Data Transmission - Aviation Australia PDF

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 necessa...

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  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