Paper 2: Logic Gates and Truth Table | O Level Computer Science 2210 & IGCSE Computer Science 0478 | Detailed Free Notes To Score An A Star (A*)

• Role of logic gates
  • The computer can only understand 0 and 1.
  • As a result, logic gates determine information based on 0 and 1.
• Main Gates
  • AND
    • 2 inputs
    • Both need to be 1 to give 1 as the answer
    • Otherwise, in any other case, would be 0
  • NOT gate
    • Only 1 input
    • It would reverse the input
    • Make 0 into 1 and 1 into 0
  • OR
    • 2 inputs
    • Both inputs 1 so 1
    • Any one of the input 1 then 1.
    • Both inputs 0 then 0
  • NAND
    • 2 Inputs
    • AND + NOT
    • Reverses output of AND
    • Therefore, two inputs
    • Both 1 then 0
    • In all other cases, it will be 1.
  • NOR
    • 2 inputs
    • It is OR plus NOT gate
    • Therefore, it reverses the output of NOT gate
    • If both zero than it gives 1 output
    • Otherwise, it will give 0 output
  • XOR
    • The XOR gate focuses on giving a 0 output if both inputs same.
    • 2 Inputs
    • If both 1 then 0
    • If both 0 then 0
    • If 1 is 0 and 1 is i1, then output is 1
• How to write gate equations
  • NOT
    • We either write NOT A or a small dash over A
  • AND
    • We either write A AND B
    • We can write A.B
  • OR
    • We can either write A OR B
    • We can write A+B
  • NAND
    • We can either write A.B and a bar on both of them
    • OR We can write A NAND B
  • NOR
    • We can write A+B and a bar on both of them or we can write A NOR B
  • XOR
    • We can either write A XOR B
    • Or we can write A + B and make a round circle around the + sign
• When making logic diagrams, remember that a straight line shows the flow of data
  • We can use a jerk in the lines to show where there is no connection between the gates.
• How to build the truth table for the logic gates
• For AND
  • How to name the variables in the truth table
  • If only two, do the following

• If there are three of them, do the following

• AND Gate

• OR

• XOR

• NAND

• NOR

• NOT

Pictures of Gates (Taken courtesy online)

Please go through the video lecture on how to create the logic circuit and fill out the truth table for difficult logic gates.