operations within a computer are carried out by means of combinations of signals passing through standard blocks of built-in circuits known as logic gates. In other words, a logic gate is an electronic circuit that operates on one or more input signals to produce standard output signals. These logic gates are the building blocks of all electronic circuits in a computer. Combinations of multiple logic gates of different types are often used to build electronic circuits in computers. Out of many types of logic gates available, we shall study here only some of the most important ones. These are sufficient to introduce the concept of electronic logic circuit with the use of logic gates.
1AND Gate
An AND gate is the physical realization of logical multiplication (AND) operation. It is an electronic circuit that generates an output signal of 1, only if all input signals are also 1.
combinations o of input signals) is represented conveniently by means of a truth table. Below Figure shows the truth table and block diagram symbol for an AND gate for two input signals. Since there are only two inputs (A and B), only four (2 2 ) combinations of inputs are possible. Notice that the output is 1 only when both inputs are in 1 state, otherwise it is 0.
OR Gate
An OR gate is the physical realization of logical addition (OR) operation. It is an electronic circuit that generates an output signal of 1, if any of the input signals is also 1. Two or more switches connected in parallel behave as an OR gate. Observe from the below Figure that the input current will reach the output point when any one of the two switches (A or B) are in ON (1) state. There will be no output only when both the switches are in OFF (0) state.
