Module: Digital techniques / electronic instrument systems (ae2336)
The function of Data buses in modern aircrafts

Done by: Prithiparajan Ganesan
KU ID: K1169901
Class: FD6
Lecturer: Mr. Yeow Khee
Contents

1. Abstract 3

2. Introduction 3

3. Five commonly used data buses on aircrafts a. ARINC 429 Specification 4 b. ARINC 629 Specification 4 c. ARINC 573 Specification 4 d. ARINC 708 Specification 5 e. MIL-STD-1553B/1773B Specification 5

4. The Operation of Data Bus Systems f. ARINC 429 Specification 6 1. Equipment connection 6 2. Protocol 6 3. Modulation 6 4. Direction of information flow 7 5. Information element 7 6. Applications 8

g. ARINC 629 Specification 8 1. Equipment connection 8 2. Protocol 8 3. Modulation 9 4. Direction of information flow 9 5. Information element 10 6. Applications 10

5. The Advantages and Disadvantages of the Systems h. Advantages 11 i. Disadvantages 11

6. Conclusion 12

7. References 13

1. Abstract
This report is focused on the various types of data bus systems incorporated in aircrafts. Specifically it will be focusing on FIVE of the most common data bus systems used in aircrafts. They are ARINC 429, ARINC 629, ARINC 573, MIL-STD-1553 and MIL-STD-177 systems. It will also focus on the behaviour and the functioning of these five data bus systems and where there would be used. Furthermore comparisons would be made between the ARINC 429 and ARINC 629 systems and the merits and disadvantages of the systems would then be analysed. As a conclusion, the data bus system’s effect on modern aircrafts would be discussed.

2. Introduction
In the earlier aircrafts, the avionic systems were analogue. Thus the number of cables used to send and receive information to and from various components was extensive. So in order to reduce the cables, the digital systems were introduced. All the analogue signals were converted to digital data and were assigned unique address labels. This ensured that there were no conflicts between data. These data (Signal) are then transmitted through a pair of wires. Thus these wires make up the data bus which allows data transmission.
Data transmission is the transfer of information from a source to a receiver. In olden days this was done through Morse code, mail or telegram. In computer terms however, it means to send a data made of streams of bits from one place to another. This can be done through many means, for example the most commonly used means is the copper wire, while the more advance and faster means is through optical fiber. Thus the data bus system makes use of these means to allow the exchange of data and communication between the various aircraft systems present in the aircraft. The language used for such communication is known as the protocol. Currently, there are different bus protocols used for the data interchange in today’s aircrafts. Some examples are the ARINC 429, ARINC 629, MIL-STD-1553, MIL-STD 1773 and ARINC 573. These buses specifications will be further discussed in the report.

3. Five commonly used Data buses Systems on Aircrafts

j. The ARINC 429 Specification
The ARINC 429 specification was also known as Digital Information Transfer System (DITS). It was first published in 1977 and since then had become the most commonly used ARINC standard for airline industries. Even though ARINC 429 acted as a medium for data transfer, it did not fit the normal definition of a data bus. This is because a typical data bus provides multidirectional transfer of information between several points through a set of wires. But ARINC 429 only allowed one way flow of information between points. Thus this greatly limited its capability. The ARINC 429 is capable of accommodating up to 20 terminals on the data bus. It as data transfer rates of either 12.5 or 100 Kbit/s. Since it was low in cost and had good integrity in installation, airlines were greatly impressed by it.

k. The ARINC 629 Specification
The ARINC 629 was first introduced in 1995. It is currently being used on aircrafts such as Airbus A330, A340 and Boeing 777. Unlike the ARINC 429, the ARINC 629 is considered a true data bus as it can operate as a multiple source and multiple sink system. This means to state that, each terminal is capable of transmitting information to and receiving information from, all other terminals that are connected to the data bus. Thus it provided more freedom in the transferring of data between aircraft systems. The ARINC 629 is also capable of accommodating up to 128 terminals on the data bus. It is much more flexible in that additional units can be accepted readily. It also has a high data rate of 2Mbps.

l. The ARINC 573 Specification
The ARINC 573 is a flight data recorder output format. It sends continuous streams of data encoded in Harvard Bi Phase. It has 12 bit words encoded in frames. The information or data in each frame contain snapshots of the many aircraft’s avionic systems. Each frame contains the same data at a different snapshot in time. It is to note that the ARINC 717 superseded the ARINC 573. The ARINC 717 added a number of different bit rates and frame sizes. It also has a secondary output data stream. This data stream is similar to the primary however it is encoded in BPRZ format (ARINC 429 format)

m. The ARINC 708 Specification
The ARINC 708 is used to transfer data from the airborne weather radar receiver to the aircrafts radar display. It utilizes a bus that is unidirectional. It also uses data that is Manchester encoded and it has a data rate of 1Mbps. The data frames are 1600 bits long. The header field of the frame consists of parameters such as range, tilt and gain. The data field is organized into 512 range-bins per scan angle value. Each range bin contains a colour value to indicate the intensity at that position.

n. The MIL-STD-1553B/1773B Specification
The MIL-STD-1553B was first introduced by the United States Department of Defence that defines the mechanical, electrical and functional characteristics of a serial bus. It was originally designed for use with military avionics, but has also become commonly used in spacecraft on-board data handling (OBDH) subsystem. It was first used on the F-16 fighter aircraft. Later many more aircraft designed adopted the standard. It consist of a dual redundant balance line physical layer, a network interface, time division multiplexing, half duplex command and respond protocol. It is capable of accommodating 31 remote terminals. It supports a bit rate of 1 Mbps. The MIL-STD-1773B is the fibre optic version of the MIL-STD-1553B. This it provides greater transfer rates as well as immunity to exposure to high intensity radiated electromagnetic fields (HIRF).

4. The Operation of the Data Bus Systems

a. Functions of the ARINC 429

1. Equipment connection

A single ARINC 429 transmitter can be connected to up to 20 receivers through a single twisted and shielded pair of wires. The shields of the wires are then grounded at both ends. It uses two signal wires to transmit 32 bit words. Transmission of sequential words is separated by at least 4 bit times of NULL (zero voltage). This eliminates the need for a separate clock signal wire. Thus this is known as a self-clocking signal.

2. Protocol

The ARINC 429 is a point to point protocol. There can be only one transmitter on a pair of wire. It is always transmitting either a 32 bit data word or a NULL state. It must be accompanied by at least one receiver on a wire pair or a maximum of 20 receivers.

3. Modulation

Return-To-Zero (RZ) modulation is used. The voltage levels are used for this modulation scheme. An example is shown in figure 1.

Figure 1
Return-to-zero Modulation

4. Direction of information flow

The information from a particular system is transmitted from a designated port to the receiving port of the other system which is in need of the information. The information would never go to a port that is designated for transmission. A separate data bus is used for each direction when the data is required to flow both ways between two systems. An example of ARINC 429 transmission is shown below.

Figure 2
ARINC 429 Transmission 5. Information element

ARINC data words are always 32 bits and the typical use the format includes five primary fields, namely Parity, SSM, Data, SDI, and Label. ARINC convention numbers the bits from 1 (LSB) to 32 (MSB).

* 32 – Parity bit – The MSB is always the parity bit for ARINC 429. Parity is normally set to odd except for certain tests. * 31 to 30 - Sign Status Matrix – BCD – This field contains hardware equipment condition, operational mode, or validity of data content. * 31 to 29 – Sign Status Matrix – BNR - This field contains hardware equipment condition, operational mode, or validity of data content * 28 to 11 – Data – This field contains data information. * 10 to 9 – Source Destination indicator – This is used for multiple receivers to identify the receiver for which the data is destined. Or It can also be used in the case of multiple systems to identify the source of the transmission * 8 to 1 – Label – It identifies the data type and the parameters associated with it

6. Applications

The ARINC 429 was first used for data transfer on the A310 and B757. Approximately 150 separate buses interconnected the various computers, radios, display, controls and sensors. ARINC 429 had two speeds. Most of the buses operated at lower speeds. Only those that are connected to critical navigation computers operated at high speeds.

b. Functions of the ARINC 629

1. Equipment connection

The ARINC 629 data bus consists of an unshielded pair of twisted wires. These wires are bonded continuously along their length. The cables can be 100 meters long. It has both voltage and current mode operation. It can be used for optical fiber interface. A unique feature of the ARINC 629 is that it uses induction coupling technique to connect the bus to the receivers/transmitters. The bus wires are fed through an inductive pick up which uses electromagnetic inductions as a means to transfer current from the bus to the user or from the user to the bus. This means that no break in bus wiring is required, thus greatly improves reliability.

2. Protocol

The ARINC 629 uses DATAC system which is a time based, collision avoidance protocol concept. Each terminal is allocated with a particular time slot to access the bus and transmit data on to the bus. The terminals would autonomously decide when the correct time slot is available through the use of certain control timers. These timers are embedded in the bus interface. Once the time slot is found only then the necessary data is transmitted.

3. Modulation

Manchester bi-phase modulation is used. An example of the Manchester bi-phase encoding is shown below.

Figure 3
Manchester Bi-phase Modulation

4. Direction of information flow

The information from a particular system is transmitted from a designated port to the receiving port of the other system which is in need of the information. Unlike the ARINC 429 information is allowed go to a port that is designated for transmission, provided that the port has finished transmitting or not transmitting at the point of transmission. The 629 used a half-duplex system which means that it can transmit and receive but only one at a time. An example of ARINC 629 transmission is shown below.

Figure 4
ARINC 429 Transmissions

5. Information element
Data is transmitted in groups called messages. The messages consist of word strings. These words strings begin with a label and followed by up to 256 data words. The first three bits are related to word time synchronization. The next 16 bits are the data contents, and the final bit is a parity bit. The data words may have a variety of formats depending on the word function. Also there is provision for general formats, systems status, function status, parameter validity, and binary and discrete data words. An example of the digital word format of the ARINC 629 is shown below.

Figure 5
ARINC 629 Word Format 6. Applications

The ARINC 629 is currently being used on the Boeing 777 and the A330 aircrafts. It seems to be superseding the ARINC 429. This is mainly due to the fact that it is bi-directional thus removing the need for additional wires for two communications. This greatly increases weights saving.

5. Merits and Disadvantages of the Systems

a. Merits

1. ARINC 429 System

* Used by a significant number of aircrafts and it has been in services for a lot more years than the ARINC 629 thus it is considered to be reliable.

2. ARINC 629 System

* Higher data rate of 2 Mbps * Capable of transmitting to 120 receivers * Due to induction coupling technique, it is not prone to single point failure. * Due to the usage of lesser wires between systems, results in high weight savings. * Has fibre optic mode

b. Disadvantages

1. ARINC 429 System

* A data rate of 100Kbps * Capable of transmitting to 20 receivers * Subjected to single point failure * Since it requires 2 wires to have a two way data transmission, it takes up more weight of the aircraft. * Does not have fibre optic mode

2. ARINC 629 System

* Recently introduced thus reliability of the system is not yet as clear as the ARINC 429.

6. Conclusion

Since the introduction of the data bus systems, there has been substantial savings in the area of weight of an aircraft. Also with faster transmission and broad bandwidth, data can be efficiently and effectively be transmitted within the aircraft systems. Through highly devised protocols, the chances of errors and failures have been significantly reduced thus improving the safety of the aircraft.

As aircraft systems become more and more sophisticated, the need for faster and more importantly more reliable data buses would be required. From copper wired connections the data buses (ARINC 429) have moved on to the next era where light, the fastest medium in the universe is used to transmit information through data buses (ARINC 629). Thus as long as there is advancement in technology there will always be continuous improvements to the performance and reliability of the data buses.

[2243 Words]

9. References

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Zwinz Technical Consulting GMBH, DIEHL Avionik Systeme, [Online], Available: http://www.techconsultzwinz.de/diehl/diehl-arinc-ueb.htm [12/02/2013]

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Condor Engineering, ARINC Protocol, [Online], Available: http://leonardodaga.insyde.it/Corsi/AD/Documenti/ARINCTutorial.pdf [12/02/2013]

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Integrated Publishing, Digital Tranmission, [Online], Available: http://www.tpub.com/neets/tm/112-2.htm [12/02/2013]

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Wikipedia, ARINC 429, [Online], Available: http://en.wikipedia.org/wiki/ARINC_429 [12/02/2013]

World Wide Web Page
Avionics Today, Databus, [Online], Available: http://www.aviationtoday.com/av/military/Databus_75122.html#.URkcYqUqaSo [12/02/2013]