Basic Logic Gates

Logic gates are the basic building blocks in digital electronics. They are intended to implement different logic functions such as the NOT, OR, NOR, AND, NAND, XOR and ENOR or XNOR.

There are many ways of describing the function of logic gates and other logic devices. The two most common are through truth tables and timing diagrams. A truth table is a tabulated list of all possible input and output combinations of a logic device. The timing diagram is similar to the truth table in that it shows the outputs for the different possible inputs. The inputs and outputs are vertically aligned and the horizontal spacing denotes changes with respect to time in periods or cycles.

Inverter/ NOT Gate

          -is one of the most popular logic gates in terms of use. It has one input and one output.

x	z
0	1
1	0

OR Gate

          -provides a high or “1” output when at least one of its inputs is “1”. The output is low or “0” when all its inputs are “0”.

x1	x0	z
0	0	0
0	1	1
1	0	1
1	1	1

NOR Gate

          -is formed by an OR gate followed by an inverter.

x1	x0	z
0	0	1
0	1	0
1	0	0
1	1	0

AND Gate

          -is another basic logic gate whose output is a high logic output only when all inputs are high.

x1	x0	z
0	0	0
0	1	0
1	0	0
1	1	1

NAND Gate

          -produces a low logic output only when all its inputs are high.

x1	x0	z
0	0	1
0	1	1
1	0	1
1	1	0

XOR Gate

          -is an "exclusive" gate that produces a high logic output only when one but not all inputs is high.

x1	x0	z
0	0	0
0	1	1
1	0	1
1	1	0

Exclusive NOR Gate

          -is the complement of the exclusive OR gate. It produces a low logic output only when one but not all its inputs is high.

x1	x0	z
0	0	0
0	1	1
1	0	1
1	1	0

References:

          Benjo Tirol. Fundamentals of Digital Electronics. Electronics Hobbyists Publishing House. 

          http://www.cs.umd.edu/class/sum2003/cmsc311/Notes/Comb/gates.html