Digital Techniques and Electronic Instrument Systems
Core concepts
Numbering systems and conversion between binary, octal, decimal and hexadecimal; binary addition and subtraction, including two's complement for negative numbers; and binary-coded decimal. Logic covers the AND, OR, NOT, NAND, NOR and exclusive-OR gates, truth tables, and simple combinational circuits. Then: sequential logic, flip-flops, counters, registers, memory types (RAM, ROM, PROM, EPROM, EEPROM, flash) and their volatility. Aircraft computing covers microprocessors, buses, input/output, analogue-to-digital and digital-to-analogue conversion, sampling, and data bus standards such as ARINC 429 and 629. Electronic instrument systems cover the CRT and LCD display, EFIS with EADI and EHSI, EICAS and ECAM, FMS, and the integration of air data and attitude systems.
Traps in questions
- Two's complement: invert bits and add one; the most significant bit becomes the sign.
- Bit numbering starts at bit 0 for the least significant bit.
- ARINC 429 is a one-way, 32-bit word bus operating at high or low speed (100 kbit/s or 12.5 kbit/s); it is not bidirectional like a two-wire data bus.
- RAM is volatile; ROM retains data without power.
- Confusing symbol rate with bit rate, and resolution with accuracy, in converter questions.
Worked example
Convert decimal 45 to binary: 45 = 32 + 8 + 4 + 1 = 101101₂. To express −45 in 8-bit two's complement: 45 = 00101101, invert to 11010010, add one → 11010011. For a converter, an 8-bit ADC over a 0–10 V range has a resolution of 10/256 ≈ 39 mV per step.
Rule of thumb
An n-bit converter has 2ⁿ steps; resolution = full-scale range ÷ 2ⁿ. A 10-bit ADC over 5 V resolves about 4.9 mV.