Logic Gates
Logic gates Logic gates (or simply gates) are the fundamental building blocks of digital circuitry. As their name implies, they functi...
https://polytechnicadda.blogspot.com/2016/05/logic-gates.html
Logic gates
Logic gates (or simply gates) are the fundamental building blocks of digital circuitry. As their name implies, they function by "opening" or "closing" to admit or reject the flow of digital information. Gates implement electronically simple logical operations on boolean (Bool’s algebra) variables, i.e. variables that can have only one of two states (0/1, low/high, false/true). From an electrical point of view and for the TTL (transistor-transistor-logic) family of digital electronics, any voltage in the range 0-0,7 V and in the range 2,5-5 V, represent logic states 0 and 1, respectively. In the following figure the accepted electronic symbols for different gates are shown, along with their corresponding "truth tables" and their symbolic logical expressions. All variables (X, A, B, …) are booleans.
The most typical logical operations are implemented by AND and OR gates. The logical expression for the AND operation is “if A is true AND B is true then X is true”, and for the OR operation is “if A is true OR B is true then X is true”. The inverted logic AND and OR gates are commonly known as NAND (Not AND) and NOR (Not OR) gates. A XOR (Exclusive-OR) gate implements the logical expression “if A is different than B then X is true”, hence sometimes this gate is called “inequality comparator”.
The buffer and the inverter are not gates but their use is closely associated with them. A buffer doesn’t change the logic state of its input. It is only occasionally used for increasing the fan-out, i.e. the capability of the output of one gate to drive a number of other gates. The inverter is much more important and it is used for inverting a logic state, i.e. for performing the logical operation of negation (NOT). The logical expressions for a buffer and an inverter are “X is A” and “X is NOT A”, respectively. AND, OR, NAND and NOR gates can have more than 2 inputs. In this case their truth tables are extended to all inputs combinations and their corresponding expressions as well. For example, the logical expression for a 4-input AND is “if A is true AND B is true AND C is true AND D is true then X is true”. The corresponding expression for a 3-input NOR gate is “if A is true OR B is true OR C is true then X is false”
