Digital Techniques / Electronic Instrument Systems

Binary (base 2), octal (8), hexadecimal (16) and BCD are used throughout avionics. Convert 1011₂ = 11₁₀; 0x2F = 47₁₀. BCD encodes each decimal digit in four bits and is common in displays and ARINC 429 labels.

AND (output high only when all inputs high), OR, NOT, NAND, NOR, XOR. NAND and NOR are 'universal' gates — any function can be built from them. Truth tables and Boolean algebra (A·1 = A, A + Ā = 1, De Morgan's theorems) let you simplify circuits.

A computer comprises CPU (ALU + control unit), memory and I/O, linked by address, data and control buses. Memory types: ROM, PROM, EPROM, EEPROM, RAM (volatile), and flash. Aircraft LRUs use BITE (Built-In Test Equipment) to store fault codes for retrieval via the CMC.

ARINC 429 — one transmitter, up to 20 receivers, single-direction twisted shielded pair, 12.5 or 100 kbit/s, 32-bit words with an 8-bit label and parity. ARINC 629 — bidirectional multi-transmitter (B777). MIL-STD-1553B — dual-redundant with a bus controller and remote terminals. AFDX (ARINC 664) — switched full-duplex Ethernet used on A380/B787.

EFIS replaces electromechanical instruments with PFD and ND displays driven by symbol generators. Related systems: ECAM (Airbus) and EICAS (Boeing) for engine and warning data, FMS for navigation and performance, and CMC/CMS for centralised maintenance. Display technologies: CRT (legacy), AMLCD and LED backlighting.

Optical fibre carries light by total internal reflection in the core; it is immune to EMI, light and high-bandwidth, but sensitive to bend radius and contamination — always fit dust caps and clean per the AMM. Electrostatic-sensitive devices require ESD handling; bonding and screening control EMI and lightning effects.