All digital systems can be constructed by only three basic logic gates. These basic gates are called the AND gate, the OR gate, and the NOT gate. Some textbooks also include the NAND gate, the NOR gate and the EXOR gate as the members of the family of basic logic gates. The descriptions of the operations of these gates are listed below: AND gate
The AND gate is a circuit which gives a high output (logic 1) if all its inputs are high. A dot () is used to indicate the AND operation. In practice, however, the dot is usually omitted. OR gate
The OR gate is a circuit which gives a high output if one or more of its inputs are high. A plus sign (+) is used to indicate the OR operation. NOT gate
The NOT gate is a circuit which produces at its output the negated (inverted) version of its input logic. The circuit is also known as an inverter. If the input variable is A, the inverted output is written as . NAND gate
The NAND gate is a NOT-AND circuit which is equivalent to an AND circuit followed by a NOT circuit. The output of the NAND gate is high if any of its inputs is low. NOR gate
The NOR gate is a NOT-OR circuit which is equivalent to an OR circuit followed by a NOT circuit. The output of the NOR gate is low if any of its inputs is high. EXOR gate
The Exclusive-OR gate is a circuit which gives a high output if either of its two inputs is high, but not both. A encircled plus sign () is used to indicate the EOR operation. The functions of these basic building blocks are summarized by means of a Truth Table. The table shows all possible input/output combinations for two inputs.