B1-05.04 DATA BUSES

• Data buses are used in aircraft systems for transmitting data
• Data buses and communication protocols have certain properties

Data Transmission: Electric Power

• Electrical power is transmitted from power stations via thick power lines to carry high voltages and current.
• Wiring diameter decreases as power is stepped down for efficiency

Electrical Data Transmission

• Instead of power, electricity transmits signals using negligible current
• Signal transmission occurs at the speed of light
• Data bus lines use small gauge wiring and signals are typically no higher than 5V DC
• Information is a series of 1s and 0s, which are data encoded as a digital signal
• Data is sent in a regulated sequence and decoded by computer processors to produce desired outputs

Digital Data Transfer

• A digital waveform ideally creates a square wave, but in practice resembles a distorted AC sine wave
• Binary data is represented by the presence or absence of voltage
• Binary 0 is typically 0 V, while binary 1 is typically 5 V
• Practical systems define binary 0 as between 0 and 0.8 V, and binary 1 as between 2 and 5 V
• A clock pulse indicates the operating speed of the data bus, synchronizing transmitter and receiver
• Transmitter detects and clocks a high signal as 1 and a low signal as 0
• Digital quantities use voltages with wide tolerance (e.g., 2-5 V for a 1), while analog voltages must be exact

Serial Data Transfer

• Serial data transfer is used to move large amounts of data in digital computers
• Each data bit gets transferred sequentially over a single line from a memory location
• The transmission is triggered by clock pulses, synchronizing the transmitter and receiver
• The data gets stored sequentially in memory before being transferred to processing circuitry
• It requires less hardware, and leads to less weight and space than parallel data transfer systems
• Multiplexing speeds up the data transfer capacity of a serial data bus

Parallel Data Transfer

• Bits are taken from a separate circuit and sent over a separate line
• It is much faster than serial transfer
• This method requires more hardware, increasing space and weight
• Serial data transmission is used for data-bus communication, while parallel processing occurs inside a computer
• Parallel buses interconnect internal devices and transmit all bits simultaneously
• An 8-bit parallel bus is 8x faster than the serial bus, and a 64-bit parallel bus is 64x faster than its serial counterpart

Multiplexing

• Combines multiple information channels onto one transmission line
• Reduces the amount of wires carrying separate signals
• Uses a "time division' technique, signals get carried by one conductor
• Benefits include weight reduction of wire bundles and circuit reliability
• Multiplexing uses synchronized rotary switches to connect input and output in sequence
• Logic gates use clock pulse signals to conduct multiplexing
• The multiplexer samples input and sends it to a transmission line when the AND gate is ON
• In an aircraft there is a Bus Controller rather than a sequencing controller to distribute outputs and processed data
• Analogue must first be converted to digital before being transmitted, then converted back afterwards

Aircraft Multiplex System

• Avionics in the 1950s and 60s were simple stand-alone analogue systems composed of subsystems
• Boxes were connected using point-to-point analogue wiring with analogue voltages, synchro-resolver signals, and switch contacts
• Weight and space constraints determined box placement in the aircraft
• The addition of more systems created complex wiring and increased weight
• Sharing information between systems became necessary to reduce black boxes
• Sensors shared heading and rate data to multiple systems
• Analogue technology still necessitated a "rat's nest" of wires and connectors
• Modifications for new functions led to integration challenges and potential system impacts
• Point-to-point wiring required hardware modifications and additional amplifiers
• Output Multiplier Boxes (OMBs) allowed a single signal to be replicated for multiple systems
• Analogue signals needed dedicated wiring to pass information between components
• In later computerized aircraft, this resulted in a ‘flying wiring loom’ due to information overload

Moving Towards Digital Systems

• Digital computers got incorporated, making the systems more digitized
• Centralized computers interfaced via A/D and D/A converters, improved computational capabilities and added easy growth
• The reduction in analogue signal conversion became a result
• Digital signals provided a benefit of transferring data bidirectionally, compared to unidirectional analogue data
• Multiplexing minimized amount of wiring to transmit data through aircraft
• Communications are managed by a Bus Controller (BC), running from the Flight Management Computer (FMC)
• Incorporation of all avionics require digital data bus for 2 way interface between sensors, computers, and indicators
• The transmission is serial to reduce wiring and receiver circuitry

Data Bus Systems

• Data buses achieve an interface between computer devices
• It is a twisted pair of shielded/jacket wires for spike & EMF protection, and accurate transmission
• May travel in one way (simplex) or in two directions (duplex)
• Accomplishes transmissions with 8-bit word
• Only one data word transits at a time

Data Bus Connectors

• Multiplexer acts like highway to connect all components using couplers
• The BC manages all data transmission
• While several terminals can perform as BC, only one can act at a time

Bus Controller (BC)

• BC sends commands to peripheral systems to request data
• ADC responds to the digital commands and relays data regarding altitude
• BC updates EFIS altitude readout by relaying data to EFIS
• BCs strictly control data transmission
• Only a few types of words are relayed over data buses, BC initiates
• It transmits the transferred data or the control to communicate the bus
• BC will either request or send data to/from remote terminal
• If sending, receiver will confirm no errors, or the status of the data sent
• BC is within a computer, like FMC or display processor

MIL-STD-1553 Data Bus

• Defines electrical and protocol characteristics for the data bus
• Allows all systems/subsystems to share a common set of wires for data transmission
• MIL-STD-1553B defines Time Division Multiplexing (TDM), with data transferred between avionics in a single transmission
• The initial standard was released in 1973

MIL-STD-1553 Data Standards and Compliance

• Standards freeze at the B level to allow manufacturers to gain experience
• Three distinct word types define the standard: command, data and status words
• All of the words are 20 bits in length
• Parity is based on odd parity calculating automatically the 1s in a words
• Command words has a terminals address/component to which address its commander
• T/R signifies whether a terminal will receive or transmit data
• The sub-address indicates memory location where data is stored in
• word count indicates the data to be sent to the remote terminal or BC
• 0 indicates the command word is the mode change
• BIT encoding is based on both Phase Manchester II
• A transition of the signal will occur at the bit time signal to transition levels
• Signalling polarity indicates info not the level and accurate timing

MIL-STD-1553 Data Transfer

• Data bus primary purpose is data exchange between systems
• Standard defines 10 message transmissions based on 3 defined word types
• The message transmissions is the mode change and transfers: without data transferred, data is transferred, and data gets requested
• There are broadcast message transmissions: It's a broadcast from BC to RT with data
• The command responses philosophy happens when all correct transmission receives a response back

Bus Controller (BC) Message Operation

• Remote terminals transmit the bus controllers message
• Commands occur following transmit command word validating the data
• It controls the word amounts transmitted back
• Immediate transmission is done where a data address memory is stored from B.C. and no transmission
• This will validate an and issues statement for receipt

ARINC (Aeronautical Radio Inc.)

• Large company that develops airline related services
• Established in 1929 for airline industries by 4 leaders
• Only airlines or companies can buy services
• They develop standards for avionics
• Specification documents are maintained by DC
• 429 is a specification which outlines how they should communicate, interconnected by a twisted pair of wires
• Employs a unidirectional data bus transmitted at 12.5 or 100kbps
• Transmission and reception are on different ports
• Needs many wires especially if they want to use big systems

ARINC 429

• Installed on most aircrafts
• Newer systems specified as ARINC 629 on 777
• Alternate systems in reducing the rates, wire and data
• 429 unidirectional systems provides high reliable wire amounts and limited data
• Its development predates 1984, and are protocols predating similar types of systems
• Standardizations benefits plane integrations
• ARINC means an avionics equipment manufacturer doesn't need to create components
• By sticking to new standards they produce standard products that create savings

ARINC 429 Characteristics

• Data bus uses 2 signal wires
• Word size of 32 bits; 1553 standard was 20 bits counting polarity and sync
• Codes with bipolar return triggered positive and negative
• Simplistic data buses; 1553 was bidirectional
• A simplex bus has 1 transmitter, multiple receivers up to 20
• There are no BCs as found in 1553; each must use it transmit bus for response or messages
• Cannot be directional on a pair

ARINC 429 Schematic Diagram

• Illustration shows output exiting on side and the input getting in on the top of the bottom
• Network wiring, has transfer to a min complex way
• Aronic 429 does not need b.c. they go via direction for data transmission
• Two wires create Mach Alt, true airspeed, AOA and test Data

ARINC 429 Signal Communications: FCC

• Flight control computers send surface position data to FMC for display on MFD
• Can send failure monitoring outputs to EICAS systems
• air data computers calculate the data in degrees and transfers it to the FMC
• It can send monitoring outputs to the CAS system
• Internal Reference system out-putted via accelerometer to yaws TMC system and display MFD and FCC
• Can also output monitoring failures to the engine indicating system
• The TMC outputted data goes to displays for MFDs, such as EPR or fuel and oil
• Finally the FMC, displays data on screens for overall system output

ARINC 429 Specifications

• Aircraft need interconnectivity electronic equipment in it depending what it is, they get number via the IDS system
• 429 details specifications for the industry aircraft equipment
• It follows a certain type of process to send and transport data, it can output a number to any type of avionics equipment

Transferring and Understanding Data

• Manchester II bi-phase coding that transfers data by sending data polarities, and aren't reliable data transfers
• The 429 uses a return for zero by not signalling relays for voltage
• It's flexible and transfers the types of data as long as all speak the same language
• Data transfers occurs over a twisted pair

Arinc 429 Transmitters

• Is sending data over bit data words or null
• Its messages make data word definitions to the labels the data that contain
• Typically measurement are repeatedly sent

Arinc 429 Words

• Specification provides codes indentifying the labels
• The port communicates over a wire
• Transfer information that has protocol set
• The data codes get integrated for the transmission networks
• It has eight bits and identifies parameter types and methods
• It uses labels that represents octal numbers
• Source and destination helps determine the data receiver
• It's used for identification or transmissions,

ARINC Continued

• Data bits contain the data that may be different formatting
• Many non standard formats been employed by many manufacturer in this situation the SDI does not get implemented
• SSM bits contains validity of data and if it has plus minus etc, its contains a type of equipment
• Data includes formats such a binary and decimal
• Although its fixed and the data bit vary, the number has different formats, and the word is transferred for display, label is important

ARINC 429 Labels Data

• Specification has 29 codes, to list the instruction and details on data multiplexing for systems
• Means single pair of wires output information via it
• Details standard data codes for transmission networks

ARINC 429: Digital Autonomous Terminal Access Communication (DATAC)

• At Boeing that was originally developed at carry single information
• All components needed exchanged with components with wired parts
• synchronized and broadcasted so each could determine whether it needs specific equipment.
• Design was perfect for Nasa, system allowed number of parts, amount of time needing required in experiments
• Required less maintenance lighter than conventional systems
• installed for aug 1894 as excellent test for the use and has became next design in the company it worked well _ A new way became standard and accepted in sept 1989

ARINC 629: Interconnection

• Databus: uses a time division system
• A bidirectional distributed bus that multiple transmissions
• A controller and interface module installed with replaceable units
• Coupler connects 120 others, all others must give way after terminal is over, that the bus is not transmitting

More on Avionics

• Transmissions: Data bus is used for connections and are also the cable Data Bus Includes: Cabling, Couplers, Module Interfaces
• Transmissions Include: Module Interface And Terminal Controller
• It's Used As The Main Connection To Transfer The Power Through The Main System

ARINC 629 Data

• They include data that moves between a coupler and the cable
• Its protected by shielded resistors at an ohm amount _ Cable consists as long 189 as 46 coupler
• Inner conductor include protective foam and skin

Structure, data types and signals

• The 629 systems moves the data as long as it is the same message and structure, and special reading skills
• Contains set numbers of word counts for the data
• Has certain 2 bit for extensions as sync pulses
• Needs a pulse to begin
• The arcing does quantity data of 69
• Its the transmission as gap which means for its
• Has 3 timers when it is in this stage

This lesson covers the fundamental principles of digital data transmission, including minimal current flow, the advantage of using electricity, and deviations from ideal square waves. It also touches on binary representation, wiring differences, and troubleshooting bit errors.