wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Embedded Networks in Civilian Aircraft Avionics Systems 24-5
executive called application executive (APEX) is an important step in developing avionics applications
without knowing the characteristics of the target processing and communicating architecture.
The first examples of IMA implementation, such as the Boeing 777 airplane information management
system (AIMS) show many potential benefits: adaptability (thanks to the modularity of the architecture,
it is easy to configure the aircraft for a given mission), maintainability (the standardization of the modules
simplifies the maintenance), cost reductions (easier maintenance and hardware evolution cut down
costs), and a decrease in weight and volume (fewer devices are needed than with previous architectures).
Considering these benefits, Airbus chose the IMA architecture for the A380, and Boeing reused the
IMA concepts for the 787 aircraft. Since the architecture is independent of the chosen communication
method used, we will present in following sections the ARINC 629 multiplexed data bus, which was
chosen for the Boeing 777 AIMS and the ARINC 664 or AFDX, which was chosen as the basis of the
IMA architecture of the A380.
24.5 arINC 629 Multiplexed Data Bus
Development of the ARINC 629 [ARI99] multiplexed data bus began when avionics systems designers realized
that the ARINC 429 single transmitter/multiple receiver concept could not cope with the increasing
amount of intersystem data transfer required for evolving commercial aircraft. The primary advantages of
a multi-transmitter data bus such as the ARINC 629 include the ability to move more data between LRUs
at higher rates using fewer wires. Also they are generally more reliable and provide an architecture allowing
the integration of complex systems. However, multitransmitter buses have been mainly used in the
context of military aircraft. The MIL-STD-1553 data bus standard [MIL78] has been developed for military
and space systems. It implements a bus architecture in which all the devices attached to the bus are capable
of receiving and transmitting data. However, it uses a command/response (centralized) mechanism that
does not satisfy the terminal independence needed for certification of commercial aircraft. Moreover, classic
Ethernet architecture and its random CSMA-CD medium access control mechanism cannot guarantee
periodic traffic. The digital autonomous terminal access communication (DATAC) project engendered
the ARINC 629 [ARI99], an avionics industry digital communications standard, which was chosen as the
primary digital communication system on the Boeing 777.
This standard allows the transmission of messages at a 2.Mbps serial data rate on twisted pair conductors.
A message has variable length and is comprised of up to 31 word strings. Each word string has variable length
and contains a 16 bit label word and up to 256 data words (each 16 bit word is actually transmitted with
3 additional bits of synchronization and a parity bit). Each LRU communicating on the ARINC 629 data bus
contains a terminal controller, which implements the bus protocol access logic that determines when the terminal
will transmit. The terminal listens to the bus and waits for series of quiet periods, one of unique duration
before transmitting (CSMA like mechanism + TDMA like allocation by the definition of preassigned
waiting times). Only one terminal is allowed to transmit at a time, and once a terminal has transmitted, it
must wait before it transmits again to give time to all other terminals which want to transmit on the bus.
The ARINC 629 standard supports two alternative data link level protocols, the basic protocol (BP) and the
combined protocol (CP), which cannot coexist on the same bus due to fundamental differences.
24.5.1 Basic Protocol
The BP provides an equal priority access to the bus for each terminal, giving the opportunity to transmit
periodic data. However, as shown in Figure 24.4, if the cycle is too short to transmit the data from all
terminals, the bus automatically loses this periodic behavior in favor of an aperiodic mode where
all messages are transmitted, without data loss.
For a given terminal i (from a set of n connected to the bus), TGi represents the unique time (terminal
gap), during which a terminal must wait after bus activity before starting its own transmission. For
all n terminals, TI represents a common time interval (transmit interval), separating two successive
© 2011 by Taylor and Francis Group, LLC