Since the 1970s, with the development of microelectronics, computers, and cybernetics, the development of avionics systems has become more rapid. In 1980, the United States specially formulated military 1553 series standards and ARINC series standards to make the data bus more standardized. At present, military and civilian aircraft with a high degree of automation, such as F-16, F-117, Phantom 2000, and Airbus A340, all use bus technology. The data bus technology has been designed and used for more than ten years in the design of China's avionics system. , Characteristics and applications are discussed and elaborated.
1 Composition of the bus
Once the designer has determined the basic flying power system structure, the most important thing is the bus layout, which has an important impact on the system performance. The bus can be unidirectional or bidirectional. The basis of the most commonly used unidirectional bus design is "ARINC429 specification MARK33 digital information transmission system". There are basically three forms of bidirectional bus layout: linear, mesh, and star. Usually according to the "MIL-STD-1553B aircraft internal time division command / responsive multiplexing data bus" regulations: the bus must have a central bus controller. Linear bidirectional bus layout design is most commonly used. When designing, special precautions should be taken, otherwise single points of failure are likely to occur (fault tree analysis technology can be used to check); the mesh layout can be used for general advanced fault-tolerant systems. The advantage is: use node controller to disconnect failure or The damaged network segment can successfully achieve fault tolerance. On other non-damaged network segments, the signal is sent according to the prescribed route, and all the functions of the system can be reconstructed; in addition to the above advantages, the layout of the star structure can also significantly reduce coupling Loss, but less flexibility.
2 Characteristics analysis of several buses
2.1 Analysis of 1553B bus characteristics
The 1553B bus provides a single data path between the bus controller and all related remote terminals, including all hardware such as twisted shielded cables, isolation resistors, and transformers. The remote terminal (RT) is the largest number of components in the 1553B bus system. In fact, there can be up to 31 remote terminals on a given bus. The remote terminals only respond to those valid commands or valid broadcast (all RTs are searched at the same time) commands that are specifically addressed by them. It can be separated from the subsystem it serves, or it can be embedded in the subsystem. The second feature of the 1553B bus is bit priority. It sends the most significant bit in the data word first, and then sends the less significant bits in order of decreasing value. The third is the transmission method. The signal transmitted by the data bus is in the form of a serial digital pulse code modulation, and it is stipulated that 10 message formats are allowed, that is, "information transmission format". The first 6 formats can only be executed under the direct control of the bus controller, and these 6 formats require a specific and unique response from the remote terminal being accessed. The last four are broadcast formats. These formats allow a terminal to send a message to all addressed terminals on the bus without the terminal receiving the message confirming its reception. Although this way of working seems extremely attractive, the 1553B standard strongly urges people not to use its capabilities because the terminal cannot detect errors and failures in the messages it receives.
2.2 ARINC and CSDB data bus characteristics analysis
ARINC429 data bus is a unidirectional transmission bus, but there can be 20 receivers. The three-state multi-channel information stream of its communication uses a 32-bit message word with parity. The signal waveform is a two-way return-to-zero code, and its bit width depends on the operating speed of the bus. The bit width is (70 ～ 80) ± 2.5% μs at low speed, and 10 ± 2.5% μs at high speed. The low-speed bus is used for general-purpose, non-critical applications; the high-speed bus is used for the transmission of large amounts of data or those vital flight information. The first 8 bits of data are used for addresses, and the last 24 bits are used for data. For example, an electronic flight instrument system in the United States has data updated at approximately 19, 9.5, and 2.4 times per second. For the synchronization of each word, it can be achieved by detecting the transition of the first bit of each word. There is a time interval of at least 4 bits between consecutively transmitted words.
The industry standard data bus (CSDB) is a binary two-state waveform. The bus is composed of twisted-pair shielded wire, and the impedance of this wire meets the requirements of the American Electronics Industry Association (EIA) RS-422A standard.
The ARINC 6-way bus (561, 568, 582) is a binary, 32-bit bus with two waveforms. Its waveform format is shown in Figure 3. The bus consists of three twisted pair shielded wires. The three lines are used for serial data, word synchronization and clock signal transmission. The serial data is compiled into binary coded data (BCD) and binary data. The first 8 bits are used as addresses and the last 24 bits are used as data. The clock signal is a 11 ± 3.5kHz rectangular wave signal, and the rise and decay time is within the range of 2 ~ 6μs.
Note: â‘ is the waveform seen by the oscilloscope; â‘¡ is the waveform displayed in the order of effective digits;
â‘¢ When 31 and 32 bits are 00, the non-test is valid; at 10, the test is valid; at 01, it is invalid; at 11, it is not specified.
The ARINC629 data bus, like ARINC429, is a hostless broadcast data bus that operates in accordance with the carrier sensing multiple access / collision detection (CSMA / CD) protocol.
Although the ARINC629 bus is intended to be the successor of ARINC429, it still has several similarities with MIL-STD-1553B. When the word length of each word is 20 bits, the data occupies 16 bits and has a parity bit. The label word has a high-low synchronization waveform when there are 3 bits, and the synchronization waveform of the data word changes from low to high, which also occupies 3 bits. A message consists of 1 to 16 words. Each string has a label word, followed by up to 256 data words. The ARINC629 bus can work in any configuration mode used by the 1553B, and its bus rate is 2MB / s. It is worth mentioning that it is easy to use an inductive coupler to connect to the bus, and it is not necessary to cut the wires when connecting. This is a very effective contribution to improving reliability and reducing electromagnetic interference.
The ARINC629 data bus is a data bus for autonomous terminal access to work, so each terminal on the bus must have its own control mechanism. This kind of control mechanism is realized by two erasable EPROMs as sending and receiving "personalized plug-ins".
2.3 Comparison of bus hardware characteristics
The transmission line of the 1553B bus is a double-stranded, shielded, and sheathed cable that requires 4 twists per foot (1ft = 0.3048m), and the shield should cover at least 75% of the cable surface. When the frequency is 1MHz, the characteristic impedance of the cable should be within 70 ～ 85Ω. Each end of the cable must be connected to a resistor equal to the cable's characteristic impedance value ± 2% resistance. The capacitance between the line and the line should be less than or equal to 30pF / ft, and the loss of the cable should be less than (or equal to) 0.015dB / ft at a frequency of 1MHz, and the cable length is not limited.
The 1553B standard specifies two coupling methods: the first uses line-to-line direct connection, commonly referred to as direct-coupled stubs. The second coupling method is the transformer coupling stub.
They use hard wires for line-to-line connections and are connected to the coupling transformer via isolation resistors. Although the length of the transformer coupling stub can be arbitrarily selected, the designer should try to make it as long as possible not to exceed 6.1m. The common mode rejection ratio should be greater than (or equal to) 45dB.
The detailed performance points of the 1553B bus regarding the terminal are as follows:
â‘ The peak-to-peak value of the transformer-coupled terminal output voltage (between lines and lines) should be in the range of 18 to 27V, and the effective value of the noise (between lines and lines) is less than 14mV;
②The transformer-coupled terminal should respond to the input signal with a peak-to-peak value in the range of 0.86 ～ 14.0V (between lines). Within the range of 75kHz ～ 1MHz, the minimum input impedance of the terminal should be 1kΩ.
ARINC429's hardware requirements are relatively harsh and easy to implement. The output impedance of the transmitter should be in the range of 75 to 85 Ω, evenly divided between the two conductors.
For the receiver, the input resistance should be greater than 12kΩ, and the differential input capacitance and the capacitance to ground should be <50pF. Therefore, the minimum input resistance of the receiver is set to 12kΩ, in order to ensure that the bus will not be overloaded when there are up to 20 receivers on the bus, and can reduce the mutual interference between the receivers in the event of a fault. In order to allow the receiver to continue to work in the event of a short circuit to ground, ARINC429 has specified that the voltage range that the receiver can receive is:
HI (high): + 6.5 ～ + 13VDC;
LO (low): -6.5 ～ -13VDC;
NULL (zero): + 2.5 ～ -2.5VDC.
Any signal outside these levels is considered invalid. In addition, when a line is shorted to ground, a differential voltage of up to + 5.5V or -5.5V will be generated. In practical applications, the maximum bypass capacitance should not exceed 30000pF.
3 Conclusion
Data bus technology has greatly improved the performance of the aircraft itself, but also expanded and improved the aircraft's ability to complete missions. Many factors that affect data bus design are not necessarily directly related to aircraft missions. In order to achieve maximum production efficiency, effectiveness, reduce life-cycle costs and ownership costs, usually additional requirements are put forward within the scope of utilization and maintenance, such as redundancy, task completion rate, ratio of maintenance hours to flight hours , MTBF and ground maintenance time, etc. The selection of the bus should be based on the task and performance requirements, and the determination of the bus design data should be based on the results of domestic and foreign data, parts, and system joint test experiments to avoid the need to re-correct the design at a high cost in the future.
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