3.2 Logical Operators and Their Symbols

1.	Logic gates process signals which represent true or false.
2.	Gates are identified by their function: NOT, AND, NAND, OR, NOR, XOR and XNOR.
3.	Logic gates are available on special ICs (chips) which usually contain several gates of the same type.
4.	There are several families of logic ICs and they can be split into two groups:

		4000 Series and 74 Series.
		4000 and 74 families are the best for battery powered projects because their power requirements are low.

5.	A Logic gate’s output is determined at anytime by the states of its inputs.
6.	Input and output signals of a logic gate can only be in one of two possible digital states, i.e., logic '0' or logic '1'.
7.	Thus, the logic state in which the output of a logic gate will be put in depends on the logic states of each of its individual inputs.

3.2 Logical Operators and Their Symbols

There are two series of symbols for logic gates:

1.	The traditional symbols, shown in Figure 3.1, have distinctive shapes making them easy to recognize. They are widely used in industry and education.

Figure 3.1: Logic Gate Traditional Symbols

3.2 Logical Operators and Their Symbols

2.	The IEC (International Electrotechnical Commission) symbols, shown in Figure 3.2, are rectangles with a symbol inside to show the gate’s function. They are rarely used despite their official status.

Figure 3.2: IEC Logic Gate Symbols

3.	Logic gates have two or more inputs, except the NOT gate which has only one input.
4.	All gates have only one output.
5.	Some gate symbols have a circle on their output which means that their function includes inverting the output.

3.2 Logical Operators and Their Symbols

	a)	It is equivalent to feeding the output through a NOT gate.
	b)	For example, the NOR (Not OR) gate symbol shown Figure 3.1 is the same as an OR gate symbol but with the addition of an inverting circle on the output.