← IGCSE Computer Science Paper 2

Drawing wires, junctions and crossings neatly

A dot drawn where wires join, so one input can feed two gates. Wires that cross with no dot are not connected. Draw inputs on the left and the output on the right.

Part of: IGCSE Computer Science 0478 — 10 Boolean Logic — Module 31

Understanding Drawing wires, junctions and crossings neatly in IGCSE Computer Science Paper 2

What it does: The circuit is read from left to right. Inputs A, B and C start on the left and the output X ends on the right. The dot marks where the wire from A splits, so the same value goes to both the NOT gate and the AND gate. Crossing wires with no dot are separate. Each signal carries on along its own wire. A junction dot lets one input feed two gates. A crossing with no dot would not connect anything. Neat straight wires make the circuit easy to read and to mark. A junction dot is not a gate. It only shows that wires are joined.

Common mistakes: The output must be drawn on the right of the circuit. It should read 'Output X on the right'. When one input feeds two gates, a junction dot must be drawn where the wire splits. It should read 'Draw a dot where A splits'.

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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
Two wires cross on a circuit diagram and there is a dot where they cross. What does this mean?
The wires are not connectedThe wires are connected
Matches
Wire from A crosses wire from B with a dot at the crossing, but the wires should be separate
↓
The dot wrongly joins A and B, so they would share one signal. If the wires are not connected, they must cross with no dot.
Applying
Meanings
Term
junction dot
Meaning
A dot drawn where wires join, so one input can feed two gates. Wires that cross with no dot are not connected. Draw inputs on the left and the output on the right.
Practice
A -> junction dot -> AND gate
Testing
Gaps
X = (A AND B) OR ( AND C)
Check Answer
Spots
1Inputs A and B on the left
2Output X on the left

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Quick Questions

1. Two wires cross on a circuit diagram and there is a dot where they cross. What does this mean?
A. The wires are connected ✓
B. The wires are not connected
A junction dot shows that wires join. The signal on the wire goes to both paths.
2. Two wires cross and there is NO dot where they cross. What does this mean?
A. The wires are connected
B. The wires are not connected ✓
Crossing wires with no dot are separate. Each signal carries on along its own wire.
3. Input A must feed both an AND gate and a NOT gate. Which is the correct way to draw it?
A. Draw one wire from A and put a junction dot where it splits to both gates ✓
B. Draw one wire from A that crosses the other wire with no dot
A junction dot lets one input feed two gates. A crossing with no dot would not connect anything.
4. Where should the inputs A, B and C be drawn on a neat logic circuit?
A. On the right, with the output X on the left
B. On the left, with the output X on the right ✓
Circuits are drawn so the signal flows from left (inputs) to right (output).
5. Which is the better way to draw a wire in an exam circuit?
A. Straight lines, drawn neatly with a ruler ✓
B. Curved, wavy lines that loop around the gates
Neat straight wires make the circuit easy to read and to mark.

Match the Pairs

A solid dot where two wires meet → The wires are connected, so input A feeds both gates
Two wires cross with no dot → The wires are not connected and carry separate signals
Inputs are drawn on the left and the output on the right → A, B, C ---> [gates] ---> X
One input can feed two gates by splitting its wire at a junction dot → A -o-- NOT gate | +-- AND gate
Wire from A crosses wire from B with a dot at the crossing, but the wires should be separate → The dot wrongly joins A and B, so they would share one signal. If the wires are not connected, they must cross with no dot.
Input A feeds two gates, but the wire splits with no dot → There is no junction dot, so it looks like a crossing and the split is unclear. A dot is needed where one wire splits to two gates.

Key Term

junction dot
A dot drawn where wires join, so one input can feed two gates. Wires that cross with no dot are not connected. Draw inputs on the left and the output on the right.

Fill in the Blanks

1. X = (A AND B) OR (A AND C) # A feeds two gates: dot on wire A
Hint: Type the input that feeds both gates, so its wire needs a junction dot.
2. A crosses B, no dot -> not connected
Hint: Type the mark that must be drawn where wires join, and which is missing here.
3. inputs: A, B on the left output: X on the right
Hint: Type the side of the diagram where the output is drawn.
4. junction on A: one input, two gates
Hint: Type the number of gates that one input feeds when it has a junction.
5. label inputs A, B, C and output X
Hint: Type the letter used to label the output wire.

Spot the Error

Question 1
A Inputs A and B on the left
B Output X on the left
C Wires run straight and level
Line B — The output must be drawn on the right of the circuit. It should read 'Output X on the right'.
Question 2
A A feeds an AND and an OR gate
B Draw no dot where A splits
C Both branches run to gates
Line B — When one input feeds two gates, a junction dot must be drawn where the wire splits. It should read 'Draw a dot where A splits'.
Question 3
A Wire A crosses wire B
B They are not joined
C Keep the wires straight
D Draw a dot where they cross
Line D — Wires that cross but are not connected must have no dot. A dot would wrongly show they are joined.
Question 4
A A feeds a NOT gate
B A also feeds an AND gate
C Dot goes after the AND gate
Line C — The junction dot goes at the point where the input wire splits, before the gates. It should read 'Dot goes where A splits'.
Question 5
A Label the inputs and output
B Write A, B, C at the right end
C Write X at the output wire
Line B — Input labels A, B and C go at the left end of the input wires. The right side is for the output X.

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