Combinational logic (A-level only) Flashcards – AQA A-level Physics (7408)

What determines the output of a combinational logic circuit?

Its present input values.

What operations do $\overline{A}$, $A\cdot B$ and $A+B$ represent in Boolean expressions?

• $\overline{A}$: NOT $A$
• $A\cdot B$: $A$ AND $B$
• $A+B$: $A$ OR $B$, not arithmetic addition

What do brackets and overbars show in a Boolean expression?

How the operations are grouped.

How can a combinational logic circuit be analysed to find its output?

Follow the signal connections from the inputs through successive gates, applying each gate’s operation to find intermediate and final expressions or binary values.

What does a truth table for a whole logic circuit show?

Every possible input combination and its corresponding final output.

How can a Boolean expression be used to construct a truth table?

Evaluate the expression for each possible input combination.

What does a circuit’s truth table specify?

The required output for every combination of inputs.

Which truth-table rows are used to construct Boolean row conditions for a circuit?

The rows where the required output is 1.

How is a Boolean condition formed for one input combination in a truth table?

Use each input directly if it must be 1, invert it if it must be 0, and combine the requirements with AND.

How are the Boolean conditions for output-1 rows combined into a circuit expression?

Combine them with OR, so the output is 1 when any required combination occurs.

How is a Boolean expression implemented as a logic circuit?

Use gates for the operations, supplying inversions where needed.

How is a constructed logic circuit checked against its truth table?

Evaluate the circuit for every input combination and compare its outputs with the truth table.

Must a valid circuit for a truth table be unique?

No. Different gate combinations can realise the same truth table.