Digital Techniques and Electronic Instrument Systems
Core content
Digital techniques includes number systems, binary arithmetic, two's complement, BCD, hexadecimal, logic gates, Boolean expressions, combinational and sequential logic, flip-flops, counters, registers, memory, buses, and analogue-to-digital conversion. You should understand the difference between serial and parallel data transfer, and between one-way and bidirectional buses. Aircraft systems use data buses such as ARINC 429, ARINC 629 and ARINC 664, and display systems such as EFIS and ECAM or EICAS. Know the meaning of redundancy, fault tolerance, built-in test, and display colour conventions: red for warnings, amber for cautions, green for normal, and white or blue for advisory information.
Traps
Two's complement questions catch candidates who forget that the most significant bit represents a negative value. Active-low signals are another trap: a line labelled with an overbar or a suffix is asserted when low. Do not assume all data buses are bidirectional; ARINC 429 is a one-way, point-to-many bus with a 32-bit word. Also distinguish a latch from a flip-flop: a latch is level-sensitive and a flip-flop is edge-triggered. Display colour meanings should not be confused with physical lamp colours.
Worked example
Convert binary 10110110 to decimal. From left to right: 128 + 0 + 32 + 16 + 0 + 4 + 2 + 0 = 182. Check by grouping: 1011 = 11 and 0110 = 6, so in hexadecimal it is B6, which is 11 x 16 + 6 = 182. Rule of thumb: group binary digits in fours from the right when converting to hexadecimal.
Key points
- Two's complement uses the most significant bit as a negative weight.
- Active-low signals are asserted at logic 0.
- ARINC 429 is a one-way 32-bit data bus.
- Red means warning, amber caution, green normal.
- Group binary bits in fours to convert to hexadecimal.