← IGCSE Computer Science Paper 2

Number of inputs on each gate

NOT has exactly one input. AND, OR, NAND, NOR and XOR each have exactly two inputs. No three-input gates in this course, so join two gates for three inputs.

Part of: IGCSE Computer Science 0478 — 10 Boolean Logic — Module 10

Understanding Number of inputs on each gate in IGCSE Computer Science Paper 2

What it does: The AND gate takes A and B, and its output goes into an OR gate with C. Each gate still has only two inputs. NOT is the one-input gate. Every other gate (AND, OR, NAND, NOR, XOR) has two inputs. AND, OR, NAND, NOR and XOR each have exactly two inputs in the IGCSE course. NOT has one input, so its truth table has one input column, A, plus the output column X. NOT is a single-input gate, so it can only be applied to one input or one bracketed result.

Common mistakes: NAND has exactly two inputs, so it cannot take A, B and C together. It should be R = A NAND B. NOT is a single-input gate, so it cannot take two inputs. It should be X = NOT U.

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This module's key terms are also in a printable crossword (Modules 539–551) and word search (Modules 539–580).

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Learning

Duo

How many inputs does a NOT gate have?

Two inputsOne input

Matches

X = A AND B AND C

↓

This uses an AND gate with three inputs. No gate has three inputs, so two gates must be combined, for example (A AND B) AND C.

Applying

Meanings

Term

NOT: 1 input, others: 2

Meaning

NOT has exactly one input. AND, OR, NAND, NOR and XOR each have exactly two inputs. No three-input gates in this course, so join two gates for three inputs.

Practice

X = NOT A

Testing

Gaps

X = A

Check Answer

Spots

2Q = A OR B

3R = A NAND B NAND C

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Quick Questions

1. How many inputs does a NOT gate have?

A. One input ✓

B. Two inputs

NOT is the only gate with a single input. It simply flips that one input: 0 becomes 1 and 1 becomes 0.

2. How many inputs does an AND gate have in this course?

A. Three inputs

B. Exactly two inputs ✓

AND, OR, NAND, NOR and XOR each have exactly two inputs in the IGCSE course.

3. Which gate is the odd one out because it has a different number of inputs?

A. NOT ✓

B. XOR

NOT has one input. AND, OR, NAND, NOR and XOR all have two inputs, so NOT is the odd one out.

4. A student draws a NOR gate with three input wires going into it. Is this allowed in this course?

A. Yes, NOR can take three inputs

B. No, NOR has exactly two inputs ✓

There are no three-input gates in this course. A NOR gate always has two inputs and one output.

5. A truth table for a single NOT gate with input A needs how many input columns?

A. One input column (A) ✓

B. Two input columns (A and B)

NOT has one input, so its truth table has one input column, A, plus the output column X.

Match the Pairs

X = NOT A → The NOT gate has exactly one input (A) and one output (X)

X = A NAND B → A two-input gate: NAND always takes exactly two inputs, A and B

Combining two gates to handle three inputs A, B and C, because no gate has three inputs → X = (A AND B) OR C

Only one gate in this course has a single input → X = NOT B

X = A AND B AND C → This uses an AND gate with three inputs. No gate has three inputs, so two gates must be combined, for example (A AND B) AND C.

X = NOT A B → NOT is given two inputs, but it has exactly one input. It should be NOT A, or NOT applied to a single input or bracket.

Key Term

NOT: 1 input, others: 2

NOT has exactly one input. AND, OR, NAND, NOR and XOR each have exactly two inputs. No three-input gates in this course, so join two gates for three inputs.

Fill in the Blanks

1. // one input only X = NOT A

Hint: Type the gate that takes just one input.

2. // NOT has 1 input

Hint: Type the number of inputs on a NOT gate.

3. // NAND, NOR and XOR each have 2 inputs

Hint: Type the number of inputs on each two-input gate.

4. // three inputs: A, B and C X = (A AND B) OR C

Hint: Type the label of the third input that the second gate combines with the first gate's output.

5. // two gates, three inputs X = (A OR B) AND C

Hint: Type the bracket that opens the group around the first two-input gate.

Spot the Error

Question 1

A P = NOT A

B Q = A OR B

C R = A NAND B NAND C

D S = A XOR B

Line C — NAND has exactly two inputs, so it cannot take A, B and C together. It should be R = A NAND B.

Question 2

A # Inputs A, B and C

B T = A AND B

C U = T AND C

D X = NOT(U, C)

Line D — NOT is a single-input gate, so it cannot take two inputs. It should be X = NOT U.

Question 3

A # Two inputs: A and B

B P = A AND B

C Q = A OR B

D X = XOR(P, Q, A)

Line D — XOR has exactly two inputs, so it cannot take three. It should be X = P XOR Q.

Question 4

A NOT gate: 2 inputs

B AND gate: 2 inputs

C OR gate: 2 inputs

Line A — NOT is the only gate with a single input. It should read NOT gate: 1 input.

Question 5

A NAND gate: 2 inputs

B NOR gate: 2 inputs

C XOR gate: 3 inputs

D NOT gate: 1 input

Line C — There are no three-input gates in this course. XOR has exactly two inputs, so it should read XOR gate: 2 inputs.

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