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              Design and Application of Multi-computer Communication Based on CAN Bus

              With the development of microprocessors, it has become a trend to use microprocessors to control industrial production processes. In the process of industrial control, due to the sharing and transmission of a large amount of data information, the traditional serial communication mode has been unable to meet the requirements. In the field of industrial control, a bus structure with strong anti-interference, high reliability, fast transmission speed and long transmission distance is needed. The CAN bus technology not only satisfies the above requirements, but also realizes information transmission between multiple points. This article uses the CAN bus on the PCI9810-cAN adapter card to form a local area network to achieve information transfer between multiple microprocessors and PC control and communication of multiple processors.

              1 Introduction to CAN bus

              CAN (Controller Area Networks) bus is the bus technology developed by German Bosch company for LAN control. The CAN bus adopts the traditional two-wire serial communication method, which has diagnostic capabilities and is resistant to electromagnetic interference. Its maximum transmission rate can reach 1 Mb ?· s-1, and the maximum communication distance can reach 10 km (the transmission rate at this time is about 40 kb ?· s-1). In the local area network composed of CAN buses, the master-slave mode is not adopted between the communication nodes, but the bus access priority is provided. The communication mode is flexible, and data can be transmitted in point-to-point, point-to-multipoint and broadcast modes.

              2 Hardware design of system communication module

              The CAN bus is provided by the PCI9810-CAN adapter card. This article mainly completes the design of the communication node. The communication node can not only exchange information with the PC, but also communicate with other nodes independently. When the microprocessor needs to communicate with the host or other nodes, it sends information to the SJA1000T register through the P0 port, and then the PCA82C250 transmits the information to the CAN bus. The host and other communication nodes judge the identifier of the received message, and will process the received information accordingly to realize the communication function, as shown in Figure 1.


              Figure 1 The principle of system communication contacts

              In the design process, in order to meet the real-time and reliability requirements of communication between multiple microprocessors, combined with the characteristics of the CAN controller, a brief introduction to Figure 1:

              (1) Pin 8 (Rs) of the PCA82C250 transceiver has three working modes: high speed, slope control and standby. The slope control method has the function of resisting radio frequency interference, so a 47 k?? resistor is used to connect to pin 8 to implement the slope control method.

              (2) Two high-speed optocoupler isolation devices 6N137 should be shown in Figure 1. Since the drive capability of the 6N137 output pin is not enough, a pull-up resistor of approximately 390 ?? needs to be connected to increase the drive capability of the output pin. The power signals of the two photocoupler isolation devices 6N137 adopt the 5 V DC-DC isolation module WRA0505P to enhance the anti-interference ability of the system.

              (3) The CANH and CANL pins of the transceiver PCA82C250 are each connected to the CAN bus through a 5 ?? resistor. The resistor plays a certain current limiting role to protect the PCA82C250 from overcurrent impact on the CAN bus.

              (4) A protection diode and a 30 pF capacitor are reversely connected between the CANH and CANL pins of the transceiver PCA82C250 and the power ground, respectively, which can play the role of overvoltage protection and overcurrent shock of the CAN bus.

              (5) There are 2 kinds of input methods of CAN controller SJA1000T: Intel input method and Motorola input method. Intel input mode is adopted here, so the MODE pin of SJA1000T is connected to high level.

              (6) The design only uses the TX0 and RX0 pins. According to the input / output logic level relationship required by the SJA1000T communication protocol, the TX1 pin of the SJA1000T is floating, and the potential of the RX1 pin must be maintained above 0.5 Vcc, so lead in TX1 The foot is connected with about 6.8 k?? and 3.6 k?? voltage divider resistors.

              (7) The pin P2.7 of the microprocessor C51 is connected to the chip select signal / CS of the CAN controller SJA1000T. It can be seen that the register first address of the CAN controller SJA1000T is 8000H. The processor C41 and the CAN controller SJA1000T share a 12 MHz crystal oscillator to increase the communication rate.

              Through the above analysis, the designed circuit schematic is shown in Figure 2.


              Figure 2 The circuit schematic of the communication node

              3 System communication module software design

              The software of the communication module is composed of 3 parts: initialization program, sending program and receiving program. Only these three parts of the program can complete the transfer of information between the communication nodes. To apply the CAN bus to more complex communication systems, we must also consider the CAN bus error handling, overload handling and other functions and the calculation method between nodes. Since each communication node has its own MCU, they can communicate freely. The CAN transceiver PCA82C250 pin CANH and CANL output to the bus, so that the bus performance "dominant", you can send information at this time. When judging that the bus is "dominant", it is necessary to prepare for receiving information.

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              女高中生上课自慰流白浆
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