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In this
assignment I will be discussing the role data buses have played in the
ever-expanding technological world, that we see in avionics and the roles in
the integration of avionics systems as well.

assignment will contain the evolution of aviation data buses and their effects
on avionics systems integration.

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The report
will also contain the operation principles and protocols of two types of data
buses and their applications in the integration of avionics systems.

I will
attempt to compare the advantage and disadvantages of data buses.


The first
generation of data buses were simply packs of wires that attached the
peripherals and memory. They were named after electrical buses or busbars.  Almost always there used to be one bus for a
memory and another for peripherals, they were retrieved by separate commands,
with totally different timings and procedures. There were issues with these data
buses, for instance, while performing an input/output, it had to wait in a que
for a peripheral to become ready and for other programs, this was found to be a
time asters other programs had to do other tasks, this potentially resulted in
lost data. As the CPU can only execute code for one peripheral at any given
time, re-arranging and prioritising the peripherals to avoid data loss improved
these data buses.


the second generation, this issue was eradicated but a new problem came to
prominence, as with the technology and speed of processors and memory increased
dramatically, the second-generation data buses, could struggling to cope with
the speed increase.


The two Data buses that will be compared are the ARINC 429 and the ARINC
629.  ARINC is short for Aeronautical
Radio, Incorporated, and is a company that develops and operates systems and
services.  It was organized in 1929 by
four major airlines to provide a single licensee and coordinator of radio
communications and its purpose was to standardise all equipment used in aircraft.

Starting its life of as the ARINC 419, the 429 was first released in
April 1978 and currently exists as ARINC 429-15 1. Part of this
standardisation is the ARINC 429, which is an important bus system being used
in commercial aircraft. The word Bus is short form of Omnibus and in computer
systems it means a free data exchange between all components of a computer
system. The system contains widespread cabling to have the data exchange system
give its best efficiency.

Modern aircraft have complicated electronics on board and their purpose
is that the whole data on board displayed to the crew. It is achieved by a
flight management computer and a display unit. The data is conveyed through a
system of data buses. Data buses permit interchange of data with computerised
systems and instruments. This system makes use of a unidirectional bus standard
called the Mark 33 Digital Information Transfer System (DITS).

The ARINC 429 is a Digital Information Transfer system
(DITS) and it used on many commercial aircraft, the likes or the Airbus A310,
Boeing, 737, 747, 757, and 777; and McDonnell Douglas MD-11. It explains both
and data formats needed for bus transmission and the hardware has a single
transmitter connected to 20 receivers (sinks) serial, twisted
shielded pair interface used by the Digital Air Data System (DADS) 1. Data
can be transmitted in simplex mode and bi-directional transmission would
require a parallel wire or a bus 2. The line replaceable units (LRUs), are normally
configured in either a Star topology or Bus-Drop topology formation as can be
seen in fig 1, This uncomplicated design, provides highly reliable data transfer.




ARINC 429 is a self-clocking, synchronising data bus, which
means that messages can start at any second along the time line.  If no messages travel down the line then the
line remains in a ‘null state’ or idle, meaning no voltage is used, and subsequently
saves power. The 429 will send or transmit data in 32-bit words at a speed of 100 kilobits/second.
This will make any information for the pilot appear to them in real time. These
data words are always 32 bits and consist of five field parities namely SSM,
Data, SDI and Label. ARINC 429 has two speeds of data transfer, a low speed
twelve Kb/s for low critical applications and high-speed bus for transferring
large and critical data. Another point is that ARINC 429 has no means of error
correction and can only communicate error detection. An error once observed
will require manual or other methods for correction.  There are three logic states, logic 0, logic 1
and null. Logic 1 is between +6.5 V and 13.0 V, while logic-0 state is the same
but for a negative voltage when no pulse is communicated, the bus is in the
null state.  If the two lines short to
ground, the voltage is halved and this in turn is deemed to be an unacceptable
range for the receiver.

The 429 transmitters are constantly transmitting either
words or a null state.  Most 429 messages
contain only one data word consisting of Binary (BNR) or Binary Coded Decimal
(BCD). 429 data words are 32 bits long made up of five primary fields as can be
seen in fig 2.

  fig 2. ARINC 429 32
bit data word.



There must be an odd number of “1” bits in the 32-bit word
that are insured by either setting or clearing the parity bit. Bits 31 and 30
contain the Sign/Status Matrix (SSM) these comprise of hardware equipment.  Bits 29 to 11 include the data, which may be
in several kinds of formats while bits 10 and 9 provide a Source/Destination
Identifier (SDI).  It is for multiple
receivers which are used recognise the receiver for which the data is intended.
Bits 8 to 1 have a label identifying the data type and the parameters
associated with it.  The 429’s data
encoding uses a Complementary Differential Bipolar Return-to-Zero (BPRZ)
transmission waveform. Pulse rise and fall times are controlled by RC circuits
built into the transmitters. This minimises overshoot ringing common with short
rise times. Allowable rise and fall times and other parameters for both high
& low speed 429 are shown in Table-1. It can be clearly seen that it’s not
merely the data speed that distinguishes between high & low speed 429
standards, but also the rise & fall time. The Bit rise & fall time for
low speed 429 are approximately 9 times slower than high speed 429.
Approximately the same is the rate to bit rate between the two categories of the
429. Table 1 describes

fig 4 The 429-transmission waveform and bit timing in a

All LRU transmitter have interface chips which have receivers,
line drivers, and logic translation from three level signals to logic zeros and
ones. Many complicated chips have lists and interface circuits for direct
application to a bus. Introduction of the 429 reduced the volume of wires in
the aircraft.  Changes in various LRU’s meant
only programming the parameters from one source to multiple receivers meaning
the amount of control panels is reduced.

The necessity for high bandwidth aerial avionics data links
that are lightweight, reduce the electromagnetic interference are always
required. This meant that optical fibre data communication was a step up from
the standard copper choice. Modern avionics require a system proficient of
transporting wave signals that carry digital data on an aircraft. The high
bandwidth to weight ratio, performance and routing flexibility accessible by
the mixture of single mode optical fibre and wavelength division multiplexing
(WDM) are among the main reasons to justify an optical network approach to
on-board avionics comm systems. The 629 is a new standard within the industry for
the transformation of digital data between.  It was first used in May 1995 and is currently
used on the Boeing 777, Airbus A330 and A340 aircraft 3. The 629 has been
developed as a successor to the ARINC 429 and it now shows that there is no
need for a bus controller and bus access is determined by each terminal
independently D.

ARINC 629 source transmits either broadcast or address
specific message to all or specific receiver or sinks. If the sinks equipment
needs to reply, each will need to be fitted with own transmitter and a specific
physical bus for the same. The single pair of wire connecting LRUs works in
full duplex mode. In 629 LRU may transmit and receive digital data using a
standard protocol. The protocol is described as Carrier Sense Multiple Access/
Collision Avoidance (CSMA/CA. 629 is a dual redundant data bus architecture
where two buses are hot standby to each other in a linear bus topology. Each
terminal can transmit 629 data to and receive data from every other terminal on
the data bus, which allows much more freedom in the exchanging of data between
units in the avionics system. The 629-data bus cable has an unshielded twisted
pair of wires and can be up to 100 meters long. Remote terminals are autonomous
and for timing synchronization each RT has independent transmitter and receiver
PROM for sequencing the time.

The first three bits are related to word time synchronisation.
 The next 16 bits are the data contents,
and the end is a parity bit.  Data groups
communicated by the 629 are called messages. Messages are comprised of word
strings, up to 31-word strings can be in a message. So, maximum 620 bits are possible
in a word string.

The 629 standard describes a multi-level protocol for inter
LRU communications along an access data bus. In the 429 stubs related to the
data bus whereas in 629 there is no stub. All the function master controller
does in 429 are done by. These RTs have both transmitter and receiver.  The 629 can be applied in three media forms, wire,
inductive or voltage coupling and optical fibre.  The optical power levels, wavelengths and
means of distributing optical power in any specific implementation must be
contained in a specification which refers this standard. This feature ensures
that the best technology is used when the system is built.


Aviation limitations, for example air speed, atmospheric
pressure, engine RPM, altitude, navigation status etc are measured in an analogue
domain and conveyed to on-board processors, displays and controls in electrical
domain.  The complexitity of avionics
systems has brought the need for more  high data bandwidth and the quicken the
process for on-board networks. Consequently, advances in the standard needed
for commercial avionics networks have to be adopted to cope with more technical
systems. This is a reason why the 629 is being implemented for optical networks
in aircraft.  In conclusion it can be
said that the 429 is an easy-to-implement and cheaper protocol were the dependability
has been satisfactory for most applications in the younger years of data buses.
So for the use of high data rates, the 629 has been useful in case of
redundancy as well and the reason why optical fibre based aerial avionics are developing
the applications for more commercial use.






3 fig 4

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