Chapter 6: Basic Computer Structure

Computer terminology

Nearly every system on a modern aircraft contains a computer: the flight management computer, air data and inertial reference units, engine controls (FADEC), flight control computers, display computers, brake and anti-skid controllers, even galley ovens and cabin lighting. They all share the same basic structure and vocabulary.

• Bit — one binary digit. Nibble — 4 bits. Byte — 8 bits, enough for one character or a number from 0 to 255. Word — the number of bits the processor handles in one operation (8, 16, 32 or 64).
• Hardware — the physical parts: circuit boards, integrated circuits, connectors, power supplies, displays.
• Software — the programs (sets of instructions) and data that tell the hardware what to do. The operating system manages the hardware and runs the application programs.
• Firmware — software stored permanently in non-volatile memory inside the hardware (in ROM or flash), such as the start-up (boot) program of an LRU.
• Programs are written in a high-level language (aviation software is often written in C or Ada) and translated by a compiler into machine code — the binary instructions the processor executes. Assembly language is a readable form of machine code.
• IC — integrated circuit: a complete circuit on one silicon chip. A microprocessor is a CPU on a single IC; a microcontroller adds memory and I/O on the same chip.

The parts of a computer

• CPU (central processing unit): the control unit fetches each instruction from memory, decodes it and sequences the operations; the ALU (arithmetic logic unit) adds, subtracts, compares and performs logic operations (AND, OR, NOT); registers are small, very fast storage locations inside the CPU — the accumulator holds results, the program counter holds the address of the next instruction, and the status (flag) register records results such as zero, carry or negative.
• Clock: a crystal oscillator that produces regular pulses to step the CPU through its operations. Its frequency (MHz or GHz) is one measure of speed.
• Memory: holds the program and the data. Each location has a numbered address. With n address lines the CPU can reach 2ⁿ locations — 16 lines address 65 536 (64 K).
• Input/output (I/O): interfaces that connect the computer to the outside world — on aircraft, ARINC 429 or AFDX bus interfaces, discrete inputs and outputs, analogue-to-digital and digital-to-analogue converters, display and keyboard interfaces, and the data-loader port.
• Buses: the address bus (one-way, from the CPU), the data bus (two-way) and the control bus (read/write, clock, interrupt and reset signals). An interrupt makes the CPU stop what it is doing to deal with an urgent event.
• How a program runs: the CPU repeats a fetch–decode–execute cycle — fetch the instruction whose address is in the program counter, decode it, execute it, then move on. (Details of microprocessor operation are B2 knowledge, sub-module 5.7.)

Memory devices

• Volatile memory loses its contents when power is removed; non-volatile memory (NVM) keeps them. Avionics computers keep their fault history, configuration and calibration data in NVM so that it survives a power-down — which is why BITE can tell you what happened on the last flight.
• Access time is the time to read a location; capacity is the number of bits or bytes stored. Some LRUs use battery-backed RAM; its battery is a maintenance item.
• Mass storage holds large amounts of data more slowly: magnetic (hard disks, tape — moving parts, sensitive to vibration and magnetic fields), optical (CD, DVD — read by laser) and solid-state (SSDs, CompactFlash, SD and PCMCIA cards — no moving parts, robust). Aircraft use them for quick-access recorders, maps, documentation, cabin entertainment and data loading.

Computer technology in aircraft systems

• LRUs: avionics computers are line-replaceable units in standard cases (ARINC 600, older ARINC 404 'ATR' sizes) that slide into racks in the electronics bay, with a rear connector and forced-air cooling. A failed unit is changed on the line and repaired in the shop.
• Architecture: in a federated system each function has its own computer. In integrated modular avionics (IMA) several functions share common processing modules (Chapter 13).
• Redundancy: critical functions use two or three computers, or command and monitor channels inside one computer, which compare their results and disconnect if they disagree. Flight control computers may use dissimilar processors and software so that one design error cannot affect them all.
• BITE (built-in test equipment): computers test themselves at power-up and continuously, store faults in NVM and report them to a central maintenance computer (Chapter 12). A watchdog timer resets or fails the computer if its software stops running normally.
• Program pins on the rear connector are wired (grounded or open) in the aircraft to tell a computer its position (left or right, engine 1 or 2) and which options are fitted, so the same part number can be used in several positions.
• Power: power supplies contain hold-up capacitors to ride through the brief interruptions that occur when electrical buses transfer; longer interruptions cause a controlled restart.
• Software loading: most computers now hold their programs in flash memory and are loaded on the aircraft with a data loader (Chapter 10). Software and hardware part numbers must match the approved configuration.
• Computers contain static-sensitive devices (Chapter 9) and need their cooling air — an overheated LRU is a common source of intermittent faults.