ENGINEERING LOGIC DIAGRAMS
There are three basic types of logic gates. They are AND, OR, and NOT gates. Each gate
is a very simple device that only has two states, on and off. The states of a gate are also
commonly referred to as high or low, 1 or 0, or True or False, where on = high = 1 = True,
and off = low = 0 = False. The state of the gate, also referred to as its output, is determined
by the status of the inputs to the gate, with each type of gate responding differently to the
various possible combinations of inputs. Specifically, these combinations are as follows.
AND gate - provides an output (on) when all its inputs are on. When any one of the
inputs is off, the gate's output is off.
OR gate - provides an output (on) when any one or more of its inputs is on. The gate
is off only when all of its inputs are off.
NOT gate - provides a reversal of the input. If the input is on, the output will be off.
If the input is off, the output will be on.
Because the NOT gate is frequently used in conjunction with AND and OR gates, special
symbols have been developed to represent these combinations. The combination of an AND
gate and a NOT gate is called a NAND gate. The combination of an OR gate with a NOT
gate is called a NOR gate.
NAND gate - is the opposite (NOT) of an AND gate's output. It provides an output
(on) except when all the inputs are on.
NOR gate - is the opposite (NOT) of an OR gate's output. It provides an output only
when all inputs are off.
Figure 3 illustrates the symbols covering the three basic logic gates plus NAND and NOR
gates. The IEEE/ANSI symbols are used most often; however, other symbol conventions are
provided on Figure 3 for information.