Digital Techniques and Aircraft Computers
Core concepts
Digital systems represent values in binary; a byte is 8 bits, and a word is the processor's natural width. Convert binary to hex in groups of four bits, and hex to decimal by place value. Logic gates - AND, OR, NOT, NAND, NOR, XOR - are described by truth tables. Combinational logic has no memory, while flip-flops and registers are sequential.
Aircraft data buses matter. ARINC 429 is a one-way, point-to-point bus using 32-bit words. MIL-STD-1553 is a bidirectional command/response bus, usually dual-redundant, with a bus controller and remote terminals. ARINC 629 is bidirectional, and ARINC 664 is Ethernet-based. LRUs communicate through these buses rather than by direct wiring.
Computers use ADCs and DACs to bridge analog and digital worlds; resolution and sampling rate limit accuracy. Memory divides into RAM (volatile) and ROM (non-volatile). Built-in test is power-up (PBIT), initiated (IBIT) or continuous (CBIT). Software is controlled by part number and configuration.
Traps
Questions often reverse bus direction or mix ARINC 429 with 1553. Binary and hex conversions and gate outputs are common, as is assuming a data bus is a power wire.
Worked example
Convert 1011 0110 to hex: split into 1011 and 0110 = B and 6, so 0xB6. Decimal = 128 + 32 + 16 + 4 + 2 = 182.