ARINC Specification 825 - The General Standardization of CAN for Airborne Use

Controller Area Network

Controller Area Network

Controller–area network is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other within a vehicle without a host computer....

 (CAN) increasingly found its way into aerospace

Aerospace

Aerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...

 applications because of its cost effective and efficient networking capability for systems employing the Line-replaceable unit

Line-replaceable unit

A line-replaceable unit is a modular component of an airplane, ship or spacecraft that is designed to be replaced quickly at an operating location. An LRU is usually a sealed unit such as a radio or other auxiliary equipment...

 (LRU) concept to share data across a common media. The ability of CAN to transmit data, across a shared shielded twisted pair

Twisted pair

Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...

 cable, has advantages in terms of weight savings at the aircraft integration level. Additionally, the CAN physical layer

Physical layer

The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....

 protocol specification provides error recovery and protection mechanisms that make this data bus standard attractive to aviation applications. Newer commercial air transport aircraft like the Airbus

Airbus

Airbus SAS is an aircraft manufacturing subsidiary of EADS, a European aerospace company. Based in Blagnac, France, surburb of Toulouse, and with significant activity across Europe, the company produces around half of the world's jet airliners....

 A380

Airbus A380

The Airbus A380 is a double-deck, wide-body, four-engine jet airliner manufactured by the European corporation Airbus, a subsidiary of EADS. It is the largest passenger airliner in the world. Due to its size, many airports had to modify and improve facilities to accommodate it...

 or the Boeing

Boeing

The Boeing Company is an American multinational aerospace and defense corporation, founded in 1916 by William E. Boeing in Seattle, Washington. Boeing has expanded over the years, merging with McDonnell Douglas in 1997. Boeing Corporate headquarters has been in Chicago, Illinois since 2001...

 787

Boeing 787

The Boeing 787 Dreamliner is a long-range, mid-size wide-body, twin-engine jet airliner developed by Boeing Commercial Airplanes. It seats 210 to 290 passengers, depending on the variant. Boeing states that it is the company's most fuel-efficient airliner and the world's first major airliner to use...

 already accommodate between 50 and 250 CAN networks for all sorts of functions including flight deck

Flight deck

The flight deck of an aircraft carrier is the surface from which its aircraft take off and land, essentially a miniature airfield at sea. On smaller naval ships which do not have aviation as a primary mission, the landing area for helicopters and other VTOL aircraft is also referred to as the...

 systems, engine control and flight control systems. In an effort to provide a common standard for the use of CAN in commercial air transport, Airbus and Boeing initiated the CAN Technical Working Group of the Airlines Electronic Engineering Committee to define the ARINC

ARINC

Aeronautical Radio, Incorporated , established in 1929, is a major provider of transport communications and systems engineering solutions for eight industries: aviation, airports, defense, government, healthcare, networks, security, and transportation...

 specification 825. The target of ARINC 825 is to ensure interoperability and to simplify interoperation of CAN subsystems with other airborne networks for all classes of aircraft including the commercial air transport segment. The CAN Technical Working Group initially consisted of members from Airbus

Airbus

Airbus SAS is an aircraft manufacturing subsidiary of EADS, a European aerospace company. Based in Blagnac, France, surburb of Toulouse, and with significant activity across Europe, the company produces around half of the world's jet airliners....

, Boeing

Boeing

The Boeing Company is an American multinational aerospace and defense corporation, founded in 1916 by William E. Boeing in Seattle, Washington. Boeing has expanded over the years, merging with McDonnell Douglas in 1997. Boeing Corporate headquarters has been in Chicago, Illinois since 2001...

, Rockwell Collins

Rockwell Collins

Rockwell Collins, Inc. is a large United States-based international company headquartered in Cedar Rapids, Iowa, primarily providing aviation and information technology systems and services to governmental agencies and aircraft manufacturers.- History :...

, GE Aerospace and Stock Flight Systems (http://www.stockflightsystems.com) and published the ARINC 825 specification in November, 2007 with supplement 1 being released by May, 2010. The ARINC specification 825 was influenced by the CANaerospace

CANaerospace

CANaerospace is a higher layer protocol based on Controller Area Network which has been developed by Stock Flight Systems in 1998 for aeronautical applications.-Background:...

 standard to a high degree.

Role of CAN in Commercial Air Transport and General Aviation Aircraft

Current commercial air transport aircraft system architectures have incorporated CAN as an ancillary subsystem bus to ARINC Specification 664, Part 7 (AFDX), networked Integrated Modular Avionics

Integrated Modular Avionics

Integrated modular avionics represent real-time computer network airborne systems. This network consists of a number of computing modules capable of supporting numerous applications of differing criticality levels....

 (IMA) architectures. For these aircraft, CAN has been used to link sensors, actuators and other types of avionics

Avionics

Avionics are electronic systems used on aircraft, artificial satellites and spacecraft.Avionic systems include communications, navigation, the display and management of multiple systems and the hundreds of systems that are fitted to aircraft to meet individual roles...

 devices that typically require low to medium data transmission volumes during operation. In this role, CAN complements higher capacity networks that support systems controlling the flight deck information flow and presentation. In contrary, General Aviation

General aviation

General aviation is one of the two categories of civil aviation. It refers to all flights other than military and scheduled airline and regular cargo flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights...

 system architectures employ CAN as one of the major avionics buses or even as the avionics backbone network. In this role CAN may have to fulfill all requirements of a flight safety critical network. The ARINC Specification 825 enhances CAN to create a network that embraces both philosophies. It may be used as a primary or ancillary avionics network and was designed to meet the following requirements:

• Easy connections of local CAN networks to other airplane networks.

• Minimal cost of implementation and cost of change over time.

• Maximum interoperability and interchangeability of CAN connected LRUs.

• Configuration flexibility: easy addition, deletion, and modification of bus nodes, without undue impact onto other LRUs.

• Simplified system and network boundary crossing for both parametric and block data transfers.

• Integrated error detection and error signaling.

• Support for system level functions such as on-board data load and airplane health management.

Physical Interface

To ensure interoperability and reliable communication, ARINC 825 specifies the electrical characteristics, bus transceiver requirements and data rates with the corresponding tolerances based on ISO 11898

ISO 11898

ISO 11898 specifies a serial communication technology called Controller Area Network that supports distributed real-time control and multiplexing for use within road vehicles.- Structure of the ISO 11898 standard :...

. The bit timing calculation (baud rate accuracy, sample point definition) and robustness to electromagnetic interference are given special emphasis. Also addressed within ARINC 825 are CAN connector and wiring considerations. The data rates supported by ARINC 825 are 1000 kbit/s, 500 kbit/s, 250 kbit/s, 125 kbit/s and 83.333 kbit/s.

Identifier Usage and Communication Layers

ARINC 825 is entirely based on CAN 2.0B using extended frames (29-bit identifiers) which provide an adequate number of bits to divide the identifier into several sub-fields. These sub-fields are key issues in employing the identifier bits not only for the data object identification and transmission prioritization inherent to CAN but also for the purpose of creating a standardized application layer. CAN communication using 11-bit identifiers may coexist on an ARINC 825 bus if it is free of potential deadlock scenarios caused by single source bus masters.

The communication mechanisms of ARINC 825 are derived from the corresponding CANaerospace

CANaerospace

CANaerospace is a higher layer protocol based on Controller Area Network which has been developed by Stock Flight Systems in 1998 for aeronautical applications.-Background:...

 mechanisms. Just like CANaerospace, ARINC 825 defines additional ISO layer 3, 4 and 6 functions to support logical communication channels, one-to-many/peer-to-peer communication and station addressing. To accomplish this, the 29-Bit CAN identifier is given a special structure for ARINC 825 (see Figure 1). Logical Communication Channels (LCCs) provide these independent layers of communication (see Figure 2).

Interoperability

To support interoperability in airborne systems, ARINC 825 includes:

• Data Endian definition (Big Endian exclusively)

• Data Type definition (Boolean, Integer, Floating-Point, ....)

• Aeronautical Axis System and Sign Convention (ISO1151/EN9300)

• Engineering Unit definitions (m, kg, ....)

• Aircraft Function definition (Flight State, Air Data, ....)

The aircraft function definitions used to identify source and destination of messages are derived from the Air Transport Association

Air Transport Association

Airlines for America , formerly known as Air Transport Association of America, Inc. , is America's oldest and largest airline trade association. A4A member airlines and their affiliates transport more than 90 percent of U.S. airline passenger and cargo traffic. Based in Washington, D.C., the...

 (ATA) aircraft system chapters. This helps system engineers to assign the proper functions for their systems based on definitions well known in aeronautics since decades.

Bandwidth Management

ARINC 825 adopted the CANaerospace

CANaerospace

CANaerospace is a higher layer protocol based on Controller Area Network which has been developed by Stock Flight Systems in 1998 for aeronautical applications.-Background:...

 bandwidth management concept known as "Time Triggered Bus Scheduling". This concept provides a means of computing the bus load based on the number of messages in a network segment and adjusting their transmission rates. Bandwidth management minimizes peak load scenarios and jitter caused by the CAN bus arbitration. Applying this concept, it can be demonstrated that ARINC 825 networks behave predictably and are able to fulfill the requirements for flight safety critical systems. For ensuring this under fault conditions the system designer has to define the behaviour under these conditions (such as high occurrence of error frames and avoidance of priority inversion

Priority inversion

In computer science, priority inversion is a problematic scenario in scheduling when a higher priority task is indirectly preempted by a lower priority task effectively "inverting" the relative priorities of the two tasks....

) .

ARINC 825 may be used for systems classified up to Design Assurance Level (DAL) A if the effect of the loss of one bus does not present a hazard exceeding the classification "major". Figure 3 shows an example of two ARINC 825 nodes operating in accordance with the Time Triggered Bus Scheduling concept.

Communication Profile Database

ARINC 825 uses a communication profile database for the description of integrated networks. A communication profile is created for each LRU in a human readable file format, since supplement 1 based on XML

XML

Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....

 1.0. The combination of all LRU communication profiles for a given network describes the entire bus traffic and provides a valuable means for specification and verification of ARINC 825 networks. An analysis of the communication profile database allows to detect potential network problems at an early stage. ARINC 825 test tools must be able to read the communication profile database and interpret network data accordingly.

Gateways between ARINC 825 and other Networks

Commercial air transport aircraft Integrated Modular Avionics

Integrated Modular Avionics

Integrated modular avionics represent real-time computer network airborne systems. This network consists of a number of computing modules capable of supporting numerous applications of differing criticality levels....

 system architectures use multiple networks with different characteristics which have to exchange data with each other using gateways. Typically, bandwidth and communication principles of the involved networks differ widely. To support the design of gateways between CAN and other networks, ARINC 825 specifies a gateway

Gateway (telecommunications)

In telecommunications, the term gateway has the following meaning:*In a communications network, a network node equipped for interfacing with another network that uses different protocols....

 model and provides substantial information about protocol conversion, bandwidth management, data buffering and fault isolation.

Design Guidelines

The ARINC specification 825 contains a design guidelines section that helps system engineers and CAN LRU designers to implement ARINC 825 properly and in a certifiable manner. This section is intended to document industry experience that led to the decisions in the ARINC Specification 825. The guidelines are general network design criteria to be considered when designing an ARINC 825 network; they are not requirements but rather the recommendations to avoid potential design traps a network designer may encounter.

Outlook

The consistency and integrity of the ARINC specification 825 was continuously verified during the standardization process using a reference hardware/software system http://www.arinc825.com. The Airlines Electronic Engineering Committee decided that all future ARINC specifications using CAN (i.e. the ARINC 826

ARINC 826

ARINC 826 is a protocol for avionic data loading over the Controller Area Network as internationally standardized in ISO 11898-1. It allows Loadable Software Aircraft Parts to be loaded in a verifiable and secure manner to avionics Line Replaceable Units and Line Replaceable Modules using...

 Data Load and ARINC 812 Galley Insert Communication Standards) shall be based on ARINC 825. Airbus Technical Design Directives already specify ARINC 825 for many systems of the new Airbus A350

Airbus A350

The Airbus A350 is a family of long-range, wide-body jet airliners under development by European aircraft manufacturer Airbus.A consortium originally comprising European aerospace companies from the UK, France, Spain and West Germany, Airbus is now fully owned by EADS and since 2001 has been known...

. CANaerospace continues to coexist with ARINC 825 as the "11-bit identifier alternative" and provides enhanced ARINC 825 compatibility starting with revision 1.8.