Digital Techniques / Electronic Instrument Systems
What you must know
Candidates should understand number systems (binary, octal, hexadecimal), logic gates and truth tables, combinational versus sequential logic, flip-flops and counters, and the architecture of digital systems: buses, memory, processors, and data conversion (ADC/DAC). Aircraft-specific content includes digital data buses (ARINC 429, ARINC 629, MIL-STD-1553), electronic displays (EFIS, EICAS/ECAM), and integrated modular avionics concepts at a descriptive level.
Traps to watch
- Binary place values: read right to left as 1, 2, 4, 8, 16...; 1011₂ = 11₁₀, not 13.
- Hexadecimal to binary: each hex digit is exactly four bits; do not drop leading zeros.
- Gate identification: NAND and NOR are inverted AND and OR; a NAND with both inputs tied together acts as an inverter.
- ADC resolution: more bits means finer resolution; resolution = full-scale range / 2ⁿ.
- Bus descriptions: ARINC 429 is a point-to-point, one-transmitter/many-receiver bus; do not confuse it with bidirectional multi-transmitter buses.
Worked example
An 8-bit ADC has a full-scale input of 10 V. Number of steps = 2⁸ = 256, so resolution = 10/256 ≈ 0.039 V (about 39 mV). A digital output of 128 represents approximately half scale, i.e. about 5 V.
Rule of thumb
For quick binary conversion, group bits in fours from the right to get hex, and remember each hex digit maps directly to four binary bits.