Digital Techniques / Electronic Instrument Systems
What you must understand
Understand number systems (binary, octal, hexadecimal and binary-coded decimal), conversion between them, and basic arithmetic in binary. Know logic gates (AND, OR, NOT, NAND, NOR, XOR) and how to read a truth table. Study sequential logic at concept level: flip-flops, registers, counters and clocks. Understand computer architecture in an aircraft context: CPU, memory, buses, input/output, and the difference between hardware and software. Know data buses used in avionics (ARINC 429, ARINC 629, MIL-STD-1553 and Ethernet-based systems) at a descriptive level: topology, directionality and typical data rates. Understand electronic instrument displays (CRT and LCD), the meaning of redundancy, built-in test equipment, and the difference between analogue and digital indication, including failure states and invalid-data flags.
Traps in questions
- Binary place values: read right to left as 1, 2, 4, 8, 16…; a common error is reversing the order.
- NAND versus NOR: both are inverted outputs; do not confuse which input combination gives a low.
- One-way versus two-way buses: ARINC 429 is typically one-way (simplex) from a transmitter to receivers; know this.
- Display failure philosophy: a blank or flagged display is not the same as a frozen one; questions test what the crew sees.
- BIT types: power-up, continuous and initiated BIT have different coverage; know the distinction.
Worked example
Convert binary 1011 to decimal: (1×8) + (0×4) + (1×2) + (1×1) = 11. To hexadecimal, group in fours from the right: 1011 = B.
Rule of thumb
For any logic question, write the truth table before reasoning; it removes almost all ambiguity.