Data Buses

• Aircraft data bus systems allow a wide variety of avionics equipment to

communicate with one anotherand exchange data
1. ARINC 427
2. ARINC 419 • The word ‘bus’ is a contraction of the Greek word ‘omnibus’ and the
3. ARINC 561 word simply means ‘to all’. Thus, in the context of computers and
4. ARINC 573 digital systems, ‘bus’ refers to a system that permits interconnection
5. ARINC 629 and data exchange between the devices in a complex system
6. ARINC 708 • With such a large number of avionic systems, a modern aircraft
7. ARINC 575 requires a considerable amount of cabling. Furthermore, some of the
8. MIL-STD-1553B/1773B cabling runs in a large aircraft can be quite lengthy Aircraft cabling
9. ARINC 575 amounts to a significant proportion of the un laden weight of an
10. CSDB and ASCB aircraft and so minimizing the amount of cabling and wiring present is
11. ARINC 615 an important consideration in the design of modern aircraft, both civil
12. FDDI and military.

Typical dimensions of a modern passenger aircraft

• Bus can be unidirectional (one way) or bidirectional (two way)

• Bus can be serial(one bit of data transmitted at time) or
parallel (where 8, 16, 32 bit of data appears as a group on a no of data lines
at the same time)
• Bus systems provide an efficient means of exchanging data between the
diverse avionic systems found in a modern aircraft
• KEY POINT
Modern aircraft use multiple redundant bus systems for exchanging data
between the various avionic systems and sub-systems. These bus systems use
serial data transfer because it minimises the size and weight of aircraft
cabling.
 Bus protocols (procedure /rules)
• Imagine for a moment that you are faced with the problem of
organizing a discussion between a large number of people sitting
around a table, all of whom are blindfolded and therefore cannot see
one another. In order to ensure that they don’t all speak at once, you
need to establish some ground rules, including how the delegates will
go about indicating that they had something to say and also
establishing some priorities as to who should be allowed to speak in
the event that several delegates indicate that they wish to speak at
the same time
• In computers and digital systems communications protocols are
established to enable the efficient exchange of data between multiple
devices connected to the same bus.

Multiple bus systems implemented on a modern

passenger aircraft

• KEY POINT
Communication protocols enable the efficient exchange of data
between a number of devices connected to the same bus. Protocols
consist of a set of rules and specifications governing, among other
things, data format and physical connections.
 • The data transferred may be synchronous ( using clock the signals generated between each
LRU) or asynchronous (self clocking)
• In the system the data is transferred using the shielded twisted pair (STP) bus cables with no
of coupler panels that are located at appropriate points in the aircraft.( i.e flight deck of
avionics bay)
• Each coupler panel allows a no of avionics units to be connected to the bus using a stub
cable

In order to transmit data using the serial data bus, information must be ARINC 429
presented in a standard format.
• A typical format for a serial data would use a word length of 32 bits , the • Unidiretional
word comprises of several discrete fields including: • Message transmitted in a packet of 32 bit at a bit-rate of 12.5 or
• An 8 bit label field 100kbps ( referred to as low and high bit respectively)
• A source/destination identifier (SDI) • Installed on wide variety of commercial aircrafts including Boeing
737,747,757,and 767; and McDouglas MD-11
• 20 bits for data
• No more than 20 receivers should be connected to a single bus.
• a number of status bits used to provide information about the mode,
hardware condition or validity of data; • Since there can be only one transmitter on a twisted wire pair, ARINC
429 uses a very simple, point-to point protocol.
• An added parity bit which provides a means for validating the data(
determining whether or not its free from error)
 ARINC 419
• It uses 32 bits words similar to ARINC429
• Some standards were based on the use of a six wire system, while others used a
• The label field is an octal value that indicates the type of data( i.e airspeed, shielded two-wire twisted pair (like ARINC 429) or a coaxial cable.
altitude, etc.) that is being transmitted.
• The SDI field is used when a transmitter is connected to multiple receivers, but
not all data is intended for use by all the receivers.in this case each receiver
will be assigned as SDI value and will look for only at label which matches its
SDI value
• The data field contains the actual data to sent. (Principal data format are BCD)
• The SSM field is used for information that assists the interpretation of the
numeric value in the data field.(i.e. north, south, plus, minus, above. etc.)
• The P field is a parity bit. ARINC429 uses odd parity

ARINC 561 ARINC 573

• Word length is 32 bit similar to Arinc 429
• Standard adapted for use with Flight Data Recorder(FDR) which uswe a
• Based on six wire system involving three pair that is used for DATA, SYNC & continuous data steam of Harvard Bi-Phase encoded 12-bit words
CLOCK
• These words are encoded into frames which contain a snapshot of the
• This system was widely used in aircraft manufactured prior to about 1970. data from each of the avionics subsystems on the aircraft
• ARINC 568 uses the same electrical interface specification as used in ARINC 561. • Each frame comprises four sub-frames.
• ARINC 717 supersedes ARINC 573 and caters for a number of different
bitrates and frame sizes.
ARINC 575 ARINC 615
• Similar to ARINC 429, this standard is a low-speed bus system that is • ARINC 615 is a software protocol that can be layered on top of ARINC
based on a single twisted pair of wires. 429-compatible systems
• Due to the low data rate supported, this bus standard is now • ARINC 615 supports high-speed data transfer to and from onboard
considered obsolete. digital systems permitting, for example, reading and writing of
• Electrically, ARINC 575 is generally compatible with low-speed ARINC 3.5-inch disks.
429. However, some variants of ARINC 575 use a bit-rate that is
significantly slower than ARINC 429 and they may not be compatible
in terms of the electrical specification and data formats.

ARINC 629 ARINC 708

• Introduced in1990s • It is to transfer data from the airborne weather radar receiver to the
• Supports a data rate of 2Mbps (20 times faster than Aritnc429) aircraft radar display
• Supports 120 connected devices and is currently used on B777, • It is unidirectional
Airbus330 and A340 • Uses Manchester- encoded data at a rate of 1 Mbps
• Bidirectional • Data words is 1600 bits long and are composed of one 64-bit status
• Another advantage it achieves the bidirectional communication words and 512 3-bit data words
without the use of a bus controller.
• The physical bus medium is the shielded twisted pair (STP)
 MIL-STD-1553B/1773B CSDB and ASCB
• Military standard 1533B is a bidirectional centrally controlled data bus The CSDB and ASCB standards are proprietary protocols from Collins and
designed for use in military aircraft. Honeywell, respectively.
• The standard uses a bus controller that can support up to 31 devices, • These systems are often used in small business and private general aviation
which are referred to as remote terminals. The standard supports a aircraft.
bit-rate of 1 Mbps. • CSDB is a unidirectional bus that permits the connection of up to ten
• MIL-STD-1773B is a fibre-optic implementation of MIL-STD-1553B receivers and one transmitter.
that provides significantly greater immunity to exposure to • The standard supports data rates of 12.5 kbps and 50 kbps.
high-intensity radiated electromagnetic fields (HIRFs).
• ASCB is a centrally controlled bidirectional bus. A basic configuration
comprises a single bus controller and two isolated buses, each of which can
support up to 48 devices.

FDDI ARINC 429

• The fibre distributed data interface (FDDI ) was originally developed by
Boeing for use on the Boeing 777 aircraft.
• FDDI is a local area network (LAN) based on a dual token-ring topology. Data
in each ring flows in opposite directions.
• The data rate is 100 Mbps and data is encoded into frames.
• CDDI (copper distributed data interface) and SDDI (shielded twisted pair
distributed data interface) are similar network bus standards based on
copper and shielded twisted pair as the physical media.
• Boeing now plans to replace the system on the 777 with a less expensive 10
Mbps copper Ethernet.
ARINC 429 ARINC 629

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