inverts the output. Input 1 is changed to 0 and
–
0 to 1. Not gate is represented as a complement (') or (bar) at the top. The gate symbol and truth table of Not gate
is given below.
A A'
0 1
Symbol of NOT gate 1 0 + A L
Truth table of NOT gate
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From the switching circuit of the NOT gate we find that the lamp glows when switch
A is opened as electricity reaches the bulb. But when the switch is closed, the current
does not reach the bulb and it does not glow. Switching circuit
diagram of NOT gate
6464 Touchpad Computer Science-XII

