So that everything also works 10000 meters above the ocean

The opening of the skies with the development of the first real commercial aircraft in the year 1914 was a milestone in the history of civilization and the cornerstone for the emergence of complex systems and high-value technologies that even offered the highest possible level of safety above the clouds. However nowadays, when looking normally at an aircraft, it is hardly possible to see the many thousands of mosaic parts that make it possible for such a means of transportation to take off. These also include the ARINC 629, a bi-directional bus system of multiple transmitters, specially developed by Boeing for one of their aircraft, the Boeing 777. The Boeing 777, frequently also called “triple seven”, was introduced in 1994, and even nowadays it is the world’s largest twinjet aircraft which is used to transport passengers. The original Boeing bus system was developed by ARINC based on the ARINC 429 – ARINC 629. In the meantime, 120 terminals can now be connected thanks to this technology.

How the ARINC 629 works

The ARINC 629 is a bus system, i.e. a system used for data transmission in the field of avionics. This is a term used for all the electronic instruments in an aircraft – excluding the cabin. Here, the data transmission takes place at exactly specified time intervals and via a two-wire line that may be either optical or electrical. For this procedure, two different protocols are available with the ARINC 629.

  • One is the basic protocol – this is a kind of protocol for which three different time intervals have been specified for sending and receiving data. On the one hand, there is the sending interval, which is also called the “transmitting interval”. This protocol of the ARINC 629 has busses of the same lengths for all terminals, and it is initialized as soon as transmission starts on this technical data bus. A device can start to transmit again only when the transmitting interval has finished with the transmission of the data. The second protocol is the synchronization in which, just like the transmission interval, the terminals of a bus always have the same length. However, the synchronization is shorter than the transmitting interval and it starts just at the moment when no data are being transmitted on the bus (binary unit system). If, during the synchronization, another device switches on in the data transmission, the timer is then set back to the initial state. Finally, with the basic protocol, there is also device recognition, Here, in a way that differs from the first two basic protocols that have been described, the terminals all have a different length. As such, this protocol type of the ARINC 629 determines the process of the other protocols, as device recognition only starts when the synchronization has ended and no other device is still transmitting via the bus. Also, the timer is reset when a device starts to transmit data before device recognition has ended.
  • The combined protocol (there are three different levels here as well) regulates the data transmission within the ARINC 629. The “combined protocol” is used especially for processing events that do not take place in a specified period. The periodic data is on the first level, which is also called level 1. This is also are transmitted in a specified order and with the same length, in the same way as the basic protocol. The short events and the more frequent nonperiodic events are especially transmitted on the second level. Also, when a deterministic cycle exists, the data are exchanged with each other at the end of the process. In the combined protocol, level 3 is then responsible for longer events and nonperiodic, infrequent events. At the end of the second level, if there is remaining cycle time, there is an exchange between these data results. Protocol levels 2 and 3 cannot be used together.

ARINC 629 – the decisive link in modern aircraft

ARINC 629 has become indispensable in avionics, as thanks to several improvements, it now reliably ensures safety and problem-free functionality of the devices in an aircraft. The high standard of this protocol is exemplified in the diversity of the place of use, even though it is naturally designed specifically for the Boeing 777 and speed. As such, data are transmitted at a rate of 2 Mbps. Based on the ARINC 429, the ARINC 629 presents itself as a high performance-oriented specification. The special feature when using an ARINC protocol is the fact that in the standard, there is no essential difference between the civilian and the military version.