DIGITAL AVIONICS ARCHITECTURE:

MIL-STD-1553:

MIL-STD-1553 is a military standard published by the United States Department of Defence that defines the mechanical, electrical, and functional characteristics of a serial data bus. It was originally designed for use with military avionics, but has also become commonly used in spacecraft on-board data handling (OBDH) subsystems, both military and civil. It features a dual redundant balanced line physical layer, a (differential) network interface, time division multiplexing, half-duplex command/response protocol, and up to 31 remote terminals (devices). A version of MIL-STD-1553 using optical cabling in place of electrical is known as MIL-STD-1773.

ARINC 429:

ARINC 429 is a specification, which defines how avionics equipment and systems should communicate with each other. They are interconnected by wires in twisted pairs. The specification defines the electrical and data characteristics and protocols, which are used. ARINC 429 employs a unidirectional data bus standard known as Mark 33 Digital Information Transfer System (DITS). Messages are transmitted at a bit rate of either 12.5 or 100 kilobits per second to other system elements, which are monitoring the bus messages. Transmission and reception is on separate ports so that many wires may be needed on aircraft, which use a large number of avionics systems.

ARINC 429:

ARINC 429 has been installed on most commercial transport aircraft including; Airbus A310/A320 and A330/A340; Bell Helicopters; Boeing 727, 737, 747, 757, and 767; and McDonnell Douglas MD-11. Boeing is installing a newer system specified as ARINC 629 on the 777, and some aircraft are using alternate systems in an attempt to reduce the weight of wire needed and to exchange data at a higher rate than is possible with ARINC 429. The unidirectional ARINC 429 system provides high reliability at the cost of wire weight and limited data rates. Military aircraft generally use a high-speed, bi-directional protocol specified in Military Specifications MIL-STD-1553.

Each aircraft may be equipped with different electronic equipment and systems needing interconnection. A large amount of equipment may be involved depending on the aircraft. These are identified in the

Specification and are assigned digital identification numbers called Equipment ID. A partial list of equipment identified in ARINC Specification 429-15 can be found in Table 1 along with their digital addresses.

The specification also identifies a number of systems, which are capable of interchanging files of data in a bit-oriented format. Such files may require the transmission of a number of messages in sequence. System is capable of bit-oriented communications.

The SAL is used to identify the recipient of a bit oriented message.

ARINC 629:

Additional ARINC standards are being developed. ARINC 629 is used on the new Boeing 777 Aircraft. It uses a high-speed bi-directional bus capable of either periodic or aperiodic transmissions. Access to the bus is controlled by a sophisticated protocol involving wait periods, quiet periods and other rules.