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ELEC4504 Avionics Systems 157 CHAPTER 14. System Integration/ Data transfer Since parts of systems are necessarily installed in different parts of the aircraft, it is necessary to provide some means of transmitting information from one to another. e.g. VOR receiver - avionics bay Control head / Indicator - cockpit Temperature sensor - outside skin Air Data Computer - avionics bay Also, as systems become more integrated more intercommunication is required e.g. INU Cockpit Display Satellite Antenna Steering attitude steering Flight Management System attitude position velocity INU aircraft attitude Figure 66: Integration of INU, Satellite Antenna and Flight Management System 14.1. Synchros Until recently much of the instrumentation in the cockpit has been of electromechanical design, with the information being presented in the form of a rotating needle and a dial. In order to transmit rotational information electrically, devices called synchros were used. These are essentially rotating transformers and motors. A A Receiver Transmitter 26VAC 400Hz B C B Figure 67: Wiring Diagram of Synchro Devices C 158 ELEC4504 Avionics Systems As shown above, each synchro consists of three static windings wound at 120˚ intervals, and a single rotor winding. As the name suggests, the static windings are fixed to the body of the synchro while the rotor winding is attached to a shaft which is free to rotate. When an excitation voltage (in aircraft usually 400 Hz, 26 Volts) is applied to the rotors, voltages are induced in each of the stator windings. The voltages are determined by the angle of rotation of the rotor. When a synchro receiver is connected as shown, its rotor will take up the same angle of rotation as the transmitter rotor. Thus the angular information has been transmitted via an electrical connection. Information available in this form includes: i) pitch, roll and heading from INU ii) altitude from an altimeter iii) relative bearing from an ADF antenna 14.1.1 Problems that can occur with synchro systems are: i) loss of the excitation voltage (circuit breaker opening) ii) interchange of wires (causes incorrect readings) 14.1.2 Disadvantages i) - relatively large and bulky ii) - require separate excitation voltage iii) - since they are mechanical, they are subject to wear and breakdown iv) - in mixed analogue/digital systems, synchro to digital and digital to synchro conversion modules are required and these are usually expensive. Ironically, since synchros are present in many current aircraft systems, new, all digital equipment may still produce information in synchro format by use of a digital to synchro converter. Some other all digital piece of equipment may provide for synchro input by means of a synchro to digital converter. 14.2. Fundamentals of digital data transmission Digital data transmission has the advantage that it is very flexible and has the potential to reduce drastically the amount of wiring required. e.g. to send pitch roll and heading information, the analogue approach would required 11 wires (3 for each synchro and 2 for a shared reference. By contrast, a digital system needs only two wires and can transmit much more data than just the attitude information. A typical process for transmitting data digitally is as follows: ELEC4504 Avionics Systems 159 16 sensor Processing A/D (16 lines) Figure 68: Typical Digital Data Transfer This does not appear to be much of an improvement, since 16 wires are required (1 bit/wire). However, it is possible to send the same information in a serial fashion in which case only two wires are needed. Note that, in the example given, the data rate would be reduced by a factor of 16, but the data rates can be increased to compensate. Example Signal byte with binary representation 010110102 is to be transmitted serially Tclock Rxclock Txclock = 8 x Tclock 0 1 0 1 1 0 1 0 0 1 0 1 serial 1 0 shift 1 register 0 01011010 Rclock 0 1 0 1 1 0 1 0 serial shift register parallel register parallel register 0 1 1 1 0 1 0 1 0 0 time Stop Start Figure 69: Diagram of Conversion between Parallel/Serial/Parallel Data Transfer Note: a shift register shifts its contents one bit for each clock pulse e.g. 160 ELEC4504 Avionics Systems Output REGISTER Input Start 1 0 1 0 1 1 Clock pulse 1 0 1 0 1 0 1 1 0 1 0 1 0 1 Clock pulse 2 Time domain multiplexing of individual bits allows transmission on a single line. The data rate, however, is lower T x clock -----------------T clock = number of bits transmitted/number of words transmitted In Figure 67. all 8 bits of the byte are clocked in to the parallel register, they are then copied to the serial shift register. Then the bits clocked from the serial shift register on to the transmission line. 14.2.1 Logic Levels The transmitter sends binary ones and zeros as specific voltage levels. However, during their travel along the transmission wires, the signals become modified by attenuation and noise, so that it can be difficult for the receiver to decide whether the signal is a one or a zero. Thus for each type of logic, the voltage levels representing ones and zeros are set far enough apart that there is little chance of misinterpretation. 5V 0V transmitted ‘high’ or ‘1’ transmitted ‘low’ or ‘0’ Figure 70: Illustration of Logic Levels voltage values receiver interprets as a ‘1’ voltage values receiver interprets as a ‘0’ ELEC4504 Avionics Systems 161 Example (TTL or transistor-transistor logic) 0.4V 0.7V driver is required to provide logic 0 signal lower than 0.4V receiver is required to accept as logic 0 all signals with amplitude ≤ 0.7V Figure 71: TTL Noise Margin In this case the noise margin is 0.3V which means that the transmitted signal can be distorted by 0.3V before a false value is detected 14.2.2 Synchronization One way by which a receiver can improve the probability of choosing the right value is to average the signal over the width of a bit. To do this, however, it must know where the boundaries of the bits are. If it were to average a signal over half a zero bit and half a one bit, the result would not be correct. Thus it is advantageous for the receiver to have a copy of the clock signal from the transmitter. This can be sent on a separate set of wires but, as we shall see, data can be sent using codes which are self-clocking in which case, only one set of wires is needed. 14.3. Overview of Digital Devices Today’s digital devices are built exclusively in the form of integrated circuits (ICs) These can be divided into the following categories: a) programmable execution devices (microprocessors, microcontrollers) b) memory devices i) SRAM (static random access memory) ii) DRAM (dynamic random access memory) iii) ROM (read only memory) iv) EPROM (erasable programmable ROM) Discrete logic devices a) logic families such as TTL (based on the methods used to implement the functions) 162 ELEC4504 Avionics Systems b) programmable logic devices c) ASICs - application specific integrated circuits 14.3.1 Microprocessors The manufacturing technology allows creation of devices containing ~ 109 (Intel Tukwila 2x109 announced 2 Feb 2008 ) transistors, but MIL spec, high reliability ICs are available only after a 5 - 10 year lag. Military/ avionics technology (MIL spec), therefore is conservative compared to the current consumer market technology 14.3.1.1 Current Standard: -MIL STD 1750A - standardized instruction set - 16 bit data bus - price ~ 10 x today’s microprocessors 14.3.2 Memory Devices RAM - random access memory in each memory cycle an arbitrary (random) location can be read or written to row decoder 2m = n2 address register 2m 2 memory array nxn 1 m indicates the number of lines selected cell 2m 2 n 1 n column decoder Figure 72: Random Access Memory ELEC4504 Avionics Systems 163 Static Memory (SRAM) uses a bistable device (flip-flop) to store binary information DATA IN STROBE D Q DATA OUT CK Q Figure 73: Static RAM Element The data out is set to whatever data in is when the strobe pulse is received The flip-flop stores the information as long as power supply voltage is provided, but it dissipates power continuously causing high power consumption for large memories. Requires at least two transistors plus resistors and capacitors. (takes up a lot of space on the chip) 14.3.3 Dynamic Memory (DRAM) Information is stored on a capacitor and unless restored, will disappear with time due to the leakage of charge from the capacitor. (leakage current) does not have to be an actual capacitor bit information control signal to open switch Figure 74: Memory Element for DRAM DRAM memory requires only one transistor for 1 bit of information. High capacity (Gbit) low power consumption memory uses DRAM technology. 14.3.4 Other types of Memory: ROM (read only memory) a) mask programmable - programmed at the factory, can’t be changed b) electrically programmable, UV erasable EPROM. can be programmed in the field. Erased by exposure to ultraviolet light c) electrically programmable and erasable EEPROM - can be erased and programmed electronically 164 ELEC4504 Avionics Systems 14.4. Avionics Data Buses Prior to the introduction of digital technology, avionics systems were connected by wires which were dedicated to one task. e.g. the wiring between an ILS receiver and the HSI synchro bearing horizontal deviation horizontal flag ILS Receiver vertical deviation vertical flag HSI course select synchro reference Figure 75: Wiring for analog ILS receiver/HSI Using data bus technology, the above function can be accomplished using two pairs of twisted, shielded wires. 14.4.1 The Twisted Shielded Pair Most data buses use a twisted shielded pair of wires for data transmission. A pair of wires is used so that each signal path has both a signal wire and a ground wire. Normally analog signals are carried on one wire with either a single separate wire serving as ground for several signals or the aircraft ground is used. In either case the ground can be noisy and can cause interference. The wires are twisted to reduce the effects of magnetic interference. Remember that a changing magnetic field passing through a conducting loop will induce a voltage in the loop. An aircraft is full of varying magnetic fields caused mainly by the electrical system which operates at 400 Hz and uses the aircraft frame as a ground conductor. ELEC4504 Avionics Systems 165 Two parallel wires and their terminations form a loop A changing magnetic field into or out of the page will induce a voltage into the loop If the wires are twisted, the loops alternate in orientation and the induced voltage tends to average out Figure 76: Effect of Magnetic Field on Untwisted and Twisted Wire Pairs The twisted wires are then shielded to reduce electrical coupling from outside sources (especially adjacent wires in a cable). 14.4.2 Data Links Data links can be divided into three main categories i) Single source/Single sink - This is a dedicated link between two units. If the data can be transmitted only in one direction it is called a half-duplex link. If data can be transmitted in both directions it is called a full duplex link ii) Single source/Multiple sink - This describes a system in which one transmitting unit can transmit to several other units. The ARINC 429 data bus to be described later is of this type iii) Multiple source/Multiple sink - In this system several transmitting units can transmit to several receivers. MIL-STD 1553 and ARINC 629 (used on the Boeing 777) are examples of this system 14.4.3 Coding Schemes Digital transmissions represent data in binary notation i.e. ones and zeros. However there are many ways to represent these values using an electrical signal and the choice of any given method depends on the requirements of the application. One consideration is the synchronization of the transmitter and receiver clocks. One means of synchronizing the clocks is to transmit the clock on a separate set of wires but this increases the wiring and weight. It is possible, however, to code the signal so that it is self-clocking. Another consideration is the requirement for transformer or magnetic coupling. This can be an advantage in the design of systems with many connections 166 ELEC4504 Avionics Systems 14.4.3.1 Common Coding Schemes 14.4.3.1.1 Bipolar Return to Zero 1 0 1 1 0 +1 0 -1 Figure 77: Bipolar Return to Zero Code This is called bipolar because the ones and zeroes are represented by positive and negative values and is called return to zero because the signal returns to zero value during each bit interval. One advantage of this system is that it is self-clocking. i.e. by rectifying the bitstream, a clock can be generated. Note: this method is complicated by the fact that three levels are required to be decoded. 14.4.3.1.2 Biphase level (also called Manchester code) 1 0 1 1 0 +1 0 -1 Figure 78: Biphase Level (Manchester) Code In this case, a one is indicated by the signal being positive in the first half of the bit period while the zero in indicated by the signal being positive in the second half of the bit period. Self clocking is provided by the fact that a transition occurs in the middle of every bit period ELEC4504 Avionics Systems 167 14.4.4 ARINC 429 ARINC 429 is probably the most prevalent data bus in commercial aviation today. As mentioned above it is a single source/multiple sink system. It operates at two clock speeds: the Low rate which clocks at 12-14.5 kHz and the High rate which clocks at 100 kHz. The data format is shown in Figure 7. Figure 79: ARINC 429 Data Word The label describes the type of data in the word. Each parameter has a unique label e.g. x acceleration, altitude, temperature The source/destination identifies where the information is coming from (e.g. INS) and where is going (e.g. Flight management system) Up to 20 receivers can be accommodated by one transmitter. A modern aircraft such as a Boeing 767 may have as many as 150 data links to interconnect different units. 168 ELEC4504 Avionics Systems A typical topology is shown below 1 2 3 4 5 Figure 80: Typical ARINC 429 Topology One disadvantage of Single Source/Single Sink system is the complexity involved in making changes to the system. This is also shown in Figure 78. The original connections are shown in solid lines while the additional connections required to add a fifth unit are shown in dotted lines. this shows a disadvantage of the single source/ multiple sink topology. The code used in ARINC 429 is bipolar RTZ and the wiring is twisted shielded pair The electrical specification is shown below: TRANSMIT 1 0 10V ± 1V 0V ± 0.5V -10V ± 1V RECEIVE 13V 6.5V 2.5V -2.5V -6.5V -13V As can be seen in the diagram the noise margin is 2.5 V (9 - 6.5 or -6.5 - 9) ELEC4504 Avionics Systems 169 14.4.5 MIL STD 1553 This data bus standard has been in effect since 1973 and has been used on all new military aircraft since then. most notably the F16. The topology of this system is completely different from that of ARINC 429 since it is a multiple source system redundant bus optional Bus Monitor 2 2 only bus Bus controller Controller can initiate a data transfer Remote Terminal Remote Terminal Remote Terminal up to 31 RTs Figure 81: MIL STD 1553 Data Bus Topology Note that adding a unit to the bus is far less complicated than in the case of the ARINC 429 system The bus controller initiates all information transfers The bus monitor provides monitoring of bus activities 14.4.6 Data Transfer In a simple data transfer, the bus controller (BC) sends a transmit command to a particular remote terminal (RT). The RT then transmits a status word followed by up to 32 data words depending on the amount of data required. A more complex transfer is one in which an RT is to transfer data to another RT. In this case the BC issues a transmit command to the first RT and then a receive command to the second RT. The first RT then transmits the status word followed by the data as above. When all the data has been transmitted, the second RT transmits a single status word to indicate that the transfer is complete and whether it was successful or not. 14.4.7 Data Transfer Formats Manchester coding described above is used. This is good for clock recovery and also allows transformer coupling to the data bus (this is an important part of the topology) 170 ELEC4504 Avionics Systems The format of the words is as follows: the time required to transfer one data word from one RT to another is around 120 μseconds which is rather an expensive overhead. If, however a maximum of 32 data words were transferred, the overheads would be the same but representing a much lower percentage of overall message time. The Allowed RT response time is 4 to 12 μsec intermessage gap 4 μsec timing watchdog (response time) 14 μsec 14.4.8 Coupling Into the Bus. Direct or Transformer coupling RT BC up to 20 ft., Transformer up to 1 ft. RT RT Figure 82: Coupling into the MIL STD 1553 Data Bus ELEC4504 Avionics Systems 171 14.4.8.1 Advantages of MIL - STD 1553 data bus a) easy to add units to the data bus b) less wiring than ARINC 429 c) will handle redundant busses and bus controllers 14.4.8.2 Disadvantages a) need to have additional units (bus controller and bus monitor) 14.4.9 ARINC 629 In the civil aviation world the disadvantages of ARINC 429 have been realized. To overcome these, a new standard, ARINC 629 has been developed and was used on the Boeing 777. This system is similar to the MIL STD 1553 system except that the bus controllers are incorporated into each unit on the bus Coupling can be made using current transformers and thus the wire does not have to be cut. 14.5. Other Databusses in use today (primarily for general aviation and business aircraft) 14.5.1 Commercial Standard Data Bus Developed by Collins Division of Rockwell International (a major avionics manufacturer) a) similar to ARIN 429 (unidirectional) b) two data rates - 12.5 kbits/sec or 50 kbits/sec 14.5.2 Avionics Standard Communications Bus Developed by Honeywell a) similar to MIL STD 1553 but uses two isolated busses b) units may listen on both busses but transmit on only one - allows less critical systems to receive data from more critical systems without being able to affect their operation c) data rate is 300 kbits/ second d) bus can support up to 48 users 172 ELEC4504 Avionics Systems 14.6. Fiber optics One of the limitations of twisted shielded pairs is the frequency of data transmission. The upper limit is dependent on the length of cable but is about 2 MHz. An alternative to twisted, shielded wire is fiber optic cable. 14.6.1 Advantages Fiber optic cable provides the possibility of data rates in the neighbourhood of 100 MHz Another advantage is the immunity to electromagnetic interference. 14.6.2 Disadvantages Fiber optic cables connections require very accurate alignment and thus it is difficult to make reliable connectors. The requirement for fiber optic connectors in the vibration and shock environment of an aircraft is especially severe. Also, glass fibers are relatively fragile which makes them susceptible to breakage under aircraft conditions. 14.7. Software Development Obviously without software none of the digital equipment we have been talking about would be of much use. In fact the design, testing and control of software is one of the biggest challenges in avionics today. 14.7.1 Determining the criticality RTCA document DO 178A “Software Considerations in Airborne Systems and Equipment Certification” defines the specific methods and techniques of software design, testing, configuration and documentation. The first step in designing software for an new avionics system is to define the criticality category. This is done by assessing the system’s application and all failures which could result from a system malfunction. The three categories are: i) flight critical ii) flight essential and iii) flight non-essential. A system’s category is defined by its most critical function. The next step is to translate the criticality of the system to the software. The levels adopted by DO 178A are Levels 1, 2 and 3 which correspond to flight critical, flight essential and flight non-essential respectively. ELEC4504 Avionics Systems 173 14.7.2 Development, Verification and Validation First a software development plan is submitted to the regulatory agency (FAA, CAA, Transport Canada). This includes: - software functions - the criticality of each function and software level - hardware and software interfaces - microprocessor characteristics - built in test (BIT) and monitoring requirements - what functional losses would occur as the result of software failure - timing, test and partitioning requirements Once written the software must be tested. This includes, module testing, module integration tests and software/ hardware integration tests As an example, module testing includes logic and computational tests to determine if the module performs its intended function. Logic testing is used to detect illogical sequences and constructs such as halted execution, executions trapped in a loop and missing input data. Computational tests consider an algorithm’s reaction to data within, outside and on the border of a specified range. For flight critical systems, all verification results must be retained and all problems logged For flight essential systems, only a Statement of Compliance is required as a summary of the testing process. System validation is intended to show that the system function properly under adverse and failure conditions 14.7.3 Software Configuration Management (SCM) and Software Quality Assurance (SQA) Any system incorporating software must undergo SCM and SQA. This requires the submission of an SCM plan which may be part of the overall SQA plan. This includes a description of how SCM will be implemented and followed throughout the certification process. It should also include how SCM will be applied during the service life of the equipment. SCM should include documentation, identification and change control and status accounting. This involves any post-certification software change 174 ELEC4504 Avionics Systems If the change does not affect interchangeability or the certification basis, it may be called a software status change. If it does affect interchangeability or certification, the system will require a new part number To comply with current standards it is necessary to use top-down programming techniques. This technique requires that the overall function of the software be broken down into a hierarchy of levels with each level performing more and more specific tasks. Ideally the smallest unit, called the module above, performs only one function and has exactly specified inputs and outputs. This structure allows testing to be done in a logical fashion starting with each module and progressing up the chain Figure 83: Top Down Software Configuration