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Questions
Draw the logic gate symbol and state the boolean expression that represents the NOR gate. (2 marks)
An accurate diagram of a NOR gate (1) A + B (1)
Show the truth table for a NAND gate. (2 marks)
| Input A |
Input B |
Output |
| 0 |
0 |
1 |
| 0 |
1 |
1 |
| 1 |
0 |
1 |
| 1 |
1 |
0 |
1 mark test all 4 input sets. 1 mark correct outputs.
Why are NAND gates referred to as universal gates? (1 mark)
Because you can build any other type of logic gates using just NAND gates. (1)
Explain how the full adder works. (3 marks)
The full adder takes three inputs, the two bits to be added and the carry bit from the previous equation. (1) From this it produces two outputs. The first is the sum (1) and the second the carry bit. (1)
Explain how you could build a circuit to add N bits using full adders (2 marks)
By concatenating together N adders (1) so the carry bit from the last becomes and input for the next. (1)
Identify the inputs of a d-type flip-flop (2 marks)
The inputs are the control input(D) (1) and the clock. (1)
Simplify the boolean expression A.B+A+B (2 marks)
= A.B+A.B (1) (DeMorgans 1st law)
= 1 (1) (X +X = 1)
If you are confused by the second line it is the result of A.B that is X in this case.
Simplify the expression (A + B).((B.C)+(C.B)) (2 marks)
= (A + B).((B.C)+(B.C)) (Commutative law)
= (A + B).(B.C) (X + X = X)
Simplify the expression 
(4 marks)