Logic Gates

   Logic gates

          process signals which represent true or false. Normally the positive supply voltage +Vs represents true and 0V represents false. Other terms which are used for the true and false states are shown in the table on the right. It is best to be familiar with them all.

          Gates are identified by their function: NOT, AND, NAND, OR, NOR, EX-OR and EX-NOR. Capital letters are normally used to make it clear that the term refers to a logic gate.

   TRUTH TABLE

         are used to help show the function of a logic gate. If you are unsure about truth tables and need guidence on how go about drawning them for individual gates or logic circuits then use the truth tablesection link.

Basic Logic Gates

OR GATE

OR gate symbol

Truth Table


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.

AND GATE

AND gate symbol

An AND gate can have two or more inputs, its output is true if all inputs are true.

NOT GATE

NOT gate symbol

Truth Table

A logical inverter , sometimes called a NOT gate to differentiate it from other types of electronic inverter devices, has only one input. It reverses the logic state. 

NOR GATE

 NOR gate symbol

Truth Table

   The NOR gate is a combination OR gate followed by an inverter. Its output is "true" if both inputs are "false." Otherwise, the output is "false.

NAND GATE

NAND gate symbol

Truth Table

    The NAND gate operates as an AND gate followed by a NOT gate. It acts in the manner of the logical operation "and" followed by negation. The output is "false" if both inputs are "true." Otherwise, the output is "true."

EXOR gate

EXOR gate symbol 

Truth table 

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.

EXNOR gate

 EXNOR gate symbol

Truth table 

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.