Knowledge Organiser: Truth Tables and Combined Logic Circuits
This topic summary covers Knowledge Organiser: Truth Tables and Combined Logic Circuits within Truth Tables for GCSE Computer Science. Revise Truth Tables in Boolean Logic for GCSE Computer Science with 15 exam-style questions and 15 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 11 of 11 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 11 of 11
Practice
15 questions
Recall
15 flashcards
Knowledge Organiser: Truth Tables and Combined Logic Circuits
Key Terms
- Truth table: A table showing all possible input combinations and their corresponding outputs
- Combined logic circuit: A circuit using multiple gates connected together
- Intermediate column: A column in a truth table showing the output of a gate within a larger circuit
- Boolean order of operations: Brackets → NOT → AND → OR (same concept as BIDMAS)
Must-Know Facts
- Number of rows in a truth table = 2ⁿ where n is the number of inputs
- 2 inputs = 4 rows; 3 inputs = 8 rows; 4 inputs = 16 rows
- Always show intermediate columns — you earn method marks even if the final answer is wrong
- Order of operations: Brackets first, then NOT, then AND, then OR (lowest priority)
- Work column by column (complete all rows for one gate before moving to the next)
- Input columns follow a binary counting pattern: rightmost alternates 0,1,0,1…
Key Concepts
- To evaluate NOT(A) AND B: Step 1 calculate NOT(A); Step 2 apply AND with B
- To evaluate (A AND B) OR C: Step 1 calculate A AND B; Step 2 apply OR with C
- Label each intermediate column clearly (e.g. "NOT A", "A AND B")
- Double-check row count before submitting — wrong row count loses all marks
Common Mistakes
- Wrong number of rows: Always use 2ⁿ rows where n is the number of inputs — 2 inputs = 4 rows, 3 inputs = 8 rows; a missing row means marks are lost for every output in that row
- Filling input columns in wrong order: The rightmost input column alternates 0,1,0,1 — the next column alternates 0,0,1,1,0,0,1,1 — this binary counting pattern must be followed exactly
- Applying operations in the wrong order: Evaluate brackets first, then NOT, then AND, then OR — skipping intermediate columns makes errors very hard to spot
- Confusing AND and OR outputs: AND only outputs 1 when BOTH inputs are 1; OR outputs 1 when ANY input is 1 — mixing these up is the most common single error in truth table questions
Revise this topic interactively on PrepWise — self-test mode, tap-to-reveal definitions, and Common Mistakes from examiners.
Try the interactive Knowledge Organiser — free →Keep building this topic
Read this section alongside the surrounding pages in Truth Tables. That gives you the full topic sequence instead of a single isolated revision point.
Practice Questions for Truth Tables
How many rows are needed in a truth table with 2 inputs (not including the header row)?
Explain why truth tables are useful when designing or testing logic circuits.
Quick Recall Flashcards
15 questions on Truth Tables — practise free
Instant marking, adaptive difficulty, and 15 spaced repetition flashcards. Free until your GCSEs.
Try PrepWise Free