ARINC 429 Data Bus PDF

Aeronautical Radio Incorporated History of ARINC Aeronautical Radio Inc. (ARINC) is a major company that develops and operates systems and services to ensure the efficiency, operation and performance of the aviation and travel industries. It was organised in 1929 by four major airlines t...

Aeronautical Radio Incorporated History of ARINC Aeronautical Radio Inc. (ARINC) is a major company that develops and operates systems and services to ensure the efficiency, operation and performance of the aviation and travel industries. It was organised in 1929 by four major airlines to provide a single licensee and coordinator of radio communications outside the government. Only airlines and aviation-related companies can be shareholders, although all airlines and aircraft can use ARINC’s services. ARINC ARINC logo ARINC has provided leadership in developing specifications and standards for avionics equipment, and one of these specifications is the focus of this lesson. Industry-wide committees prepare the specifications and standards. ARINC Specification 429 was developed and is maintained by the Airlines Electronic Engineering Committee (AEEC) comprising members that represent airlines, government and ARINC. The General Aviation Manufacturers Association (GAMA) in Washington, DC, also maintains a specification document with ARINC 429 labels: ARINC 429 General Aviation Subset. What is ARINC 429? ARINC 429 is a specification which defines how avionics equipment and systems should communicate with each other. They are interconnected by wires in twisted pairs. The specification defines the electrical and data characteristics and protocols used. ARINC 429 employs a unidirectional data bus. Messages are transmitted at a bit rate of either 12.5 or 100 kb per second to other system elements, which are monitoring the bus messages. Transmission and reception are on separate ports, so many wires may be needed on aircraft which use a large number of avionics systems. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 96 of 444 CASA Part 66 - Training Materials Only  ARINC 429 ARINC 429 Usage ARINC 429 has been installed on most commercial transport aircraft, including Airbus A310/A320/A330/A340; Bell helicopters; Boeing 727, 737, 747, 757 and 767; and McDonnell Douglas MD-11. Boeing is installing a newer system specified as ARINC 629 on the 777. Some aircraft are using alternate systems in an attempt to reduce the weight of wire needed and to exchange data at a higher rate than is possible with ARINC 429. The unidirectional ARINC 429 system provides high reliability at the cost of wire weight and limited data rates. ARINC 429 characteristics Development of ARINC 429 A number of digital transmission system building blocks were available prior to 1984. Many protocols predate ARINC 429, such as ARINC 561, 582, 573, 575 and 419. The variability of standards does not matter when a single user is involved, but is important when equipment from different suppliers must interact. Standardisation is beneficial not only to the aircraft integrator but to the equipment supplier, who can have greater assurance of product acceptability so long as it is ‘on spec’. ARINC 429 is the most widely applied digital data transmission specification for modern transport aircraft. The existence of ARINC 429 means avionics equipment manufacturers need not to make components specific to certain aircraft or manufacturer types. The alternative, with all air carriers utilising different data busses, would mean manufacturers of, say, a laser ring gyro would then need to manufacture variants to comply with each carrier’s data bus specifications. By sticking to a standard, manufacturers can produce standard products which can be incorporated into any aircraft, thus keeping manufacturing costs down, resulting in massive savings to the airlines purchasing aircraft and spare parts. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 97 of 444 CASA Part 66 - Training Materials Only  ARINC 429 Characteristics Some of the major characteristics include: Data bus using two signal wires Word size of 32 bits (1553 standard was 20, counting polarity and sync) Bit encoding with bipolar return to zero (1553 standard was Manchester II Bi-Phase, triggered by positive [+ve] and negative [-ve] going pulses) Simplex data bus (1553 standard was a bidirectional data bus). ARINC ARINC-429 bus A simplex bus is one on which there is only one transmitter but multiple receivers (up to a maximum of 20 in the case of 429). There are no BCs as found in 1553 buses. Since each bus is unidirectional, a system needs to have its own transmit bus if it is required to respond or to send messages. ARINC 429 specifies that bi-directional data flow on a pair of wires is not permitted. Requirements for minimum weight and maximum flexibility drove 1553 to operate at 1 MB on a bidirectional bus. Certification requirements drove ARINC 429 to operate at either 12 to 14.5 kb or 100 kb on a simplex bus. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 98 of 444 CASA Part 66 - Training Materials Only  ARINC 429 Schematic Diagram Ⓒ Aviation Australia ARINC 429 schematic diagram This illustration is drawn with outputs represented exiting the side of the boxes and inputs entering the top or bottom. This wiring network looks more complex than the 1553 data bus because data transfer is only simplex or one-way. But the ARINC 429 data bus system does not require a BC. Because all data outputs are transmitted over two output wires, wiring is still kept to a minimum. For example, two wires carry indicated airspeed, true airspeed, mach number, altitude, AOA, OAT and built-in test data. If two components need to exchange information, for example, the Flight Controls Computer (FCC) sends flight control surface position information to the Flight Management Computer (FMC), which transmits autopilot commands to the FCC. In just this case, the FCC output is sent to the FMC, and the FMC output is sent to the FCC. Following is a list of the basic signal transfer considered in developing this diagram for instruction purposes: Flight Control Computer (FCC) Output Surface Positions and maintenance data sent to FMC for eventual display on a Multifunction Display (MFD). Failure monitoring output sent to Engine Indicating and Crew Alerting System (EICAS), for example, aileron or flap, FCC Channel Fail. Air Data Computer (ADC) Output 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 99 of 444 CASA Part 66 - Training Materials Only  Altitude, airspeed, AOA, total temp and so on, sent to: FCC for gain scheduling Thrust Management Computer (TMC) for thrust management calculations FMC for eventual display. Failure monitoring output sent to EICAS, for example, AOA sensor fail. Internal Reference System (IRS) Output Accelerometer outputs detecting yaw or sideslip sent to TMC to maintain symmetrical flight. Roll, pitch, yaw to FCC for autopilot, and to FMC for display on an MFD. Failure monitoring output sent to EICAS, for example, sensor overheat. Thrust Management Computer (TMC) Output To FMC for display on an MFD: Engine Pressure Ratio (EPR), Exhaust Gas Temperature (EGT) and so on. Failure monitoring output sent to EICAS, for example, oil pressure low, N2 RPM high. Flight Management Computer (FMC) Output Sent to all avionics components for overall system control utilising FMC display/select panel. Engine Indicating and Crew Alerting System (EICAS) In this diagram, EICAS has no outputs to other avionics systems, but purely monitors inputs. Because it has no outputs, it has no need of an ARINC 429 output data bus. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 100 of 444 CASA Part 66 - Training Materials Only  ARINC 429 Specifications Each aircraft may be equipped with different electronic equipment and systems needing interconnection. A large amount of equipment may be involved, depending on the aircraft. These are identified in the specification and are assigned digital identification numbers called Equipment Identification (ID). A partial list of equipment identified in ARINC Specification 429 is illustrated below, along with their digital addresses. Eq. Eq. Equipment Type Equipment Type ID ID 001 Flight Control Computer (701) 029 ADDCS (729) and EICAS 002 Flight Management Computer (702) 02A Thrust Management Computer 003 Thrust Control Computer (703) 02B Perf. Nav. Computer System (Boeing 737) 004 Inertial Reference System (704) 02C Digital Fuel Gauging System (A310) Altitude and Heading Ref. System 005 02D EPR Indicator (Boeing 757) (705) 006 Air Data System (706) 02E Land Rollout CU/Landing C &LU 007 Radio Altimeter (707) 02F Full Authority EEC-A 008 Airborne Weather Radar (708) 030 Airborne Separation Assurance System 009 Airborne DME (709) 031 Chronometer (731) Passenger Entertain. Tape Reproducer 00A FAC (A310) 032 (732) 00B Global Positioning System 033 Propulsion Multiplexer (PMUX) (733) ARINC 429 details a significant number of specifications, all of which are intended to set a standard for avionics data bus systems installed in aviation industry equipment. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 101 of 444 CASA Part 66 - Training Materials Only  ARINC 429 Data Transfer The Manchester II Bi-Phase coding of the 1553 data bus transferred data (1s and 0s) by the shifting polarity of a signal (+ve to -ve = 1 and -ve to +ve = 0), and hence was not reliant upon voltage levels. The ARINC 429 data bus uses Return to Zero (RTZ), where no signal is relayed by a signal voltage of 0. Aviation Australia Manchester II bi-phase code The intricacies of the method of data transfer do not really matter to us as long as you understand the concept and realise that both data bus systems (1553 and ARINC 429) have different signalling methods. The fact that signals are transferred by different methods provides evidence of the flexible nature of digital communications, and as long as all components are speaking the same language and are synchronised, communication will result. Aviation Australia Transmitter communication through twisted shielded pair to receiver ARINC 429 is a very simple, point-to-point protocol. There can be only one 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 102 of 444 CASA Part 66 - Training Materials Only  transmitter on a wire pair. The transmitter is always transmitting either 32-bit data words or the NULL state. There may be up to 20 receivers on a wire pair. In most cases, an ARINC message consists of a single data word. The label field of the word defines the type of data that is contained in the rest of the word. Typically, messages are sent repetitively. For example, measured airspeed is transmitted from the sensor to the instrument at intervals not less than 100 ms or greater than 200 ms. Messages may also be sent in repetitive word sequences or frames. Messages from each fuel tank level sensor are sent in sequence, and then the sequence is repeated after a specified time. Once the 63-word sequence to relate all the fuel tank levels is completed, it repeats, starting over with word 1. Most of the data are in binary format, but some words are in BCD. Communications on 429 buses use 32-bit words with odd parity. A low-speed bus (12 to 14.5 kb/s) is used for general-purpose, low-criticality applications, and a high-speed bus (100 kb/s) is used to transmit large quantities of data or flight-critical information. ARINC 429 Words ARINC Specification 429 specifies, among other things, the codes used as identifying labels for instructions and the standard types of data used in an aircraft multiplexing system. It also specifies that the information from the output port of an avionics system element (e.g. the Navigation Computer) be ‘communicated’ over a single twisted and shielded pair of wires to all other systems elements requiring the information. This means the information protocol is specified in ARINC 429, detailing standard data codes (labels) and formats which are to be incorporated into the data transfer network. Label – The label is the first 8 bits of a word and identifies the data type and the parameters associated with it. The label is an important part of the message; it is used to determine the data type of the remainder of the word and therefore the method of data translation to use. Labels are typically represented as octal numbers. SDI – Bits 10 and 9 provide a Source/Destination Identifier, or SDI. This is used for multiple receivers to identify the receiver for which the data are destined. It can also be used in the case of multiple systems to identify the source of the transmission. In some cases, these bits are used for data. ARINC 429 can have only one transmitter on a pair of wires, but up to 20 receivers. Aviation Australia ARINC 429 words Data - Bits 29 through 11 contain the data, which may be in a number of different 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 103 of 444 CASA Part 66 - Training Materials Only  formats. Many non-standard formats have also been implemented by various manufacturers. In some cases, the data field overlaps down into the SDI bits. In this case, the SDI field is not used. SSM - Bits 31 and 30 contain the Sign/Status Matrix, or SSM. This field contains hardware equipment condition, operational mode or validity of data content. This refers to plus, minus, north, south, left, right and so on in the Binary Coded Decimal (BCD) numeric data. Parity - The parity bit functions as explained earlier. For odd parity, the parity bit will be a 1 if there is an even number of 1s in the preceding part of the data word. The parity bit will be a 0 if there is already an odd number of 1s in the word. This means there will always be an odd number of 1s in each data word when the parity bit is included. Although the label, SDI, SSM and parity bits remain somewhat fixed in each word, bits 11-29, which contain the data, may be laid out in several formats. Some data are sent in BCD format, where each set of four binary bits represents a decimal number. This could be a word transmitted to an MFD to display altitude, for example, 25 786 ft. Remember, it is the label identifying the type of data that tells the receiving unit which format the data are sent in, hence the importance of the label. The label and the data encoding method are described in ARINC 429. Aviation Australia ARINC 429 data words In the second example, a purely binary value is transmitted 0 100 001 100 000 000 000, which equals 13721610 (Base 10). This could represent total fuel remaining in pounds; again, the label identifies the data contained and what it is encoded in. Data label and encoding are described in ARINC 429. All ARINC data is transmitted in 32-bit words. The data type may be BCD, two’s complement, Binary Notation (BNR), Discrete Data, Maintenance Data and Acknowledgment, or American Standard Code for Information Interchange (ASCII). 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 104 of 444 CASA Part 66 - Training Materials Only  ARINC incorporated ASCII code into ARINC 429. ASCII code is accepted worldwide as the International Standard Organisation Code No. 5 (ISO No. 5). It translates all alphanumeric inputs from the keyboard into the representing binary numbers required by the system. ARINC 429 Data Types There are several basic word formats in Specification 429 for numerical, discrete and alphanumeric data, which are encoded using ISO No. 5. All are based on the standard arrangement with a label, Sign/Status Matrix (SSM) and parity, but there are minor variations between them depending on the data being transmitted. The label indicates the data type so all LRUs receiving the word can interpret the information contained in each word. Aviation Australia ARINC 429 data types 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 105 of 444 CASA Part 66 - Training Materials Only  ARINC 429 Labels ARINC Specification 429 specifies, among other things, the codes used as identifying labels for instructions and the standard types of data used in an aircraft multiplexing system. It also specifies that the information from the output port of an avionics system element (for example, the Navigation Computer) be ‘communicated’ over a single twisted and shielded pair of wires to all other systems elements requiring the information. This means the information protocol is specified in ARINC 429, detailing standard data codes (labels) and formats which are to be incorporated into the data transfer network. The label is the first 8 bits of a word and identifies the data type and the parameters associated with it. The label is an important part of the message; it is used to determine the data type of the remainder of the word and, therefore, the method of data translation to use. Labels are typically represented as octal numbers. Aviation Australia 18 ARINC 429 labels Labels may be associated with more than one equipment type, and the equipment IDs associated with the examples are illustrated (above). Thus BCD label 010 is always present latitude, but it can pertain to three different sources: the Flight Management Computer (002), the Inertial Reference System (004), or Air Data and Inertial Reference System (ADIRS; 038). BCD label 014 is either Magnetic Heading from the Inertial Reference System (004), Attitude and Heading Reference System (005), or Air Data and Inertial Reference System (ADIRS). These examples also provide additional specifications for the data transmissions, number of digits used to transfer the data and present position, where a +ve data signal (SSM or just a positive digital number) indicates latitude north, whereas a negative number indicates latitude south. Data transmission rate means the parameter is transmitted from the source at a minimum of once every 250 ms (four times per second) up to a max of once every 500 ms (twice per second). 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 106 of 444 CASA Part 66 - Training Materials Only  ARINC 629 The new technology, called Digital Autonomous Terminal Access Communication (DATAC), was originally being developed at Boeing. It was a design for a single global data bus that would carry all the information between the different components of the aeroplane systems. The data bus consisted of a single twisted pair of wires to which all the components that needed to exchange information were connected. To keep the data from each component from getting jumbled with the other information being exchanged, each component's data were coded and ‘broadcasted’ in a synchronised order. All the information was transmitted on the data bus, and each computer or component could be programmed to pull off whatever information it needed. The DATAC system was a perfect design for the NASA 737. It allowed a far greater number of components to be integrated into the aircraft systems, and it greatly reduced the amount of time required to add or exchange experimental equipment. Since the data bus had fewer wires and components, it was also lighter and required less maintenance than a conventional system. By the beginning of August 1984, DATAC was installed and working in the aircraft. The 737 made an excellent test bed for a new data bus because the equipment in the front cockpit remained conventional. Later, the DATAC technology was incorporated into Boeing's next jet transport design, the B777. DATAC worked so well, in fact, that ARINC used it as the basis of a new industry data bus standard. The specification for the new data bus, called ARINC 629, was adopted in September 1989. Boeing B777 flight deck 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 107 of 444 CASA Part 66 - Training Materials Only  ARINC 629 Interconnection The ARINC 629 data bus is a time-division multiplex system. It is a bidirectional, distributed control bus capable of supporting up to 120 users at a transmission rate of 2 Mbps. It includes multiple transmitters with broadcast-type, autonomous terminal access. Terminals listen to the bus and wait for a quiet period before transmitting. The users communicate with the bus using a coupler and terminal. Only one terminal is allowed to transmit at a time. After a terminal has transmitted, three different protocol timers are used to ensure that it does not transmit again until all of the other terminals have had a chance to transmit. The ARINC 629 terminal controller and Serial Interface Module (SIM) are installed on a circuit board within each Line Replacement Unit (LRU). The SIM interfaces with the stub cable via a connector on the LRU. The stub cable is then coupled to the global data bus via a current mode coupler. Aviation Australia ARINC 629 interconnection 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 108 of 444 CASA Part 66 - Training Materials Only  ARINC 629 Interfaces The ARINC 629 data bus system includes these parts: Data bus cable Current-mode couplers Stub cables. The ARINC 629 system also includes these components in the LRUs: Serial interface modules Terminal controllers. Aviation Australia 629 data bus current coupler and ARINC 629 LRU The current-mode coupler connects the bus cable to the stub cable. The stub cables are for bidirectional data movement between the LRU and the current-mode coupler. The stub cables also supply power from the LRUs to the current-mode couplers. A stub cable has four wires: two to transmit and the other two to receive. An ARINC 629 LRU contains a SIM and a terminal controller. These move data between the LRU and the current-mode coupler. Each LRU has a personality that identifies its purpose and operation. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 109 of 444 CASA Part 66 - Training Materials Only  The personality data are in two parts: Transmit personality PROM (XPP) Receive personality PROM (RPP). The terminal controller uses the personality PROMs to control the flow of data between the LRU and the data bus. ARINC 629 Data Bus Cable The bus cable moves data between LRUs. A current-mode coupler and a stub cable attach each LRU terminal to the data bus cable. A bus cable is a pair of twisted wires with a termination resistor at each end. Each resistor has a value of about 130 Ω. The left and right systems bus cables have production break connectors in the middle for easy replacement. The parts of the system bus cable that are external to the coupler panels have shielding outside. A bus cable in the 777 may be as long as 180 ft. It connects as many as 46 current-mode couplers. The cable has a centre conductor covered by a layer of foam. A Teflon skin covers the foam. Aviation Australia Data cable 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 110 of 444 CASA Part 66 - Training Materials Only  ARINC 629 Message Structure Data in the ARINC 629 system move through the data bus cable and other components as messages. Between each message is a Terminal Gap (TG). A message is a group of word strings. Each word string has a label word followed by data words. Each message has a special structure that allows the LRUs to select and read the message. Message Structure A message has up to 31 word strings. There is a 4-bit time gap between each word string. A word string begins with a label word and has up to 256 data words. There is no gap between words in a word string. The minimum length message has one label and no data words. The maximum length message has 31 labels, with 256 data words following each label and 30 time gaps of 4 bits each. Label Word Structure A label word is a 20-bit word. It has: A 12-bit label field A 4-bit label extension field A single parity bit A 3-bit time hi lo sync pulse. A pulse of 1/2-bit time, called the Pre-Sync Sync Pulse (PSSP), comes before the first label word of a message. An approximately 1/2-bit time Pre-Pre-Sync Sync Pulse (PPSSP) comes before the PSSP. The PPSSP and the PSSP occur prior to the 3-bit time hi lo sync pulse. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 111 of 444 CASA Part 66 - Training Materials Only  Data Word Structure A data word is also a 20-bit word. It has: A 16-bit data field A single parity bit A 3-bit time lo hi sync pulse. Aviation Australia ARINC 629 message structure ARINC 629 Timing Because of the quantity of data that may be on the bus, ARINC 629 uses a time procedure to prevent accidental signal mixture. ARINC 629 uses three timers: Transmit Interval (TI) timer Synchronisation Gap (SG) timer Terminal Gap (TG) timer. The timers are part of the LRU personality. Each LRU uses all three timers to isolate data messages. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 112 of 444 CASA Part 66 - Training Materials Only  Transmit Interval (TI) The TI for any LRU begins the moment the terminal starts to transmit. After the terminal transmits a message, it must wait the length of time equal to the TI before it transmits again. All LRUs on a bus have the same TI. Synchronisation Gap (SG) After the TI, the SG is the longest timer. The SG begins when there is no signal on the bus. The SG is the same for all LRUs. It has a value greater than the value of the longest terminal gap used on a given bus. If a signal comes on the bus before the SG completes, the SG stops. When the SG completes, it stays reset until the LRU transmits again. Terminal Gap (TG) Each LRU on the bus has a special TG. The TG begins after the SG is complete and no signal is on the bus. If there is a signal on the bus before the TG completes, the TG stops. It starts again when there is no signal on the bus. The TG and SG cannot overlap in time, but must occur in sequence. Aviation Australia ARINC 629 timing 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 113 of 444 CASA Part 66 - Training Materials Only  ARINC 629 Timing Mode The three ARINC 629 timers operate in these two ways: Periodic mode Aperiodic mode. The periodic mode makes sure that an LRU transmits at a regular time sequence, in the power-up order. If an LRU message length increases because of a non-normal condition, the system changes to aperiodic operation. In aperiodic operation, the LRUs transmit in a different time sequence, in order of shortest TG to longest TG. Periodic Mode The periodic mode is the normal mode of operation. In the periodic mode, an LRU transmits once every TI. The examples show the timing diagram for three LRUs in the periodic mode. Aviation Australia Timing diagram for three LRUs in the periodic mode Events At event 1, all three timers (TI, SG and TG) for LRU 1 are complete and LRU 1 starts to transmit a message (M). LRUs 2 and 3 stop their TGs when LRU 1 starts to transmit. At event 2, LRU 1 no longer transmits, and LRU 2 and 3 start their TG timers. At event 3, the TG timer for LRU 3 is complete. However, TI still continues. LRU 3 does not transmit. The TG timer for LRU 2 continues. At event 4, the TG timer for LRU 2 is complete. All three timers for LRU 2 (TI, SG and TG) are complete and the LRU 2 starts to transmit a message, while LRU 3 waits for its TI to complete. LRU 3 stops its TG when LRU 2 starts to transmit. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 114 of 444 CASA Part 66 - Training Materials Only  At event 5, LRU 2 stops transmission and LRU 2 starts its TG timer. At event 6, the TG timer for LRU 3 completes. For LRU 3, all three timers (TI, SG and TG) are complete and it starts to transmit a message. At event 7, LRU 3 no longer transmits, and all three LRUs start their SG timers. At event 8, all three SG timers are complete and the TG timers start. At event 9, the TG for LRU 1 completes. TI continues, so it does not transmit. At event 10, TG for LRU 3 is complete. TI continues, so the LRU does not transmit. At event 11, the TG for LRU 2 completes. TI continues, so the LRU does not transmit. Back at event 1, all three timers (TI, SG and TG) for LRU 1 are complete and it starts to transmit a message. LRU 2 and 3 stop their TGs when LRU 1 starts to transmit. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 115 of 444 CASA Part 66 - Training Materials Only  Aperiodic Mode If the sum of all the TGs, transmission times and SGs is greater than the TI, then the system operates in aperiodic mode. Aperiodic data are a direct result of a discrete event. They are data that are asynchronous and updated at a non-uniform rate. For example, aperiodic data can be a position report in landing gear systems. Aperiodic data transfers data about events important to aircraft operation. In this example, TG1 < TG2 < TG3 so that LRU 3 transmits its message first. These data are categorised into two classes: Data to control tasks, such as landing gear sensors and flight deck switches Data for status information. Aperiodic data also transmits large blocks of data for these functions: Database loads Operational software BITE information. Aviation Australia Aperiodic mode A comparison of data buses is shown below. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 116 of 444 CASA Part 66 - Training Materials Only  Aviation Australia Data bus summary 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 117 of 444 CASA Part 66 - Training Materials Only