Digital Electronics and Logic Basics
This domain tests binary number sense, logic gates, truth tables and simple combinational and sequential circuits. Questions often ask you to predict an output or build a function from NAND gates.
Gates and truth tables
- AND: output 1 only when all inputs are 1.
- OR: output 1 when any input is 1.
- NOT: inverts the input.
- NAND and NOR: inverted AND and OR; both are universal gates.
- XOR: output 1 when inputs differ; useful for parity and adders.
- Binary place values are 1, 2, 4, 8, 16, ...; convert by summing set bits.
Worked example
A 2-input NAND gives output 0 only for A = 1, B = 1; all other combinations give 1. To make a NOT gate, tie both NAND inputs together: input 1 gives 0, input 0 gives 1. To make an AND gate, feed A and B into a NAND, then invert the result with a second NAND whose inputs are tied. This demonstrates why NAND is called universal.
Sequential logic
Flip-flops store one bit; counters and shift registers are built from them. A clock edge changes state, so these circuits have memory, unlike combinational gates.
Traps that cost marks
Confusing NAND with NOR; forgetting that an active-low signal is asserted at 0 V; misreading binary place values; assuming a gate output is always 5 V rather than a logic level; ignoring propagation delay in timing questions.
Exam focus
Practise drawing truth tables from a circuit description and converting between binary, decimal and hexadecimal.