Digital Electronics and Logic Gates
This domain tests number systems, logic gates, Boolean algebra, combinational blocks and simple sequential circuits. Typical questions require a truth table, a simplification, or identification of a gate function.
Core rules
- Binary place values are powers of 2: 8 4 2 1. Hexadecimal groups four bits.
- Basic gates: AND (A·B), OR (A+B), NOT (A′). NAND and NOR are universal gates.
- XOR gives 1 when inputs differ; XNOR gives 1 when they are equal.
- De Morgan: (A·B)′ = A′ + B′ and (A+B)′ = A′·B′.
- Combinational circuits include adders, comparators, encoders, decoders and multiplexers; their output depends only on present inputs.
- Flip-flops store one bit. A clocked D flip-flop copies D to Q on the active edge. Counters and shift registers are built from flip-flops.
- A multiplexer selects one of several inputs under control of select lines.
Worked example
Simplify Y = A′B + AB. Factor B: Y = B(A′ + A). Since A′ + A = 1, Y = B. The two-input truth table confirms that Y follows B regardless of A. A common exam follow-up is to realise Y = B with a buffer or with a NAND gate wired as an inverter.
Marks-losing traps
Do not treat XOR as OR: XOR is 1 only when inputs differ. An active-low signal is asserted at 0, not 1. In a truth table, list all 2ⁿ input combinations. For edge-triggered flip-flops, the output changes only at the clock edge, not throughout the high level. Do not confuse a latch and a flip-flop.