Data Buses

Data Buses (5.4)

• Data buses in aircraft systems are described
• Data bus communication protocols are described, including MIL-STD 1553, ARINC 429, ARINC 629, Ethernet, and AFDX.

Electric Power

• Electricity is transmitted over heavily constructed power lines from power stations.
• Power lines carry very high voltages and a large electrical current.
• As power is stepped down, the diameter of the wiring decreases to conduct electricity efficiently.
• Different applications require different power amounts.
• Navigation lights and aircraft starter motors both use DC power but require different current levels.

Electrical Data Transmission

• Electricity is an ideal medium for transmitting signals because it travels at the speed of light.
• Data lines ideally have no current flow but can have very small current levels to forward and reverse bias semiconductor P and N junctions.
• Electrical signals are typically small gauge
• Electrical signals transmitted through ICs and data cables (or wireless)
• Information transmitted is in a regulated sequence.
• Computers at either end of the data bus decode and use the data to produce the desired outputs, ranging from displaying location to controlling various mechanisms in an aircraft.

Digital Data Transfer

• Ideal digital signals are square waves.
• Practical digital signals will be distorted due to the nature of transistors.
• In electronic digital systems, binary data is represented by the presence or absence of a voltage.
• Binary 0 is typically represented by 0 V, and binary 1 by 5 V.
• Voltages between 0 V and 0.8 V represent binary 0; between 2 V and 5 V, binary 1.
• Clock signals are used to synchronise data transfer between components

Serial Data Transfer

• Parallel data transfer and serial data transfer are two basic ways to move enormous amounts of data in digital computers.
• In serial transfer, each bit is transmitted sequentially along one line.
• Serial bus is used for long-distance communications and uses less hardware than parallel transfer.
• Serial data transfer is typical of data bus communications
• Multiplexing is used to increase the capacity of serial data buses.

Parallel Data Transfer

• In parallel transfer, each bit is transmitted over its own separate line.
• Parallel data transfer is much faster than serial transfer.
• Parallel transfer requires more hardware requiring more space and weight in aircraft design.

Multiplexing

• Multiplexing combines multiple information channels onto a common transmission medium on aircraft.
• Reduce the number of wires to improve circuit reliability.
• Multiplexing uses time division technique to carry multiple signals on one conductor.

Aircraft Multiplex System

• In the 1950s and 1960s, avionics systems were primarily composed of many separate boxes requiring a complex wiring system.
• The weight and space constraints and the overall complexity of wiring increase as more systems are added.
• The new digital systems were designed to share information among systems through a common digital data bus and have a significant reduction in the weight and wiring complexity.
• New digital technology and the introduction of microprocessors improved computational capability making the systems much more efficient.

Data Bus Systems

• The interface between each computer and external device is accomplished via the digital data bus.
• The bus consists of shielded, twisted wires to eliminate electromagnetic interference.
• Data may travel in one way (simplex) or two directions (duplex), depending on the system design.

Data Bus Connectors

• Multiplexer bus functions like an arterial highway.
• Components are connected to the bus using breakouts and couplers.
• Each computer can have several terminals and can be active at any one time.

Bus Controller

• The bus controller manages all communication on the data bus
• Only one BC is active at any one time
• The BC issues commands controlling data transfer or bus management.

Mil-Std-1553 Data Bus

• Data transmission medium for aircraft communication between systems and subsystems using a single common set of wires
• Time division multiplexing is used.
• Three distinct word types: Command words, Data words, and Status Words

Mil-Std-1773

• Fibre optic cabling is used as a transmission medium in the MIL-STD-1553B bus protocol
• The standard repeats MIL-STD-1553
• The standard does not specify power levels, noise levels, spectral characteristics, optical wavelength, electrical/optical isolation or means of distributing optical power, these will be defined in the specifications of the intended use.
• The standard has differing environmental concerns compared to traditional wire conductors.

Aeronautical Radio Incorporated (ARINC)

• ARINC 429 data bus system is a unidirectional communication protocol
• The system uses a low bit rate (either 12,5 or 100 kb/s)
• The system is used for transmission on separate ports ensuring many separate wires are not needed.
• There are various specifications and standards for avionics equipment, including ARINC 429 and general aviation subset.

ARINC 629

• ARINC 629 is a more recent design for a global data bus within an aircraft, offering faster transfer rates and greater integration capabilities.
• Designed to carry all data between various components.
• Uses a single twisted pair of wires for communication.
• Data is structured to avoid transmission issues.