Circuits and Gates

• These are the very basic building blocks of computers
• Built out of transisters
• Operate on 2-5 volts for 1
• 0-2 volts for 0
• A signal passes along a wire.
• And is received by a gate, a basic operator.
• gates are combined to form circuits
• These operate acording to the rules of Boolean Algebra
• NOT
  • Takes a single input
  • Inverts or flips the input to produce a single output
  • Truth Table:

    Input	Output
    0	1
    1	0

  • Logic Diagram Symbol
  • Boolean Expression X = A'
• Or
  • True when one or the other or both inputs are true

  • X	Y	X+Y
    0	0	0
    0	1	1
    1	0	1
    1	1	1

  • 
  • A = X + Y
• And
  • True only both inputs are true

  • X	Y	XY
    0	0	0
    0	1	0
    1	0	0
    1	1	1

  • 
  • A = XY
• Exclusive Or
  • True when one or the other but not both inputs are true

  • X	Y	X xor Y
    0	0	0
    0	1	1
    1	0	1
    1	1	0

  • 
  • A = X xor Y (plus with a circle around it)
• nand (not and)
  • True when both inputs are not true

  • X	Y	(XY)'
    0	0	1
    0	1	1
    1	0	1
    1	1	0

  • 
• nor (not or)
  • True when both inputs are false

  • X	Y	(X + Y)'
    0	0	1
    0	1	0
    1	0	0
    1	1	0

  • 
• Gates can have multiple inputs, and operate as expected
  • Three input and gate only true when all three are true
  • Three input or gate only false when all three are false.
• All gates can be constructed from transistors.
• Do exercises 26 and 30 page 111