Digital Logic 1/Digital Signals and Logic Symbols

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Basic Ideas and Concepts[edit | edit source]

  • Learn what is referred to as a "digital" circuit
  • Learn the respective logic gates and symbols associated
  • Learn the functionality of each logic gate

Digitization[edit | edit source]

When we say we have a "digital" circuit, it means that we create an entire new domain that is purely represented by an analog signal. What that means is, referring back to our basic circuit of a voltage source and a resistor, we can say that the different voltage levels can represent a "low" or "high". Using electrical and computer engineering conventions, we classify a logic "high" as anywhere from 4.5 volts to 5.0 volts and a logic "low" as anywhere from 0.1 to 0.5 volts.

So basically, we say that a 0 is inputted when the voltage is between 0.1-0.5 volts and a 1 if the voltage is 4.5-5 volts. Ideally however, we want 0.0 and 5.0 volts to represent 0 and 1 respectively and pictorially is represented below.

high 1 or onlow 0 or off

X 100M
X 100K
X 100
X 100m
X 100μ
Ω
A
DC
AC
C L

X 100M
X 100K
X 100
X 100m
X 100μ
Ω
A
DC
AC
C L

With this in mind, we can now move on and discuss logic gates which form the foundation of combinational circuits and analysis.

Logic Gates[edit | edit source]

Below are all the different logic symbols that are used to design and implement digital circuits.

Type Distinctive shape Rectangular shape Mathematical
Representation
Truth table
AND Gate AND symbol AND symbol
INPUT OUTPUT
A B A AND B
0 0 0
0 1 0
1 0 0
1 1 1
OR Gate OR symbol OR symbol
INPUT OUTPUT
A B A OR B
0 0 0
0 1 1
1 0 1
1 1 1
NOT Gate NOT symbol NOT symbol
INPUT OUTPUT
A NOT A
0 1
1 0
NAND Gate NAND symbol NAND symbol
INPUT OUTPUT
A B A NAND B
0 0 1
0 1 1
1 0 1
1 1 0
NOR Gate NOR symbol NOR symbol
INPUT OUTPUT
A B A NOR B
0 0 1
0 1 0
1 0 0
1 1 0
XOR Gate XOR symbol XOR symbol
INPUT OUTPUT
A B A XOR B
0 0 0
0 1 1
1 0 1
1 1 0
XNOR Gate XNOR symbol XNOR symbol or
INPUT OUTPUT
A B A XNOR B
0 0 1
0 1 0
1 0 0
1 1 1