Logic Gates

• Performs a basic operation on electrical signals
• Accepts one or more inputs and produces a single output signal
• Easiest gates to make are NOT, NAND, and NOR gates
  • 

NOT

• Accepts an input
• Output is inverted of input
• An inverter
• Boolean algebra:
  • A'
  • The mark comes after a value being negated
  • Sometimes shown as horizontal bar over value being negated
• Logic diagram:
  • Small circle after a transistor

NOT Truth Table

AND

• Accepts two inputs
• Outputs 1 if both inputs are 1
• Outputs 0 if either input is 0
• Boolean algebra:
  • A • B or A * B or AB or A ^ B

AND Truth Table

OR

• Accepts two inputs
• Outputs 0 if both inputs are 0
• Outputs 1 otherwise
• Boolean algebra:
  • A + B or A v B

OR Truth Table

XOR

• Exclusive OR
  • Only differs from an OR if both inputs are 1
  • "When I say or, I mean one or the other, not both."
• Accepts two inputs
• Outputs 0 if the two inputs are the same
• Outputs 1 otherwise
• Boolean algebra:
  • Circle with a plus in it

XOR Truth Table

NAND

• NOT + AND
  • It's an AND gate that gets inverted
• Accepts two inputs
• Outputs 0 if both are 1
• Outputs 1 otherwise
• Boolean algebra:
  • (A • B)'

NAND Truth Table

NOR

• NOT + OR
  • It's an OR gate that gets inverted
• Accepts two inputs
• Outputs 1 if both input is 0
• Outputs 0 otherwise
• Boolean algebra:
  • (A + B)'

NOR Truth Table

Computer Science