LOGIC GATES
INTRODUCTION
An implementation of a logic operation is called a logic gates. It is used to implement such devices are diodes, electronic switches, transistors and integrated circuits. An AND gate, for instance, is an implementation of the AND operation. Suppose it is necessary to implement a logic gate for the function
R=F (A, B, C…)
Where F is a Boolean function and R, A, B, C and so on are Boolean variable. The arguments A, B, C and so on will be the inputs to the logic gate R, the value of the function, will be its output. The structure of the gate will naturally depend on the nature of the function F.
It is the normal practice to design logic gates to implement only a small number of elementary function such as AND, OR and NOT. Complex functions which are not available in the form of these ‘standard’ logic gates can be expressed in terms of the elementary functions and implement using networks of standard gates.
Mainly, gate INPUTS are driven by voltages e.g. 0v and 5v representing logic 0 and logic 1 respectively.
And gate OUTPUTS are driven by voltages only e.g.0v and 5v representing logic 0 and logic 1 respectively. In general, there is only one output to a logic gate except in some special cases.
TYPES OF LOGIC GATES ARE:
1) AND GATE
2) OR GATE
3) NOT GATE
4) NAND GATE
5) NOR GATE
6) EXOR GATE
7) EX NOR GATE
EXPLANATION:
1) AND GATE: The gate is an electronic circuit that gives a high output (1) only if all its inputs are high. A dot (.) is used to show the AND operation i.e. A.B. Bear in mind that this dot is sometimes omitted i.e. AB
TRUTH TABLE OF AND GATE
LOGIC DIAGRAM
2) OR GATE: The OR gate is an electronic circuit that gives a high output (1) if one or more of its inputs are high. A plus (+) is used to show the OR operation.
TRUTH TABLE OF OR GATE
LOGIC DIAGRAM
3) NOT GATE: The NOT gate is an electronic circuit that produces an inverted version of the input at its output. It is also known as an inverter. If the input variable is A, the inverted output is known as NOT A.
TRUTH TABLE OF NOT GATE
LOGIC DIAGRAM
4) NAND GATE: This is a NOT-AND gate which is equal to an AND gate followed by a NOT gate. The outputs of all NAND gates are high if any of the inputs are low. The symbol is an AND gate with a small circle on the output. The small circle represents inversion.
TRUTH TABLE OF NAND GATE
LOGIC DIAGRAM
5) NOR GATE: This is a NOT-OR gate which is equal to an OR gate followed by a NOT gate. The outputs of all NOR gates are low if any of the inputs are high. The symbol is an OR gate with a small circle on the output. The small circle represents inversion.
TRUTH TABLE OF NOR GATE
LOGIC DIAGRAM
6) EXOR GATE: The 'Exclusive-OR' gate is a circuit which will give a high output if either, but not both, of its two inputs are high. An encircled plus sign ( ) is used to show the EOR operation.
TRUTH TABLE OF EXOR GATE
LOGIC DIAGRAM
7) EXNOR GATE: The 'Exclusive-NOR' gate circuit does the opposite to the EOR gate. It will give a low output if either, but not both, of its two inputs are high. The symbol is an EXOR gate with a small circle on the output. The small circle represents inversion.
TRUTH TABLE OF EXNOR GATE
LOGIC DIAGRAM
NOTE: The NAND and NOR gates are called universal functions since with either one the AND and OR functions and NOT can be generated.
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