5.4 Data Bus


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5.4 Data Bus

  1. 1. Module 5: Digital Techniques and Electronic Instrument Systems 5.4 Data Buses
  2. 2. What is a bus?  Bus: A collection of wires through which data is transmitted from one part of a computer (or one computer) to another.  PC: Connects e.g. CPU, DVD- ROM, RAM, PCIe card etc.  Mobile device: Connects CPU, GPU, WiFi controller, etc.  Aircraft: Data highway which links one LRU (Line Replaceable Unit) to another.
  3. 3. Address and Data Bus  All busses consist of two parts:  Address bus: To which device connected to the bus, the data should go?  Data bus: The actual data to be transferred.  However, the address and the data bus can be incorporated in one, by transmitting a single data word which contains the address information. (This is the case in the aircraft busses).
  4. 4. Serial vs. Parallel bus  Parallel bus:  Each bit of the data word is transferred via a specific wire.  Requires a lot of wiring.  Examples: conventional PCI.  Serial bus:  Each bit of the data word is transferred via the same wire.  Examples: PCIe, USB, ARINC, I2C, …
  5. 5. Serial vs. Parallel bus  Parallel buses should have been faster than serial.  However:  Parallel busses suffer from clock skew. (i.e. a bit can reach the destination before or after other bits: lack of synchronization).  Require more wiring.  Cannot be synchronized as fast as the serial busses. (lower data rate).  Parallel buses are rare today. Most busses architectures are serial.
  6. 6. Communication between Components  Single source – single sink: One LRU communicates with a single LRU.  Single source – multiple sink: One LRU communicates with multiple LRUs at the same time.  Multiple source – multiple sink: Multiple computers communicate with multiple LRUs at the same time..
  7. 7. Communication Direction  Simplex: A data bus can transmit only in one direction (Any LRU can only transmit or receive data at any time).  Half – duplex: Can transmit in both directions, but not at the same time (LRUs can take turns transmitting or receiving).  Full duplex (Duplex): All LRUs can send and receive data at the same time.
  8. 8. Other bus characteristics  Width: How many bits can be transmitted at the same time.  e.g. a 16-bit bus can transmit 16 bits simultaneously.  Clock speed (in MHz): How often the bus can transmit data. Faster clock speed means faster bus.  All these characteristics form the “bus architecture and protocol”. Civil aircraft busses are defined and standardized by ARINC.
  9. 9. ARINC  ARINC: Aeronautical Radio Incorporated.  A company that develops and operates aviation systems and services.  ARINC, ACARS (datalink between aircraft and ground), LRU standards.  Develops also solutions for defense, networks, security, …  Founded in 1929.
  10. 10. Error detection  Most common technique: Parity Check  How it works?  An extra bit is used called parity – bit in every data word.  We can use 2 kinds of parity that a bus can use:  Odd parity: Parity bit should have such a value that the total number of “1s” is odd.  Even parity: Parity bit should have such a value that the total number of “1s” is even.  The transmitter sends the data (along with the parity bit) to the receiver.  The receiver counts the “1s” and if the number does not agree with the Parity, error is detected and the word is sent again.  It is the simplest form of error detection. It is also used in PCs for data transfer between RAM or HD and CPU.
  11. 11. Binary Encoding Formats  Binary Encoding: How to represent “1” and “0” in a bus?  3 basic techniques are used in aircraft data buses:  Bipolar Return to Zero (BPRZ)  Harvard Bi-Phase  Manchester II Non Return to Zero (NRZ)  Self-clocking techniques:  The clock is embedded in the transmitted signal.  The receiver LRU does not need a clock to decode the data.
  12. 12. Bipolar Return to Zero (BPRZ)  “1” is a positive voltage and return to Null at half bit time.  “0” is a negative voltage and return to Null at half bit time.  The return to Null is the way the receiver identifies every single bit.  Self-clocking: No clock is needed in the receiver.
  13. 13. Harvard Bi-phase  “1” is positive voltage and return to zero (or the opposite).  “0” is positive or zero voltage.  In case of sequentially “0” the voltage level changes from positive to zero or zero to positive.
  14. 14. Manchester II Non Return to Zero (NRZ)  “1” is a change at half bit time from positive to negative voltage.  “0” is a change at half bit time from negative to positive voltage.
  15. 15. ARINC 429  The most commonly used data bus in commercial aircrafts.  Defines how avionics equipment and systems communicate on the aircraft.  Characteristics:  Unidirectional transition (simplex).  32 bits word transmitted over 2 wires (twisted pairs).  Bipolar RZ encoding. (“1”: +10V, “0”: -10V).  Messages are transmitted at 12.5 – 14.5 (low speed mode) or 100 Kbps (high speed mode).  Up to 20 LRUs can be connected in a single ARINC 429 bus.  Sequential words are separated by 4 bit times Null voltage.  High reliability, low weight and low cost. However, limited data rates.  Is installed in:  Airbus A310 / A320 / A330 / A340  Boeing 727 / 737 / 747 / 757 / 767.  Boeing 777 uses ARINC 629.
  16. 16. ARINC 429  Slew rate:  The time needed for a signal to rise from 10% to 90% of its maximum voltage. Parameter High Speed Low speed Bit rate 100 Kbps 12.5 – 14.5 Kbps 1 bit time 10μsec ± 2.5% 1/(bit rate) μsec ± 2.5% 1/2 bit time 5μsec ± 5% 1/(bit rate/2) μsec ± 5% Pulse rising time 1.5μsec ± 0.5% 10 ± 5 μsec Pulse fall time 1.5μsec ± 0.5% 10 ± 5 μsec
  17. 17. ARINC 429  2 kinds of word formats:  BNR (Binary): Data are encoded in binary numerical system.  example: 23  00010111  In BNR bit 29 indicates a positive / negative number, or North / South, West / East, Above / Below. Bit 28 is the MSB.  BCD (Binary Coded Decimal): Each decimal digit is encoded to the corresponding binary digit.  example: 23  0010 0011  BCD word encoding:  BNR word encoding:
  18. 18. ARINC 429  Word Fields:  P: Parity bit: Normally odd parity is used.  SSM (Sign / Status Matrix): Information about the data characteristics (data content – e.g. test, validity of data, etc.)  Data: In BNR or BCD format.  Bits 11-13 can be used as an equipment identifier, if necessary, to determine the equipment that transmitted the data. e.g. 00216 is the FMC (Flight Management Computer).  SDI (Source / Destination Identifier): Source or Receiver identification (e.g. does the data word targets a specific LRU or every LRU connected on the bus?).  Label: The type of data and how to be translated. Usually expressed in Octal. It is always sent first.
  19. 19. ARINC 429  SSM for BCD data SSM for BNR data Labellist(forBNR)
  20. 20. ARINC 429  BNR encoding example: positive 256 128 64 32 16 8 4 2 1odd parity Normal operation 00.26841812561 Padded bits Source / Destination (00 means to everybody) 1038: Selected airspeed  268 knots selected airspeed is transmitted through the ARINC 429 bus to every LRU connected in the bus.
  21. 21. ARINC 429  BCD word format example:  Data transmitted: 25786 in decimal.
  22. 22. ARINC 429  Equipment identifier is optional.  Represents the source of the data word.  BCD labels  BNR labels  Equipment Identifiers:
  23. 23. ARINC 429  TCAS Intruder Altitude Word
  24. 24. ARINC 429  Examples:  Decimal digits in BNR are encoded as 1/2x
  25. 25. ARINC 629  Used in Boeing 777.  Characteristics:  Half Duplex.  Up to 120 LRUs can be connected. (46 are connected in Boeing 777).  Clock speed: 100MHz.  Inductive coupling is used to connect the LRUs on the bus.  Data transferred to / from the bus using electromagnetic induction.  Improved reliability since no break in the bus wires is needed.
  26. 26. ARINC 629  Data are transmitted to the bus in groups called messages.  Each message consists of up to 31 word strings.  Each word string begins with a label word, followed by up to 256 data words.  Each label word and data word is 20 bits.  Only one LRU is allowed to transmit data through the bus each time.  One or more LRUs can receive data.
  27. 27. ARINC 629  Terminal Interval.  A time period common to all transmitters.  Every transmitter can make only one transition per terminal Interval.  Terminal Gap.  A time period different to each transmitter. (Priority assignment).  Any transmitter is inactive until the terminal gap for that transmitter has ended.  Synchronization Gap.  A time period common to all transmitters, longer than the terminal interval.  Will occur when all transmitters have had the chance to transmit.  Each transmitter can make only one transmission. Then, it must wait until the synchronization gap has occurred, before it can make a new transmission.  When an LRU is not willing to send data, the synchronization gap decreases.
  28. 28. ARINC 629  2 modes of operation:  Periodic: LRUs transmit in order of power-up. (Normal operation).  Aperiodic: LRUs transmit in priority order. Takes place when a discrete event takes place.  e.g. Landing gear system down.
  29. 29. Other ARINC protocols  ARINC 573  Used in Flight Data Recorder.  Harvard Bi-Phase encoding.  12 bit words of data.  Data are a snapshot of may avionics subsystems on the aircraft.  Each frame contains the same data at a different snapshot in time.  ARINC 717 is an alternative and extended protocol to ARINC 573.   ARINC 575  Older specification of 429, now obsolete.  ARINC 708  Used in airborne weather radar systems.  Simplex bus with 2 wires.  Manchester encoding.  1 Mbps clock speed.  Data words 1600 bits long (64bit status word + 3x512 bits data).
  30. 30. MIL-STD-1553B  A military half-dublex ARINC protocol.  2 twisted wires  Up to 30 terminals can be connected.  1MHz clock speed.  Word length: 20 bits. (16 bits are the data).  Manchester II bi-phase encoding.
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