Data Buses (5.4) PDF

Data Buses (5.4) Learning Objectives 5.4.1 Describe the operation of data buses in aircraft systems (Level 2). 5.4.2 Describe the properties of data bus communication protocols including: MIL-STD 1553, ARINC 429, ARINC 629, Ethernet, AFDX (Level 2). 2024-11-05 B2-05a...

Data Buses (5.4) Learning Objectives 5.4.1 Describe the operation of data buses in aircraft systems (Level 2). 5.4.2 Describe the properties of data bus communication protocols including: MIL-STD 1553, ARINC 429, ARINC 629, Ethernet, AFDX (Level 2). 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 72 of 444 CASA Part 66 - Training Materials Only  Data Transmission Electric Power In electricity delivery grids from power stations, power is transmitted over heavily constructed, thick power lines designed to carry very high voltages and a large electrical current. As the power is stepped down, so is the diameter of the wiring necessary to conduct it efficiently. Adapted from the National Energy Education Development Project (Public Domain) Electricity generation, transmission and distribution Different applications require differing amounts of power. For example, navigation light runs on 28-V DC, and so does an aircraft starter motor. The navigation light draws only a very small current, as is evident by the narrow-gauge wiring providing the power to it. The starter motor, on the other hand, draws a very high current, so a thick cable is necessary for the motor to operate efficiently and generate the torque required to turn over an aircraft engine. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 73 of 444 CASA Part 66 - Training Materials Only  An example of a battery connected to an aircraft starter Both of these aircraft applications use electricity to perform work, hence there is significant current flow, necessitating appropriately sized wiring to carry the current required. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 74 of 444 CASA Part 66 - Training Materials Only  Electrical Data Transmission An entirely different use for electricity is to transmit signals. Electricity is the ideal means to transmit information because it travels at the speed of light. Wiring utilised to transmit data carries only a negligible current, just sufficient to switch a transistor ON or OFF or to carry an audio signal, which is then amplified at its destination. This lesson deals entirely with transmission of data. Ideally no current flows on a digital data line, although in reality there is a small flow of current sufficient to at least forward and reverse bias semiconductor P and N junctions. But data bus lines are typically very small gauge, and the electrical signal transmitted over them is typically no higher than 5-V DC and looks similar to an AC sine wave, although without any uniformity or sequence. The information transmitted is actually all the 1s and 0s which represent data encoded as a digital signal. The data are sent in a regulated and uniform sequence between components, where computer processors at either end decode and utilise the data to produce the desired outputs, whether it be to display present latitude and longitude on a horizontal indicator or to drive a servo motor to regulate fuel flow to an engine. Aviation Australia Electrical signals through ICs and transmitted through data cables (or wireless) 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 75 of 444 CASA Part 66 - Training Materials Only  Digital Data Transfer An ideal digital waveform is a square wave. The illustration below demonstrates an example of an ideal waveform (top) compared to how a waveform is likely to present in practice (bottom). Both waveforms represent 1010 0110. It is important to remember that the voltage produced is a result of the transistor switching ON and OFF, and transistors are not perfect in respect to instantaneously switching states from OFF to fully saturated. They are continually forward and reverse biased, so the wave shape is in reality more like a distorted AC sine wave as shown below. Although the wave shapes are not perfect, they do function as intended and any computer is a testament to how well the digital data transfer works. Aviation Australia Ideal vs practical digital signal In electronic digital systems, data in binary form is represented by the presence or lack of a voltage for each bit at the inputs and outputs of the various circuits. Typically binary 0 is represented by 0 V, and binary 1 is represented by 5 V. In practical systems, any voltage between 0 and 0.8 V (not sufficient to saturate a transistor) represents binary 0 and any voltage between 2 and 5 V represents a binary 1. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 76 of 444 CASA Part 66 - Training Materials Only  Aviation Australia Practical data signal The clock pulse represented in the diagram is basically a representation of the operating speed of the data bus, and the transmitter and receiver are synchronised by the same clock pulse. When the transmitter is outputting a high, the receiver detects it and clocks it through as a 1 to processing circuitry. When data are next sampled by the receiver, the transmitter is outputting a low, and a 0 is clocked through to the processing circuitry. In digital, quantities are represented by voltages which have a wide tolerance (for example, 2–5 V for a 1) whereas in analogue, voltages must be exact – any deviation causes errors. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 77 of 444 CASA Part 66 - Training Materials Only  Serial Data Transfer In digital computers, enormous amounts of data move between parts of the system. The two basic ways of doing this are by parallel data transfer and serial data transfer. In serial transfer, each bit of data is transferred from a store (or memory location; illustrated is 2 bytes of data, or 16 bits) in sequence over the same line. The data is triggered by clock pulses as explained in the previous slide, and the transmitter and receiver are synchronised with reference to the same clock pulse. When the data arrives at the receiver, it is sequentially stored in memory (2 bytes’ worth in this case) before being transferred to processing circuitry in the receiving component. The serial bus is one on which the data are transmitted sequentially, one word following another word. It is commonly used for long-distance transmissions. Advantages of serial data flow: less hardware, therefore less weight and space for an installation compared to parallel data transfer systems. Serial data transfer is typical of data-bus communications. Multiplexing is a typical method of speeding up the data transfer capacity of a serial data bus. Aviation Australia Serial data transfer 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 78 of 444 CASA Part 66 - Training Materials Only  Parallel Data Transfer In parallel transfer, each bit is taken from a separate circuit (for example, a processing or calculating circuit) and is transmitted over a separate line. Advantage of parallel data transfer: much faster. In the example in the slide, it would be 16 times faster. When considering the time taken to download data from the internet over the serial connection, imagine how quickly everything would run if there was a parallel connection. The downside of parallel is, of course, you need much more hardware, which takes up space and increases weight, two things we do not want to do in an aircraft. Aviation Australia Parallel data transfer and associated hardware Serial data transfer is typical of data-bus communications, whereas once a signal is inside a computer, it is typically processed in parallel. A parallel bus typically interconnects the internal devices of a computer and has enough wires to transmit all bits of the word simultaneously. An 8-bit parallel bus is 8 times faster than the serial bus, and a 64-bit parallel bus is 64 times faster than its equivalent serial bus. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 79 of 444 CASA Part 66 - Training Materials Only  Multiplexing Multiplexing is combining two or more information channels onto a common transmission medium. On aircraft, multiplexing greatly decreases the number of wires carrying separate signals. Using a digital ‘time division’ technique, many different signals can be carried by one conductor. Benefits include a significant reduction in the weight of wire bundles and improved circuit reliability. Aviation Australia Multiplexing where two rotary switches are synchronised The basic principle of multiplexing is that two rotary switches are synchronised in their switching as they rotate around a series of contacts. The synchronised rotating contacts connect matching input and output lines in sequence, and data are transmitted over the common transmission line. © Aviation Australia Time domain multiplexing 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 80 of 444 CASA Part 66 - Training Materials Only  In reality, multiplexing is usually done by logic gates responding in sequence to clock pulse signals. At the multiplexing end, the signal on each input line is sampled and passed to the common transmission line, when the inputs AND gate is clocked ON. The sequenced gate outputs are serially transmitted to the demultiplexer, where the inverse happens. As each AND gate is clocked on, it passes the signal that is on the transmission line at that time. This has the effect of transmitting eight separate inputs through to eight separate outputs over the same transmission line. In aircraft, analogue signals may be multiplexed, but they must first be converted to digital, transmitted over the multiplexer network and then converted back to analogue form once demultiplexed. In an aircraft, the sequencing controller is replaced by a Bus Controller (BC), which typically receives all the inputs and distributes outputs and processed data (after processing data from several inputs) to systems requiring the information. For example, it can distribute digitised data to be displayed on a multifunction display or calculated air density data for transmission to a thrust computer. Aviation Australia Logic gate multiplexing 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 81 of 444 CASA Part 66 - Training Materials Only  Aircraft Multiplex System In the 1950s and 1960s, aviation electronics, referred to as avionics, were simple stand-alone systems. The navigation, communications, flight controls and displays consisted of analogue systems. Often these systems were composed of multiple boxes, or subsystems, connected to form a single system. Various boxes within a system were connected with point-to-point (analogue) wiring. The signals mainly consisted of analogue voltages, synchro-resolver signals and switch contacts. The location of these boxes within the aircraft was a function of operator need, available space, and aircraft weight and balance constraints. Aircraft digital, analogue and airframe - engine sensors, engine control unit As more and more systems were added, cockpits became more crowded, wiring became more complex, and the overall weight of aircraft increased. By the late 1960s and early 1970s, it became necessary to share information between the various systems to reduce the number of black boxes required by each system. A single sensor, for example, that provided heading and rate information, could provide those data to the navigation system, the flight control system and the pilots’ display system. However, the avionics technology was still basically analogue, and while sharing sensors did reduce the overall number of black boxes, the connecting signals became a ‘rat's nest’ of wires and connectors. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 82 of 444 CASA Part 66 - Training Materials Only  Moreover, functions or systems that were added later became an integration nightmare, as additional connections of a particular signal could have potential system impacts. Additionally, as the system used point-to-point wiring, the system that was the source of the signal typically had to be modified to provide the additional hardware to output to the newly added subsystem, such as additional amplifiers, or Output Multiplier Boxes (OMBs). Because a single parameter may be required by several boxes or systems, it was necessary to incorporate OMBs where a single signal (or range of signals, for example, ADC OMB) would be fed into analogue multipliers. This allowed the signal to be replicated many times for output to the associated systems requiring the information, for example, attitude for display, autopilot, radar transmitter stabilisation and so on. Output multipliers were large, heavy boxes and added to aircraft weight and space problems, as well as adding complexity to systems operation. The analogue signals all require dedicated wiring to pass the information from one box to another. In later computerised aircraft, it would likely be impossible to design or construct an aircraft with analogue wiring because too much information is typically transmitted for routine operation of the avionics systems. The aircraft would be a flying wiring loom. Aviation Australia Digital aircraft system By the late 1970s, with the advent of digital technology, digital computers had made their way into avionics systems and subsystems. They offered increased computational capability and easy growth compared to their analogue predecessors. However, the data signals, inputs and outputs from the sending and receiving systems were still mainly analogue in nature. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 83 of 444 CASA Part 66 - Training Materials Only  This led to the configuration of a small number of centralised computers (typically only one or two) interfaced to other systems and subsystems via complex and expensive A/D and D/A converters. As time and technology progressed, the avionics systems became more digitised. With the advent of the microprocessor, things really took off. A benefit of this digital application was the reduction in the number of analogue signals, and hence the need for their conversion. An additional benefit was that digital data could be transferred bidirectionally, whereas analogue data were transferred unidirectionally. Multiplexing is a technique that minimises the amount of wiring required to transmit information or commands throughout an aircraft. It is not unique to aircraft; motor vehicles have also been using multiplexing for many years. Avionics data bus layout in an aircraft By multiplexing, a Bus Controller (BC) manages all communications over the multiplexing bus. The BC works similarly to a base station radio operator. In the illustration, the BC is run by the Flight Management Computer (FMC). The mission computers control all data transmitted over the multiplexer busses. The incorporation of a fully integrated digital avionics system requires a digital data bus to provide a two-way interface between various navigation sensors, computers and indicators. Serial rather than parallel transmission of the data is used to reduce the number of interconnections (wires) within the aircraft and the receiver/driver circuitry required with the black boxes. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 84 of 444 CASA Part 66 - Training Materials Only  Data Bus Systems The interface between each computer and external device is accomplished via the digital data bus. The bus is made up of a twisted pair of wires which are shielded and jacketed. The shielding provides spike protection, eliminates electromagnetic field (EMF)-induced errors and virtually guarantees accurate transmissions from each transmitter to its receiver. The wires are twisted so the magnetic fields induced by the currents flowing through them will cancel each other out, eliminating electromagnetic interference (EMI). Aviation Australia Data bus systems Data may travel one way (simplex) or in two directions (duplex), depending on the system design. Transmission of data within micro-computers and external transmissions between other components are accomplished with 8-, 16-, 32- or 64-bit digital words. Regardless of which system is employed, only one data word will be transmitted on the data bus at any time. It is not a free-for-all, as then the data bus would simply be a jumble of 1s and 0s and would be meaningless. All communication is controlled by either BCs or timing regimes, like in a radio communications system. If several stations transmit over the same frequency simultaneously, none can be understood. Each transmission must be timed to transmit one at a time, and then all radio traffic can be understood. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 85 of 444 CASA Part 66 - Training Materials Only  Data Bus Connectors The multiplexer bus functions like an arterial highway and is not designed to connect to any specific components. The highway is laid, and a BC is connected to manage all data transmitted over the highway. All the peripheral components are connected to the highway by breakouts (couplers) and perform similarly to telephone extensions connected to an exchange or computers connected to a Local Area Network (LAN). Aviation Australia Data bus connectors Bus Controller While several terminals may be capable of performing as the BC, only one BC may be active at any time. The BC is the only one allowed to issue commands on the data bus. Commands may be for transfer of data or control and management of the bus. Aviation Australia Data bus systems and bus controller 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 86 of 444 CASA Part 66 - Training Materials Only  BCs manage this activity by sending out commands to peripheral systems requesting data stored in the memory of the peripheral components, for example, altitude from the Air Data Computer (ADC). The ADC responds to the digital command and transmits binary data representing aircraft altitude. The BC then updates the Electronic Flight Instrument System (EFIS) altitude readout by transmitting the digitised altitude value to the EFIS, where the new altitude value is displayed. Data transmission is strictly controlled by the BCs. Typically only a few types of words are transmitted over digital data busses, and all communications on specific types of data busses are initiated by the BC. The commands may be for the transfer of data or the control and management of the bus (referred to as mode commands). The BC commands a remote terminal to transmit data to it. The remote terminal responds with the information requested. The BC may command a remote terminal to receive data. It sends the data, for example, digitised information for display by a multifunction display, and the remote terminal responds when the data transfer is complete to confirm that it received the information and that no errors were detected in the transmission. In this way enormous amounts of information are transferred along the data bus, all under the control of the Bus Controller. Typically, the BC is a function that is contained within some other computer, such as a Flight Management Computer (FMC) or a display processor. Aviation Australia Bus controller and backup bus controller 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 87 of 444 CASA Part 66 - Training Materials Only  MIL–STD–1553 Data Bus MIL-STD-1553 – Military Standard (MIL-STD) defines electrical and protocol characteristics for a data bus. The advent of the digital data bus alone was still not enough. A data transmission medium which would allow all systems and subsystems to share a single and common set of wires was needed. By sharing the use of an interconnect, the various subsystems could send data among themselves, and to other systems and subsystems, one at a time and in a defined sequence, hence a data bus. MIL-STD-1553B defines the term Time Division Multiplexing (TDM) as ‘the transmission of information from several signal sources through one communications system with different signal samples staggered in time to form a composite pulse train’. This means data can be transferred between multiple avionics units over a single transmission medium, with the communications between the different avionics boxes taking place at different moments in time – hence time division. MIL-STD-1553 (USAF) was released in August 1973. The primary user of the initial standard was the F-16 Fighting Falcon. Further changes and improvements were made and a tri-service version, MIL- STD-1553A, was released in 1975. The first users of the A version of the standard were the U.S. Air Force's F-16 and the U.S. Army's new attack helicopter, the AH-64A Apache. With some real-world experience, it was soon realised that further definitions and additional capabilities were needed. The Society of Automotive Engineers (SAE) spent three years of concentrated effort to produce 1553B, which was released in 1978. At that point, the government decided to ‘freeze’ the standard at the B level to allow component manufacturers to develop products and to allow the industry to gain some additional real-world experience before determining the next set of changes to be made. Aviation Australia MIL-STD-1553 schematic diagram - data bus layout of aircraft system 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 88 of 444 CASA Part 66 - Training Materials Only  MIL-STD-1553 Data Words Three distinct word types are defined by the standard. These are: Command words Data words Status words. Each word type has a unique format, yet all three maintain a common structure. Each word is 20 bits in length. The first 3 bits are used as a synchronisation field, allowing all device clocks to re-sync at the beginning of each new word. The next 16 bits are the information field and are different between the 3-word types. The last bit is the parity bit. Parity is based on odd parity for the single word. The encoder automatically calculates parity. Odd parity means there is always an odd number of 1s in a word. Command words contain a terminal address which tells the remote terminals which component the command is addressed to. T/R (Transmit/Receive) bit signifies whether the remote terminal will prepare to receive or transmit data. Sub-address mode indicates the memory location the remote terminal will either store transmitted data in or transmit data from. Word count indicates how many data words are about to be sent to the Remote Terminal (RT) or how many words the RT must transmit back to the BC. If the sub-address area contains all 0s, this indicates the command word is a mode change, and then the word count block contains data indicating which mode the RT is to switch to, for example, to switch an Inertial Navigation Unit from alignment mode to navigation mode, or to command an ADC to perform a Built-In Test (BIT). 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 89 of 444 CASA Part 66 - Training Materials Only  Aviation Australia MIL–STD–1553 data words Data words contain purely data and are always preceded by a command or status word to effectively label what data is contained in the data words. Status words contain the terminal address from where the status word is sent so the BC knows who it is talking to. The remainder of the word basically tells the BC that the data transfer was completed successfully and that the remote terminal is serviceable and operating correctly. BIT encoding for all words is based on Bi-Phase Manchester II format. A transition of the signal occurs at the centre of the bit time. A logic 0 is a signal that transitions from a negative level to a positive level. A logic 1 is a signal that transitions from a positive level to a negative level. Aviation Australia Manchester II Bi-Phase waveform 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 90 of 444 CASA Part 66 - Training Materials Only  It is important to note that the voltage levels on the bus are not the signalling media and that it is strictly the timing and polarity of the zero crossings that convey information on the bus. That is, the ramps up or down indicate a 0 or a 1, not the magnitude of voltage. For this reason, the 1553 bus is extremely forgiving of conditions that cause the voltage levels on the bus to vary. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 91 of 444 CASA Part 66 - Training Materials Only  MIL–STD–1553 Data Transfer The primary purpose of the data bus is to provide a common medium for the exchange of data between systems. The exchange of data is based on message transmissions. The standard defines 10 types of message transmission formats. All of these formats are based on the three word types just defined. The message formats are: Mode change transmissions: BC commands mode change of RT, without any data being transferred BC transmits Mode Change command of RT and transmits data to the RT BC transmits Mode Change command of RT and requests data be transmitted from RT to BC Broadcast message transmissions: BC to RTs, BC transmitting data BC commands RTs to transmit data to other RTs BC transmits Mode Change command to RTs without any data being transferred BC transmits Mode Change command to RTs and transmits data to RTs. There are two message format groups, the information transfer formats and the broadcast information transfer formats. The information transfer formats are based on the command/response philosophy in that all error-free transmissions received by an RT are followed by the transmission of a status word from the terminal to the BC. This handshaking principle validates the receipt of the message by the RT. Each of the message formats is summarised in the sections which follow. Aviation Australia Bus controller transmitting data to remote terminal The Bus Controller to Remote Terminal (BC-RT) message is referred to as the receive command since the remote terminal is going to receive data. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 92 of 444 CASA Part 66 - Training Materials Only  The BC outputs a command word to the terminal defining the sub-address of the data and the number of data words it is sending (the sub-address informs the receiver of the memory location where the data are to be stored). Immediately (with no transmission gap), the number of data words (up to 32) specified in the command word is sent. The RT, upon validating the command word and all data words, issues a status word indicating the message was received and was valid. Aviation Australia Remote terminal transmitting data to bus controller The Remote Terminal to Bus Controller (RT-BC) message is referred to as a transmit command. The BC issues only a transmit command word to the RT. The terminal, on validating the command word, transmits its status word followed by the number of data words requested by the command word. Remote Terminal (RT) Transmitting Data to Remote Terminal (RT) The Remote Terminal to Remote Terminal (RT-RT) command allows a terminal (the data source) to transfer data directly to another terminal (the data sink) without going through the BC. However, the BC may also collect the data and use them. The BC issues a command word to the receiving terminal, immediately followed by a command word to the transmitting terminal. Aviation Australia Remote terminal transmitting data to remote terminal 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 93 of 444 CASA Part 66 - Training Materials Only  The receiving terminal is expecting data, but instead of data after the command word it sees a command synchronisation (command sync, the second command word). The receiving terminal ignores this word and waits for a word with a data synchronisation (data sync). The transmitting terminal ignored the first command word (it did not contain the appropriate terminal address). The second word was addressed to it, so it processes the command as a RT-BC command by transmitting its status word followed by the required data words. The receiving terminal, having ignored the second command word, again sees a command (status) synchronisation (sync) on the next word and waits. The next word (the first data word sent) now has data sync and the receiving RT starts collecting data. After receipt (and validation) of all of the data words, the terminal transmits its status word. MIL–STD–1553 Specifications Aviation Australia Example of the standards specified by MIL-STD-1553 The 1553 bus is bidirectional, meaning data flow in both directions (not simultaneously) from BC to RT and from RT to BC. This means the bus must be managed by a BC coordinating all the data traffic. ARINC 429 is simplex operation. A component transmits data to up to 20 terminals, but data do not flow in reverse. As time and technology have progressed, avionics systems have become more digitised. With the advent of the microprocessor, things really took off. Small analogue sensors could incorporate a microprocessor, thus providing a digital output and negating the requirement for A/D and D/A converters. Standard 1553 systems incorporated many A/D and D/A converters, increasing the cost of installations. With avionics systems now typically producing outputs in digital format, the 1553 standard of installation has fallen behind contemporary digital avionics system installations. So although MIL-STD-1553 was a pioneer in digital data bus development, Aeronautical Radio Inc. (ARINC) standard installations are now more typically incorporated in modern commercial aircraft. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 94 of 444 CASA Part 66 - Training Materials Only  MIL-STD-1773 MIL-STD-1773 contains the requirements for utilising a fibre optic ‘cabling’ system as a transmission medium for the MIL-STD-1553B bus protocol. As such, the standard repeats MIL-STD-1553 nearly word-for-word. The standard does not specify power levels, noise levels, spectral characteristics, optical wavelength, electrical/optical isolation or means of distributing optical power. These must be contained in separate specifications for each intended use. Data encoding and word format are identical to MIL-STD-1553, with the exception that pulses are defined as transitions between 0 (off) and 1 (on) rather than between positive and negative voltage transitions since light cannot have a negative value. Since the standard applies to cabling only, the bus operates at the same speed at which it would utilise a wire. Additionally, data error rate requirements are unchanged. Different environmental considerations must be given to fibre optic systems. Altitude, humidity, temperature and age affect fibre optics differently than wire conductors. Power is divided evenly at junctions which branch, and connectors have losses just as wire connectors do. Fibre optic 'cabling' 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 95 of 444 CASA Part 66 - Training Materials Only  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