Logic Gates: Schematic and Truth Table

Logic Gates - perform basic logical functions and are the fundamental building blocks of digital integrated circuits. Most logic gates take an input of two binary values, and output a single value of a 1 or 0. Some circuits may have only a few logic gates, while others, such as microprocessors, may have millions of them.

Schematic - is a representation of the elements of a system using abstract, graphic symbols rather than realistic pictures. A schematic usually omits all details that are not relevant to the information the schematic is intended to convey, and may add unrealistic elements that aid comprehension.

Truth Table - are used to help show the function of a logic gate.

1. AND 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. 

   - True if A and B are both True

Schematic                                                                       Truth Table

                       

2. 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.

   - True if either A or B are True

Schematic                                                                    Truth Table

3. 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.  This is also shown as A', or A with a bar over the top, as shown at the outputs. The diagrams below show two ways that the NAND logic gate can be configured to produce a NOT gate. It can also be done using NOR logic gates in the same way.

  -  Inverts value: True if input is False; False if input is True

Schematic                                                                    Truth Table

4. NOT-AND or NAND 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.

  - AND followed by NOT: False only if A and B are both True

Schematic                                                                      Truth Table

5. NOT-OR or NOR 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.

  - OR followed by NOT: True only if A and B are both False

Schematic                                                                      Truth Table

6.  'Exclusive-OR' or EXOR 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.

  - True if either A or B are True, but False if both are True

Schematic                                                                      Truth Table

7.  'Exclusive-NOR' or EXNOR 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.

  - XOR followed by NOT: True if A and B are both True or both False

Schematic                                                                    Truth Table

Quad 2-input gates




7400 quad 2-input NAND
7403 quad 2-input NAND with open collector outputs
7408 quad 2-input AND
7409 quad 2-input AND with open collector outputs
7432 quad 2-input OR
7486 quad 2-input EX-OR
74132 quad 2-input NAND with Schmitt trigger inputs












References: http://en.wikipedia.org/wiki/Logic_gate

           http://en.wikipedia.org/wiki/Schematic

         http://www.ee.surrey.ac.uk/Projects/CAL/digital-logic/gatesfunc/index.html

         http://www.techterms.com/definition/logicgate

         http://electronicsclub.info/74series.htm