Digital Techniques / Electronic Instrument Systems
Core concepts
You need number systems (binary, octal, hexadecimal and conversions), logic gates and truth tables, combinational and sequential logic (flip-flops, counters, registers), buses and data transmission (serial versus parallel, ARINC 429, ARINC 629, MIL-STD-1553, Ethernet), analogue-to-digital and digital-to-analogue conversion, displays (CRT, LCD, LED), and electronic instrument systems including EFIS, EICAS/ECAM, FMS, and glass cockpit architecture with redundancy and reversionary modes.
Traps to watch
- Bit significance: most significant bit is the leftmost in written binary; get the place values right (1, 2, 4, 8, 16...).
- Gate identification: NAND and NOR are inverted AND and OR; an exclusive-OR outputs 1 only when inputs differ.
- Serial vs parallel: serial is slower per word but needs fewer wires and suits long runs.
- ADC resolution: more bits means finer resolution, not necessarily a faster conversion.
- Redundancy: know the difference between duplicate, triple redundant and dissimilar redundancy, and what a reversionary mode does when a display fails.
Worked example
Convert binary 1011 to decimal: (1×8) + (0×4) + (1×2) + (1×1) = 11. In hexadecimal, group bits in fours from the right: 1011 = B. An 8-bit ADC with a 5 V reference has a resolution of 5 ÷ 256 ≈ 19.5 mV per step.
Rule of thumb
For logic questions, draw the truth table rather than reasoning in your head. For data-bus questions, first identify whether the system is serial or parallel and who is the bus controller.