Digital Techniques Data Buses PDF

Here is the converted markdown format of the provided document: # Aviation AUSTRALIA ## 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-STD1553...

Here is the converted markdown format of the provided document: # Aviation AUSTRALIA ## 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-STD1553, ARINC 429, ARINC 629, Ethernet, AFDX (Level 2). ## 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. Below is an illustration of electricity generation, transmission, and istribution. The figure includes a power plant that generates electricity for transmission lines carry electricity long distances. Distribution lines then cary electricity to houses; transformers step up voltage for transmission, while neighborhood transformers bring it down. Pole transformers also sit between the distribution lines and houses. 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. Below is an illustration of an example of a battery connected to an aircraft starter The diagram is labelled with the components of the system including the starter, starter switch, master switch, battery, bus, starter solenoid, and master solenoid all connected together. 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. ### 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 utilized 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 utilize 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. Below is an image of a system of electronic components The diagram includes integrated circuits, data transmission and data logic circuits. It is labelled as electrical signals through ICs and transmited through data cables. ### Digital Data Transfer An ideal digital waveform is a square wave. The illustration below demonstrates an example of an ideal waveform (top) comapred 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, 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. Below is an illustration which compares an ideal digital signal versus a realistic digital signal. Clock pulses are labeled on a graph as the system works. The label Ideal vs practical digital signal is at the bottom. 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. Continuation of a graph showing clock pulses in a digital system. It is labelled "How a digital signal would probably look in reality". 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. ### 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. Below is an illistration of Serial data transfer, Which includes the following 0101001011001001 transmitter, reciever, data bus and bus terminater ### 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 of 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. Below is an illistration of Parallel data transfer and associated hardware Transmitter Reciever Hard drive 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 compouter and has enough wires 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. ## 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. Below is a diagram of Multiplexing where two rotary switches are synchronised Inputs and outputs along with 0's and 1's. The image is labelled Serial data transmission line Below is a diagram of Time domain multiplexing Inputs and outputs along with 0's and 1's. 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, wwhere 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 digitial, 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 recieves all the inputs and distributes outputs and processed data (after processing data from several inputs) to systems requiring informations. 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. Below is a diagram of Logic gate multiplexing Inputs and outputs along with 0's and 1's with clocks. ## 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 voltage, 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. Below is a diagram of aircraft instruments connected in a complex system. The components are Navigation systems, Air data systems, Airframe & Engine sensors and Flight control signal processing & autopilot As more and more systems were added, cockpit 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. 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 a 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 multipiers 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. Below is a diagram of digital aircraft system The components inlcude the navigation computer data computer and SYMBOL GENERATOR 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. 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 A/A converters. As time and technology progressed, the avionics system 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 vehocles have also been using multiplexing for many years. Below is an image of avionics data bus layout in an aircraft