5.4 Data buses Flashcards
data buses were designed and evolved to meet what requirements
efficient communication between their subsystems and components. must be as lightweight and reliable as possible, which means keeping wiring and connectors to a minimum
Avionics system designers used Analogue-to-Digital Converters (ADC) and Digital-to-Analogue Converters (DACs) .These require only two twisted pairs of wires to connect single data transmitters (sources) and single data receivers (sinks)
.
what is a single source and multiple -sink bus called
ARINC 429 (Aeronautical Radio Incorporated)
multiple-source with multiple-sinks buses
ARINC 629 and its military equivalent, MIL-STD-1553B.
time - division multiplexing
enables different forms of information to be transmitted through a single communication system.
when was ARINC founded
1929 in the United States
what data bus remains the standard for all aircraft
ARINC 429
ARINC 629 is how much faster than ARINC 429
20 times faster
data rate of 2 megabits per second
can handle 120 terminals instead of (ARINC429’s) 20
simplex means
the transmission can flow only in one direction, i.e., the communication is unidirectional. can only send data but not receive it vice versa
half duplex means
One transmitter, one receiver; the transmission can flow in both directions, but only one at a time (bi-directional with limitations)
full duplex means
One transmitter, one receiver; the transmission can flow in both directions at the same time (bi-directional)
the ARINC 429 specification describes the bus as an open loop transmission model
.
ARINC 429 is a
simplex bus with multiple receivers and can be thought of as a ‘shout’ or ‘broadcast’ bus
the transmitting LRU is known as the
source
each receiving LRU is called a
sink
ARINC 429 transmits sets of information at
periodic intervals
advantages of serial transmission
only a single line is required for transmission
only one set of digital circuitry is needed to process the data
ARINC 429 information is transmitted via a uni-directional bus, which is made up of
two 78 ohm twisted shielded wires
ARINC 429 defines two different transmission speeds:
Low-Speed (LS BUS) (12-14 kbit/s)
High-Speed (HS BUS) (100 kbit/s)
Low-speed and high-speed transmission can never occur on the same data bus. It is either a low-speed or high-speed bus.
.
Important data is transmitted often; less important data is transmitted less frequently. This is known as
information rate
BCD data (Binary Coded Decimal) is used for displays only, it is transmitted
twice per second on average
BNR (Binary) is processed by other LRUs and is therefore required to be transmitted 6 to 20 times a second.
.
the use of SDI and SSM is not mandatory
.
the label …
The Most Significant Bit (MSB) of the actual label is in the Least Significant Bit (LSB) of the ARINC 429 word and is transmitted first onto the bus; this causes the label to be transmitted onto the bus in reverse bit position order.
the source/ destination identifier (SDI) function is used
When it is necessary to indicate the source of information
when the information is directed to a specific location (which has a minor function),
The data bus cable is a pair of twisted wires which are not shielded.
.
The ARINC 629 bus employs an inductive coupling technique used to connect the bus to receivers/transmitters
.
Each LRU or Line Replaceable Module (LRM) is connected to its coupler by
a stub cable
The stub cable contains two sets of
shielded twisted pairs of wires
cabling arrangements of ARINC 629 data bus
data bus cable
LRU
current mode coupler
serial interface module
stub cable
ARINC 629 system can be thought as a
party line
ethernet terminology
medium =
Ethernet devices attach to a common medium that provides a path along which the electronic signals will travel
ethernet terminology
segment =
Refers to a single shared medium as an ethernet segment. Segmentation is overcome by a bridge.
node =
Devices that attach to that segment are stations or nodes.
frame =
The nodes communicate in short messages called frames, which are variably sized chunks of information.
jitter =
Is when the signal drifts from its reference position.
frames =
The method that is used to stream data over ethernet systems.
full duplex ethernet cable =
This allows data to be sent and received at the same time.
CSMA/CD =
The acronym Carrier Sense Multiple Access with Collision Detection (CSMA/CD) describes how the Ethernet protocol regulates communication among nodes.
collision detection
In Ethernet terminology, a collision occurs when they both spoke at once.
When stations detect a collision, they cease transmission, wait a random amount of time, and attempt to transmit when they again detect silence on the medium
repeaters =
Repeaters connect multiple Ethernet segments, listening to each segment and repeating the signal heard on one segment onto every other segment connected to the repeater. By running multiple cables and joining them with repeaters, you can significantly increase your network diameter.
the controller area network (CAN) bus is
a multi-master broadcast serial bus standard to connect electronic control units which is an advantage over ARINC 429.
CAN data bus is a serial communications protocol that supports distributed real-time control with a high level of security.
the CAN bus was introduced when
1980s by Robert Bosch GmbH
A CAN Bus operates at data rates of up to 1 Megabyte per second (Mbps) for cable lengths of less than 40 m.
if the cable length increases, what happens to the data rate
typically falls to 125 kilobytes per second (kb/sec) for 500 m (1640 ft) in length.
The CAN Bus data frame consists of seven different bit fields:
Start of frame
Arbitration
Control
Data
CRC
ACK
End of frame
A CAN base frame message begins with the start bit called
Start of Frame (SOF). It is a single dominant bit
The arbitration field, which consists of the identifier and the Remote Transmission Request (RTR) bit,
is used to distinguish between the data frame and the data request frame called a “remote frame”.
Cyclic Redundancy Check (CRC)
The integrity of the frame is guaranteed by the following CRC sum
Advantages of the CAN Bus include:
No separate master bus controller
no need to retransmit the message; only the priority message is on the bus.
no need for any node to wait for the token (like a token ring network) to transmit the message.
With only the maximum capacity of the bus as a speed-limiting factor, the CAN Bus does not collapse or lock up.
Error detection is done by:
Monitoring data transmitted on the bus. Every transmitter compares the bit level it transmitted with the bit level it received from the bus
Cyclic redundancy check
Message frame check
Bit stuffing
Full duplex operation mode can only be used when:
The connecting medium is capable of simultaneous transmission and reception (full duplex).
The two computers communicating have a full duplex connection.
Both communicating computers are configured for full duplex connection.
Full duplex systems have numerous advantages.
Data transfer is bi-directional
Data transfer is high speed (100 Mbit/s)
High data transfer rate
Virtual link concept
It involves one transmitter and multiple receivers
ARINC 664
The Aircraft Data Communication Network (ADCN) systems communicate with each other using AFDX switches and full duplex quad cables.
ARINC 664 (2)
For system redundancy, two networks are used: Network A and Network B. All systems are connected to both networks. Each system sends data to the receiving system via both networks. The receiver uses the first network to provide valid data
