3.2 Logic Gates & Circuits
AS Level · 3 questions found
What this topic covers
Section titled “What this topic covers”- Gate symbols and functions: NOT, AND, OR, NAND, NOR, XOR
- Construct truth tables for each gate
- Build a logic circuit from a problem statement, expression or truth table
- Build a truth table from a circuit, expression or problem statement
- Write a logic expression from a circuit, truth table or problem statement
Past paper questions
Section titled “Past paper questions” Q1
1
(a) Tick (✓) one box to identify the correct logic statement for this truth table.
A
B
C
X
0
0
0
1
0
0
1
0
0
1
0
0
0
1
1
0
1
0
0
0
1
0
1
0
1
1
0
1
1
1
1
0
NOT (A AND B AND C)
(A XOR B) NOR C
(A OR B OR C) NOR C
NOT A AND NOT B AND NOT C
[1]
(b) Draw a logic circuit for the logic expression:
X = NOT (NOT A AND (NOT B XOR C))
A
X
B
C
[2]
Show mark scheme
1(a) [2 marks]
1 mark
for:
(A XOR B) NOR C
for:
(A XOR B) NOR C
1(b)
1 mark
for NOT B XOR C
1 mark
for NOT A and final AND plus NOT
for NOT B XOR C
1 mark
for NOT A and final AND plus NOT
Q1
1
(a) Describe the operation of each of the following logic gates:
NAND .......................................................................................................................................
NOR ..........................................................................................................................................
XOR ..........................................................................................................................................
OR ............................................................................................................................................
[4]
(b) Draw a logic circuit for this logic expression:
X = NOT ((A AND B) OR (C AND D))
A
X
B
C
D
[2]
Show mark scheme
1(a) [2 marks]
1 mark
for each correct answer:
NAND
The output is 0 when both inputs are 1, otherwise the output is 1
NOR
The output is 1 when both inputs are 0, otherwise the output is 0
XOR
The output is 1 when one of the inputs is 1 and the other input is 0,
otherwise the output is 0
OR
The output is 0 when both inputs are 0, otherwise the output is 1
for each correct answer:
NAND
The output is 0 when both inputs are 1, otherwise the output is 1
NOR
The output is 1 when both inputs are 0, otherwise the output is 0
XOR
The output is 1 when one of the inputs is 1 and the other input is 0,
otherwise the output is 0
OR
The output is 0 when both inputs are 0, otherwise the output is 1
1(b) [4 marks]
1 mark
for both AND gates with correct inputs
1 mark
for correct OR and NOT gates with correct inputs and no superfluous
gates:
for both AND gates with correct inputs
1 mark
for correct OR and NOT gates with correct inputs and no superfluous
gates:
Q6
6
Each of the following truth tables has three inputs (A, B and C) and one output (X).
Draw one line to match each truth table with its logic expression.
Truth table
Logic expression
A
B
C
X
0
0
0
0
0
0
1
1
0
1
0
0
0
1
1
0
1
0
0
1
1
0
1
1
1
1
0
0
1
1
1
0
A
B
C
X
0
0
0
0
0
0
1
1
0
1
0
0
0
1
1
0
1
0
0
0
1
0
1
0
1
1
0
0
1
1
1
1
A
B
C
X
0
0
0
1
0
0
1
1
0
1
0
1
0
1
1
1
1
0
0
1
1
0
1
1
1
1
0
0
1
1
1
1
[3]
NOT (A XOR B) AND C
(A OR C) AND NOT B
A NAND B NAND C
(A NAND B) OR C
NOT (A AND B AND C)
,
,
Show mark scheme
6 [3 marks]
1 mark
for each correct line:
for each correct line: