Abstract: The application status of CAN bus in the aerospace field is introduced. Based on the research on CAN bus technology and DSP chip function, a two-layer data acquisition system based on CAN bus and DSP is designed. This data acquisition system provides a reference for the widespread application of CAN bus technology in the aerospace field. Keywords: CAN bus DSP data acquisition 1 Introduction CAN (Controller Area Network) is a serial communication network designed by BOSCH in Germany to realize data communication between automotive measurement and execution components, supporting distributed control and real-time control. CAN BUS fieldbus has been approved by ISO/TC22 technical committee as international standard ISO11898 (communication rate less than 1Mbps) and ISO11519 (communication rate less than 125kbps). CAN bus was initially mainly used in automotive engine components, sensors, anti-skid systems and other applications in the field of automation electronics. However, with the popularization of CAN application, the advantages of CAN bus such as real-time performance and strong anti-interference ability have also been gradually recognized by the aerospace field [1]. This paper introduces the application of CAN bus in the aerospace field and designs a two-layer data acquisition system based on CAN bus and DSP technology research. The paper focuses on the structural composition of the two-layer data acquisition system and the design of the CAN bus interface. 2. Application Status of CAN Bus in the Aerospace Field Initially, CAN bus was mainly used in the field of automation electronics, such as automotive engine components, sensors, and anti-skid systems. However, with the popularization of CAN applications, its scope is no longer limited to the automotive industry and is gradually being recognized by the aerospace field. In 1995, SSTL (Surrey University Satellite Technology) used CAN as an onboard telemetry/remote control channel, and subsequently developed a CAN-based distributed solution. To date, SSTL has applied CAN bus networks in six LEO satellites, including UoSAT-12, SNAP-1, AISAT-1, UKDMC, NigeriaSAT-1, and BilSAT-1, to realize communication between onboard computers and other mission nodes; ESA also uses CAN as the system bus and payload bus on SMART-1 to realize data exchange and control command transmission. ESA’s research on CAN technology shows that using a high-speed serial bus with differential signal transmission for data transmission between spaceborne devices can ensure timely communication, reduce the power consumption of spaceborne devices, and help obtain advantages such as low noise, strong anti-electromagnetic interference, low EMI, and signal unaffected by power switch state changes, and has good prospects for aerospace applications. With the development of aerospace electronic technology, the integration level of aerospace electronic equipment is getting higher and higher, and the amount of information that needs to be exchanged between devices is getting larger and larger. CAN bus technology has begun to be used more and more widely in the field of aerospace electronics [3]. In China, CAN bus technology has been practically applied in small satellites. With the development of aerospace information integration technology, CAN bus will be widely used in spacecraft measurement, control and other systems. The adoption of CAN interface will greatly simplify the cable network of the measurement system and improve the flight reliability of spacecraft. 3 Structure and function of dual-layer data acquisition system The overall structure of dual-layer data acquisition system is shown in Figure 1. Dual-layer data acquisition system consists of acquisition unit, DSP intermediate controller, top-level control center, etc., and its composition structure is shown in Figure 1. This data acquisition system can simultaneously acquire and manage information from multiple areas and units. It employs a hierarchical, regional control optimization approach, using a DSP intermediate controller as the core controller for each acquisition area to facilitate data exchange between upper and lower layers. The acquisition unit is the foundation and key component of the data acquisition system. It connects directly to the parameter acquisition actuator to acquire field parameters, including voltage, current, temperature, and speed. Each acquisition unit has its own microcontroller and memory. It serves both as an important part of the system, participating in the implementation of system functions, and as an independent unit to perform data acquisition. Even in the event of communication failures, the acquisition unit can still independently perform data acquisition and data storage, improving system reliability. The acquisition unit connects to the Bot-CAN bus via a standard CAN bus interface to acquire and transmit field data. The DSP intermediate controller is the communication hub of the entire acquisition system, providing dual interfaces for communication with both upper and lower network layers. It communicates with the top-level control center via the upper network (Top-CAN BUS) and with each acquisition unit via the lower network (Bot-CAN BUS). The DSP intermediate controller is the area controller for data acquisition, realizing the collection and processing of data in its local area, and communicating with the top-level control center via the CAN bus, enabling the top-level control center to collect and control information from each data acquisition area and acquisition unit. 4. Design of CAN Bus Network Interface The main interfaces of the dual-layer CAN bus network include the CAN bus interface of the acquisition unit, the dual CAN bus interface of the DSP intermediate controller, and the CAN bus interface of the top-level control center. The CAN bus interface of the acquisition unit uses a standard CAN bus interface, which will not be elaborated here. The top-level control center is generally an industrial control computer, which can be directly connected to the CAN bus network via a CAN communication card, so it also requires no further explanation. The following focuses on the design of the dual CAN bus interface of the DSP intermediate controller with dual CAN bus interfaces. 4.1 Introduction to the DSP Chip The design of the data acquisition system must consider both speed characteristics and stability. The TMS320LF2407A is a DSP chip from TI with a built-in CAN module, operating at 3.3V. It features inherent operational flexibility and high-speed computing capabilities. The DSP intermediate controller uses this chip as its main control chip. The TMS320LF2407A's CAN module fully supports the CAN 2.0A/B protocol. The CAN controller module is a complete CAN controller with programmable bit timers, programmable interrupt configuration, programmable CAN bus wake-up function, automatic response to remote requests, bus error diagnosis, and other functions. It can operate in standard and extended modes, and has six built-in mailboxes for data transmission and reception. It can perform self-tests. The structure and functions of each part of the CAN module are basically the same as the popular PHILIPS enhanced CAN controller SJAl000. 4.2 DSP Intermediate Controller Upper-Layer CAN Bus Network Interface Design In the upper-layer CAN bus network interface design, the core chip TMS320LF2407A's CAN module fully supports the CAN 2.0A/B protocol. Only one CAN transceiver is needed to easily implement the CAN bus interface. The design uses the TI SN65HVD230D 3.3V series CAN transceiver. The SN65HVD230D is a TI product specifically designed for the interface between the CAN controller and the physical bus in the 240X series DSPs. Its power supply voltage is the same as the TMS320LF2407A, only 3.3V. The SN65HVD230D CAN data line transceiver is designed for reliable and efficient bidirectional data transmission between controllers, conforming to the CAN bus architecture standard ISO11898. This series of devices supports differential signaling with transmission rates up to 1Mbps, while also being compatible with existing signaling systems. The device uses the industry-standard PCA82C250 package, suitable for dual-terminal transmission lines and half-duplex operation. The device's output conversion time, or conversion rate control, is programmable, which helps designers reduce electromagnetic interference and thus improve system reliability. Its interface design is shown in Figure 2. 4.3 DSP Intermediate Controller Lower-Layer CAN Bus Network Interface Design The lower-layer CAN bus hardware interface circuit consists of a main control chip, a CAN controller, and a CAN transceiver. Since the main control chip has been selected as TMS320LF2407A, only a suitable CAN controller and transceiver need to be selected to implement this interface design. The common design approach is to use the SJA1000 produced by PHILIPS as the CAN controller and the PCA82C250 chip of PHILIPS as the CAN transceiver. Therefore, the focus of this interface design is to realize the interface design between the DSP and the SJA1000. The external pins of the TMS320LF2407A chip generally adopt the design method of separating the address line and the data line, no longer using the address and data time-division multiplexing line, and there is no ALE address valid signal, which brings certain difficulties to the interface between the CAN controller and it. The TMS320LF2407A does not provide a direct interface signal with the SJA1000 CAN controller. Using the INTEL method of SJA1000, the following design points are adopted to meet the interface requirements between the TMS320LF2407A and the CAN controller and the SJA1000 [2]. a. Address/Data Multiplexing Line Design: The TMS320LF2407A's data lines D0-D7 are used as the CAN's address/data multiplexing lines. The TMS320LF2407A's data lines are used to select the CAN's internal ports and transmit data. b. Generation of Address Valid Signal (ALE): The TMS320LF2407A's ALE signal is generated using a logical combination of address lines AO, write strobe signal, and port strobe signal. c. Generation of Read/Write Signals: The SJA1000's read strobe signal is generated using a logical combination of read/write signals and A0, and the SJA1000's write strobe signal is generated using a logical combination of write signals and A0. The logical relationships are shown in Table 2. d. Generation of Chip Select Signal: The CAN chip select is generated using a logical combination of the TMS320LF2407A's I/O space strobe signal and the high-order address decoding signal. This method modifies the DSP's data lines to suit the CAN controller's data and address lines; therefore, the DSP's A0 is used as the address/data selection line. When AO=1, the address is valid; when A0=0, the data is valid. That is, odd-numbered addresses are used to select ports, and even-numbered addresses are used to transmit data. Simultaneously, through logical signal combinations, no read/write signals are generated during the address validity period, but a CAN address valid signal (ALE) is generated; during the data validity period, read and write logic signals satisfying CAN are generated. Interface logic conversion is implemented using a single GAL chip in the SJA1000 and TMS320LF2407A, as shown in Figure 3. 5. Conclusion This paper, based on the application of CAN bus in the aerospace field and the research on CAN bus technology and DSP chip functions, designs a dual-layer data acquisition system based on CAN bus and DSP. This system fully utilizes the advantage of the DSP's built-in CAN controller, designing a DSP intermediate controller with dual CAN bus interfaces. It adopts a hierarchical, layered, and region-combined approach to achieve dual-layer, multi-region data acquisition. The paper presents the structure of this data acquisition system and designs the hardware circuit of the dual CAN bus network interface of the DSP intermediate controller, which will provide a positive reference for the widespread application of CAN bus in the aerospace field. References: 1. CAN Fieldbus Principles and Applications, Rao Yuntao, Zou Jijun, Zheng Yongyun, Beijing University of Aeronautics and Astronautics Press, June 2003. 2. Interface between CAN Bus Controller and DSP, Liao Chuanshu, Li Chong, Wuhan University of Technology, November 2002. 3. Application of Industrial Field CAN Bus Technology in Aerospace, Zhou Xinfa, Shang Zhi, Liu Qun, Fieldbus and Network Technology, 2006, No. 1.