Application of CAN bus in locomotive monitoring system

1 Introduction
For the locomotive monitoring system, the speed, accuracy, reliability and flexibility of data measurement/transmission are all crucial. In the past, locomotive data monitoring only provided on-site data display and alarm. The intelligent instruments added to various equipment were relatively scattered, which made it inconvenient to observe the locomotive operation and conduct timely maintenance. The locomotive operation data could not be saved, and these data were important bases for finding out the cause of locomotive failure and maintenance. Various operations of intelligent instruments, such as zeroing, limit value and accuracy, were affected by environmental and human factors, which affected the display and alarm functions of simple instruments. In addition, the harsh working environment (high temperature, vibration, electromagnetic wave radiation, etc.) led to poor real-time performance and accuracy. With the development of domestic fieldbus technology, it is necessary to update the original detection system in a timely manner.
The locomotive monitoring system itself has dozens of data acquisition nodes (using single-chip microcomputer AT89C51), and a host industrial computer is responsible for data storage and recording. Due to the large number of acquisition points and high acquisition frequency (20Hz), the data on the bus is usually crowded, and the system has particularly high requirements for the accuracy of data transmission and the real-time nature of alarm data. In view of this, we chose the CAN field bus to build a data platform, which has the advantages of flexibility, real-time, accuracy, and reliability.
CAN is the abbreviation of Controller Area Net. It is a serial communication protocol developed by the German Mercedes-Benz Automobile Company in the 1980s. It is mainly used for data communication between multiple control devices and multiple instruments in automobiles. The link layer uses the CAN2.0B protocol. Now, it has been widely used in various industrial sites, especially for systems for optimization, analysis and maintenance. In the 1990s, China began to study the application of CAN bus, and CAN bus technology has been applied in many fields.
This system is a local area network built according to CAN2.0B. The bus controller uses SJA1000 from Philips of Germany, and the driver is PCA82C250 that matches it. The lower computer and the upper computer communicate bidirectionally through CANH and CANL twisted pair cables. The entire hardware structure of this system can also be used as a standard node of the CAN network to form a tree network, which is the minimum structure and typical application of the CAN bus system.
2 Introduction to the characteristics
of the CAN bus The biggest feature of the CAN bus is that each node in the network works in a multi-master mode. Each node can send information to other nodes point-to-point or point-to-multipoint at any time, regardless of master and slave, and has good flexibility.
The data transmitted simultaneously on the CAN bus uses non-destructive arbitration, with priority transmission for small IDs and delayed transmission of low-priority data. This is very effective in solving network paralysis, network congestion and improving efficiency for networks with heavy loads.
CAN also has a strong verification function, and the erroneous data is automatically resent, with high reliability.
In addition, the CAN communication medium uses twisted pair cables, and the space on the locomotive site is narrow, so the on-site wiring, installation and disassembly of twisted pair cables are relatively simple. The maximum communication distance is up to 10km, the bit rate can reach 1Mbps (when the communication distance is larger, the bit rate will be reduced), and 16 messages can be transmitted simultaneously. The operating temperature range of Philips' SJA1000 is: -40℃～+125℃, and the storage temperature range is: -65℃～+150℃.
For the SJA1000CAN controller selected by the system, its functional modules mainly include: Interface Management Logic, Receive FIFO Stack, Acceptance Filter, Transmit Buffer and CAN Core Block, and the structure is shown in Figure 1 (see Philips Semiconductors' information: Application Note - SJA1000 Stand-alone CAN Controller AN97076, 1997).