Analysis of application characteristics of several main data buses in avionics systems

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 the 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, Mirage 2000, Airbus A340, etc., all use bus technology. Data bus technology has been designed and used in the design of avionics systems in my country for more than ten years. This article discusses and explains the commonly used MIL-STD-1553B, ARINC429, CSDB, ARINC 6-way bus (561, 568, 582) and ARINC629 bus from the aspects of composition, characteristics and application.

　　1 Bus composition

　　Once the designer has determined the basic structure of the flying electric system, the most important thing is the bus layout, which has a significant impact on the system performance. The bus can be unidirectional or bidirectional. The most commonly used unidirectional bus design is based on the "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/response multiplexed data bus", the bus must have a central bus controller. The linear bidirectional bus layout design is the most commonly used. When designing, pay attention to taking special precautions, otherwise it is easy to produce single point failure (which can be checked by using fault tree analysis technology); the mesh layout can be used for general advanced fault-tolerant systems. The advantages are: using the node controller to disconnect the failed or damaged network segment can successfully achieve fault tolerance, and the signals are sent according to the specified route on other undamaged network segments, and the full function of the system can be reconstructed; in addition to the above advantages, the star structure layout can also significantly reduce coupling loss, but the flexibility is poor.

　　2 Characteristics Analysis of Several Buses

　　2.1 1553B Bus Characteristics Analysis

　　The 1553B bus provides a single data path between the bus controller and all relevant remote terminals, including all hardware such as twisted-pair shielded cables, isolation resistors, transformers, etc. The remote terminal (RT) is the largest component in the 1553B bus system. In fact, there can be up to 31 remote terminals on a given bus. The remote terminal only responds to those valid instructions or valid broadcast (all RTs are visited at the same time) instructions for their specific addressing inquiries. It can be separated from the subsystem it serves or embedded in the subsystem. The second characteristic of the 1553B bus is bit priority. It first sends the highest bit in the data word, and then sends the lower significant bits in descending order. The third is the transmission method. The signal transmitted by the data bus is in the form of modulation of a serial digital pulse code, and it is stipulated that 10 message formats, namely "information transmission formats", are allowed. The first 6 formats can only be executed under the direct control of the bus controller, and these 6 formats require the remote terminal being visited to make a specific and unique response. The last four are broadcast formats, which 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 working method seems very attractive, the 1553B standard strongly advises people not to use this capability because the terminal cannot detect errors and failures in the messages it receives.

　　2.2 Analysis of ARINC and CSDB data bus characteristics

　　The ARINC429 data bus is a unidirectional transmission bus, but it can have 20 receivers. The three-state multiplexed information stream of its communication uses a 32-bit message word with parity check. The signal waveform is a bidirectional return-to-zero code, and its bit width depends on the operating rate 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 to transmit large amounts of data or critical 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 updates its data 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 Industrial Standard Data Bus (CSDB) is a binary two-state waveform. The bus consists of twisted-pair shielded wires whose impedance meets the RS-422A standard requirements of the American Electronics Industry Association (EIA).

　　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 shielded pairs. The three lines are used for serial data, word synchronization and clock signal transmission respectively. The serial data is encoded 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 an 11±3.5kHz rectangular wave signal with a rise and decay time within the range of 2 to 6μs.

　　Note: ① is the waveform seen by the oscilloscope; ② is the waveform displayed in the order of effective bits;

　　③When bits 31 and 32 are 00, non-test is valid; when bits 10, test is valid; when bits 01, invalid; when bits 11, unspecified.

　　The ARINC629 data bus, like the ARINC429, is a host-less broadcast data bus that operates according to the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) protocol.

　　Although the ARINC629 bus is intended as a successor to ARINC429, it still has several similarities with MIL-STD-1553B. Each word is 20 bits long, with 16 bits of data and one parity bit. The label word has a high-low synchronization waveform of 3 bits, while the data word has a synchronization waveform from low to high, also occupying 3 bits. A message consists of 1 to 16 strings. Each string has a label word followed by up to 256 data words. The ARINC629 bus can operate in any configuration used by 1553B, with a bus rate of 2MB/s. A noteworthy feature is the ease of connecting to the bus using an inductive coupler without having to cut the wire when connecting, which is a fruitful contribution to improving reliability and reducing electromagnetic interference.

　　The ARINC629 data bus is an autonomous terminal access data bus, so each terminal on the bus must have its own control mechanism. This control mechanism is implemented 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 twisted-pair, shielded, and sheathed cable, requiring 4 twists per foot (1ft=0.3048m), and the shielding 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 with a resistance value equal to the characteristic impedance value of the cable ±2%. The capacitance between the lines should be less than or equal to 30pF/ft, and the cable loss should be less than (or equal to) 0.015dB/ft at a frequency of 1MHz. The cable length is not limited.

　　The 1553B standard specifies two coupling methods: the first method uses direct connection between lines, usually called direct coupled stubs. The second coupling method is transformer coupled stubs.

　　They use hard wires to connect the lines and are connected to the coupling transformer through isolation resistors. Although the length of the transformer coupling short stub can be selected arbitrarily, the designer should try to make it no longer than 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:

　　① The peak-to-peak value of the transformer-coupled terminal output voltage (between lines) should be within the range of 18 to 27V, and the effective value of its noise (between lines) should be less than 14mV;

　　② The transformer-coupled terminal should respond to input signals with a peak-to-peak value range of 0.86 to 14.0 V (line to line). In the range of 75 kHz to 1 MHz, the minimum input impedance of the terminal should be 1 kΩ.

　　ARINC429 has relatively low hardware requirements and is easy to implement. The output impedance of the transmitter should be in the range of 75 to 85Ω, evenly divided between the two wires.

　　For the receiver, the input resistance should be greater than 12kΩ, and the differential input capacitance and capacitance to ground should be less than 50pF. Therefore, the minimum input resistance of the receiver is set to 12kΩ to ensure that the bus will not be overloaded when there are up to 20 receivers on the bus, and to reduce mutual interference between receivers in the event of a fault. In order to ensure that the receiver can continue to work in the event of a fault such as a short circuit between one line and the 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 capacitor should not exceed 30000pF.

　　3 Conclusion

　　Data bus technology has greatly improved the performance of the aircraft itself, and has 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, and reduce life cycle costs and ownership costs, some additional requirements are usually put forward within the scope of utilization and maintenance, such as redundancy, mission completion rate, ratio of maintenance hours to flight hours, MTBF and ground maintenance time. The selection of the bus should be based on mission and performance requirements, and the determination of the bus design data should be based on domestic and foreign data, and the results of partial and system joint tests to avoid the need to revise the design at a high cost in the future.