Do you know about logic gates? There are two types of logic gates. Let’s discuss what is primary and secondary logic gates with their detailed classification.
Primary logic gates
The common use of logic elements is to act as switches, although they have no moving parts. They open to pass on a pulse of electricity or are close to shutting it off.
This is why they are known as gates. The primary gates are
- OR Gate,
- AND Gate and
- NOT Gate
An OR gate has an output of 1 if any of the inputs are 1. The symbol and truth table for a two-input. Here output, Y=A+B, where + denotes OR operation.
The figure illustrates, the close relationship between the 2-input OR gate and an electrical switching circuit. Such a circuit normally contains some source of every (a battery), an
Output device (a lamp), and one or more switches are all connected by wires. A switch is a two-state device that is either closed (on) or opens (off). In Figure 3.2 switches A and B, are connected in parallel.
The lamp will light if switch A is closed, if switch B is closed, or if both switches are closed. This is the property described by the truth table for the Or gate, where 1 denotes that the switch or lamp is on and 0 indicates that it is off.
An AND gate has an output of 1 if all of its inputs are 1. The symbol and truth table for two input AND gates are shown in the figure. Here output, Y=A.B. where denotes AND operation.
The figure is a circuit showing two switches. A and B, are connected in series. The lamp will light only when both A and B are close. This is exactly the property described by the truth table for the AND gate, where 1 denotes that the circuit element is on and 0 denotes that it is off.
A NOT gate has one input and one output. It has the effect of reversing the input signal and is also called an inverter. The symbol and truth table for a NOT gate are shown in the figure. Here output Y=A, where-indicates NOT operation.
Secondary Logic Gates
Some secondary gates are
- NAND Gate
- NOR Gate
- EXOR Gate
NAND and NOR gates are called universal gates because anyone can be used to realize any logical expression.
A NAND gate has the same effect as an AND gate followed by a NOT gate, Hence the output will be opposite of the AND gate. The symbol and truth table for a two-input NAND gate is shown in Figure.
A NOR gate has the same effect as an OR gate followed by a NOT gate. Hence the output will be the opposite of the OR gate. The symbol and truth table for a two-input NOR gate are shown in Figure.
It is widely used in digital circuits. The symbol and truth table for a two-input EXOR gate are shown in Figure 3.8. Here output Y=A + denotes EXOR operation.
Basic gates (AND, OR, NOT) and universal gates (NAND, NOR) can be used in combination to make up digital computer circuits.
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