← IGCSE Computer Science Paper 2

Telling similar gate symbols apart

A small circle on a gate's output means NOT: AND becomes NAND and OR becomes NOR. XOR has no circle; its symbol is OR with an extra curved line at the input side.

Part of: IGCSE Computer Science 0478 — 10 Boolean Logic — Module 15

Understanding Telling similar gate symbols apart in IGCSE Computer Science Paper 2

What it does: The extra curved line marks XOR and separates it from OR. The circle is for NAND, NOR and NOT only. The circle on the triangle's output is the same NOT circle used on NAND and NOR. The output circle means NOT, so OR plus a circle is NOR. XOR does not use a circle. A plain OR shape has no circle and no extra curve. The extra curve would make it XOR.

Common mistakes: NAND is AND with a small circle on the output. Line 2 should say NAND: AND shape + circle, because without the circle it is just an AND gate. The NOT circle goes on the gate's output, not the input. Line 3 should say Add a circle on the output.

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This module's key terms are also in a printable crossword (Modules 552–561) and word search (Modules 539–580).

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Learning

Duo

A gate has the AND shape with a small circle on its output. Which gate is it?

ANDNAND

Matches

Labelling an OR shape with a circle on the output as XOR

↓

A circle on the output means NOT, so OR with a circle is NOR, not XOR. XOR has an extra curved line at the input side instead.

Applying

Meanings

Term

output circle

Meaning

A small circle on a gate's output means NOT: AND becomes NAND and OR becomes NOR. XOR has no circle; its symbol is OR with an extra curved line at the input side.

Practice

AND + circle on output = NAND

Testing

Gaps

AND symbol + output circle =

Check Answer

Spots

1AND: flat back, round front

2NAND: AND shape, no circle

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Quick Questions

1. A gate has the AND shape with a small circle on its output. Which gate is it?

A. AND

B. NAND ✓

The small circle on the output means NOT, so AND plus a circle is NAND.

2. A gate has the OR shape with a small circle on its output. Which gate is it?

A. XOR

B. NOR ✓

The output circle means NOT, so OR plus a circle is NOR. XOR does not use a circle.

3. Which gate symbol is an OR shape with an extra curved line at the input side?

A. XOR ✓

B. NOR

The extra curved line at the inputs marks XOR. NOR has a circle at the output instead.

4. Which describes the NOT gate symbol?

A. A triangle with a small circle on its output ✓

B. A triangle with an extra curved line at its input

NOT is a triangle with an output circle. The curved line at the inputs belongs to XOR.

5. What does the small circle on a gate's output mean?

A. The output is inverted (NOT) ✓

B. The gate has three inputs

The circle always means the output is NOT-ed. It says nothing about the number of inputs.

Match the Pairs

AND gate symbol + small circle on the output → A NAND gate: it gives the opposite of AND, so the output is 0 only when both inputs are 1

OR gate symbol + small circle on the output → A NOR gate: it gives the opposite of OR, so the output is 1 only when both inputs are 0

The XOR gate has no output circle. Its symbol is the OR shape with an extra curved line at the input side. → OR symbol + extra curved line at the inputs (no circle) = XOR

The NOT gate is drawn as a triangle with a small circle on its output, and it has only one input. → Triangle with a circle on the output tip = NOT (A -> X)

Labelling an OR shape with a circle on the output as XOR → A circle on the output means NOT, so OR with a circle is NOR, not XOR. XOR has an extra curved line at the input side instead.

Labelling an AND shape with no circle on the output as NAND → NAND needs a small circle on the output. An AND shape without a circle is just AND.

Key Term

output circle

A small circle on a gate's output means NOT: AND becomes NAND and OR becomes NOR. XOR has no circle; its symbol is OR with an extra curved line at the input side.

Fill in the Blanks

1. AND symbol + output circle = NAND

Hint: Type the name of the gate made by adding a circle to the AND symbol

2. OR symbol + output circle = NOR

Hint: Type the name of the gate made by adding a circle to the OR symbol

3. XOR symbol = OR symbol + extra curved line

Hint: Type the word that describes the shape of the extra line at the input side

4. NOT symbol = triangle + small circle

Hint: Type the shape drawn on the output of the triangle to show NOT

5. NAND gate: AND with output circle X = NOT (A AND B)

Hint: Type the operator that the output circle stands for

Spot the Error

Question 1

A AND: flat back, round front

B NAND: AND shape, no circle

C OR: curved back, pointed tip

Line B — NAND is AND with a small circle on the output. Line 2 should say NAND: AND shape + circle, because without the circle it is just an AND gate.

Question 2

A Draw the NOR symbol:

B Start with the OR shape

C Add a circle on the input

Line C — The NOT circle goes on the gate's output, not the input. Line 3 should say Add a circle on the output.

Question 3

A NOT gate symbol:

B Shape: triangle

C Point faces the output

D Small circle on the input

Line D — The NOT symbol is a triangle with a small circle at its output tip. Line 4 should say the circle is on the output.

Question 4

A XOR: OR shape + small circle

B Extra line: input side

C XOR has no output circle

Line A — XOR is OR with an extra curved line at the input side, not a circle. A circle on the output would make it NOR.

Question 5

A AND: no circle

B NAND: AND + circle

C OR: no circle

D NOR: AND + circle

Line D — NOR is OR with a circle on the output. AND with a circle is NAND. Line 4 should say NOR: OR + circle.

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