← IGCSE Computer Science Paper 2

Logic circuit: gates, wires and junctions

A diagram of logic gates joined by wires. Inputs are on the left, the output is on the right, and each wire carries 0 or 1. A dot where a wire splits is a junction.

Part of: IGCSE Computer Science 0478 — 10 Boolean Logic — Module 24

Understanding Logic circuit: gates, wires and junctions in IGCSE Computer Science Paper 2

What it does: Each wire in a logic circuit holds a Boolean value, either 0 or 1, so a wire can be shown as a value of 0 or 1. In a circuit diagram the inputs A, B and C are on the left and the output X is on the right, with gates joined by wires between them. Gates can be chained: a wire can join the output of one gate to the input of another gate. A junction dot means the wire splits, so the same 0 or 1 goes along each branch. A NOT gate is drawn as a triangle with a small circle.

Common mistakes: A wire in a logic circuit carries only a 0 or a 1, so line 3 should say 'Wire W carries 0 or 1'. A NOT gate has only one input, so it cannot take both A and B. The circuit needs a two-input gate here: 'A and B go into an OR gate'.

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This module's key terms are also in a printable crossword (Modules 562–580) and word search (Modules 539–580).

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Learning
Duo
In a logic circuit diagram, where are the inputs normally drawn?
On the leftOn the right
Matches
Output X drawn on the left, inputs A and B drawn on the right
↓
The inputs and output are on the wrong sides. Inputs go on the left and the output goes on the right.
Applying
Meanings
Term
logic circuit
Meaning
A diagram of logic gates joined by wires. Inputs are on the left, the output is on the right, and each wire carries 0 or 1. A dot where a wire splits is a junction.
Practice
P = A AND B
Testing
Gaps
X = W C
Check Answer
Spots
2A and B feed an AND gate
3Wire W carries 0 to 9

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Quick Questions

1. In a logic circuit diagram, where are the inputs normally drawn?
A. On the right
B. On the left ✓
Circuits are read from left to right: inputs (such as A, B and C) are on the left and the output X is on the right.
2. In a logic circuit diagram, where is the output X normally drawn?
A. On the left
B. On the right ✓
The signal flows from the inputs on the left through the gates to the output on the right.
3. Which statement about a wire in a logic circuit is correct?
A. A wire carries either 0 or 1 ✓
B. A wire can carry any number from 0 to 9
Logic circuits use Boolean values only, so every wire carries 0 or 1 at any moment.
4. Which statement about a logic circuit is correct?
A. The output of one gate can be the input of the next gate ✓
B. Each gate must take its inputs only from A, B or C
Gates can be chained: a wire can join the output of one gate to the input of another gate.
5. What does a dot where wires meet on a circuit diagram show?
A. A junction where one wire splits into two ✓
B. A NOT gate that flips the value
A junction dot means the wire splits, so the same 0 or 1 goes along each branch. A NOT gate is drawn as a triangle with a small circle.

Match the Pairs

A ──┬──[AND]── X └──[NOT]── → A junction dot where the wire from input A splits, so the same 0 or 1 goes into two gates
(A AND B) OR C → The AND gate output is wired into the OR gate as one of its inputs
A wire carries only one of two values at any moment: 0 or 1 → wire = 1
Inputs are drawn on the left and the output on the right, so the signal flows left to right → A, B, C -> gates -> X
Output X drawn on the left, inputs A and B drawn on the right → The inputs and output are on the wrong sides. Inputs go on the left and the output goes on the right.
A wire from the output of an AND gate cannot go into another gate → This is false. The output of one gate can be joined by a wire to the input of the next gate.

Key Term

logic circuit
A diagram of logic gates joined by wires. Inputs are on the left, the output is on the right, and each wire carries 0 or 1. A dot where a wire splits is a junction.

Fill in the Blanks

1. # X is 1 if W is 1 or C is 1 W = A AND B X = W OR C
Hint: Type the gate name that gives 1 when either of its inputs is 1.
2. # Gate 1 output W is input to gate 2 W = A AND B X = W OR C
Hint: Type the wire name that carries the first gate's output into the second gate.
3. # Inputs A, B on left; output on right X = A AND B
Hint: Type the label of the single output on the right of the circuit.
4. # Junction: A goes to both gates X = (A AND B) OR (NOT A)
Hint: Type the input whose wire splits at the junction and feeds both gates.
5. A = 1, B = 1 W = A AND B # The wire W carries 1
Hint: Type the value (0 or 1) that the AND gate puts onto wire W.

Spot the Error

Question 1
A Inputs A, B are on the left
B A and B feed an AND gate
C Wire W carries 0 to 9
D W goes to a NOT gate
E Output X is on the right
Line C — A wire in a logic circuit carries only a 0 or a 1, so line 3 should say 'Wire W carries 0 or 1'.
Question 2
A Inputs: A, B, C (left side)
B A and B go into a NOT gate
C OR output and C go to AND
D AND output is X (right side)
Line B — A NOT gate has only one input, so it cannot take both A and B. The circuit needs a two-input gate here: 'A and B go into an OR gate'.
Question 3
A A is wired to two gates
B A dot shows where A turns to 1
C One copy goes to a NOT gate
D One copy goes to an AND gate
Line B — A junction dot only shows where a wire splits into copies. It does not change the value. Each copy carries the same 0 or 1 as A.
Question 4
A A = 1, B = 0
B Gate 1: A AND B gives W
C Gate 2: NOT W gives X
D W = 0, so X = 0
Line D — The NOT gate reverses its input. W is 0, so X = NOT 0 = 1. Line 4 should be 'W = 0, so X = 1'.
Question 5
A Inputs are on the right
B The output is on the right
C Gates are joined by wires
Line A — In a logic circuit the inputs are drawn on the left and the output on the right, so line 1 should say the inputs are on the left.

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