Digital Techniques / Electronic Instrument Systems
What the module tests
This module covers number systems, data conversion, logic circuits, computers, data buses, fibre optics, displays and electronic instrument systems such as EICAS/ECAM and integrated avionics. Expect binary/hex conversion, gate logic and bus architecture questions.
Core rules
- Binary place values are powers of 2; group four bits to convert to hexadecimal.
- In two's complement, invert all bits and add 1 to represent a negative number.
- Logic gates: AND outputs 1 only if all inputs are 1; OR if any input is 1; XOR if inputs differ; NOT inverts.
- ARINC 429 is a point-to-point, single-transmitter bus; ARINC 629 and AFDX are shared buses with different arbitration.
- Fibre-optic cable carries data as modulated light and is immune to EMI, but is sensitive to bend radius and contamination.
Worked example
Convert 10110110₂ to decimal and hexadecimal, and interpret it as an 8-bit two's complement number. Binary: 128 + 32 + 16 + 4 + 2 = 182. Hex: group 1011 0110 = B6. As two's complement, the leading 1 means negative; invert to 01001001, add 1 to get 01001010 = 74, so the value is -74.
Traps
Reading bits in the wrong order; forgetting that hex B6 is not decimal 182 when the question asks for a signed value; confusing a bus with a protocol; assuming fibre optics are unaffected by contamination; mixing display symbology with the underlying system.
Exam approach
Write the place values above the bits, then group for hex. For logic, build a truth table rather than guessing. For buses, link the architecture to its aircraft role.