1. NOCTI Review Lesson 4 Objectives:
Explain the principles of Basic gates.
Describe the principles of the other logical gates.

2. Basic Logic Gates
There are three basic logic gates each of which performs a basic logic function, they are called NOT, AND and OR.

3. All other logic functions can ultimately be derived from combinations of these three.

4. The NOT gate
The input to the NOT gate is inverted i.e the binary input state of 0 gives an output of 1 and the binary input state of 1 gives an output of 0.

5. is known as "NOT A" or alternatively as the complement of A .
The truth table for the NOT gate appears as below
        1

6. The AND gate
The AND gate has two or more inputs. The output from the AND gate is 1 if and only if all of the inputs are 1, otherwise the output from the gate is 0.

7. The AND gate is drawn as follows

8. The truth table for a two-input AND gate looks like
       
                1

10. The OR gate
The OR gate has two or more inputs. The output from the OR gate is 1 if any of the inputs is 1. The gate output is 0 if and only if all inputs are 0.

11. The OR gate is drawn as follows                                                                                                    

12. The truth table for a two-input OR gate looks like
       
                 1

13. Other Logic Gates
The three basic logic gates can be combined to provide more complex logical functions. Namely NAND, NOR, XOR and XNOR.

14. The NAND gate
The NAND gate has two or more inputs. The output from the NAND gate is 0 if and only if all of the inputs are 1 otherwise the output is 1.

15. The NAND gate is drawn as follows
                                                                                                    

16. The truth table for a two-input NAND gate looks like
       
                1

17. The NOR gate
The NOR gate has two or more inputs. The output from the NOR gate is 1 if and only if all of the inputs are 0, otherwise the output is 0.

18. The NOR gate is drawn as follows
                                                                                                   

19. The truth table for a two-input NOR gate looks like
       
                  1

20. The eXclusive-OR (XOR) gate
The exclusive-OR or XOR gate has two or more inputs. An XOR gate gives an output value of 1 when there are an odd number of 1's on the inputs to the gate.

21. The XOR gate is drawn as
                                                                                                   

22. The truth table for a two-input XOR gate looks like
       
                 1

23. The eXclusive-NOR (XNOR) gate
The exclusive-NOR or XNOR gate has two or more inputs. An XNOR gate gives an output value of 1 when there are an even number of 1's on the inputs to the gate.

24. The XNOR gate is drawn using the same symbol as the XOR gate with an invert bubble on the output line as is illustrated below                                                                                                    

25. The truth table for a two-input XNOR gate looks like
       
                 1