Key word: CAN main line ECN J1939 agreement communications platform
Introduction
Along with the integrated circuit and monolithic integrated circuit’s on automobile widespread application, on modern automobile’s electronic controller’s quantity are getting more and more, common has engine’s electron fuel injection installment, against to hold the dead arresting gear (ABS), the security aerocyst installment, the electrically operated windows and doors installment, the driving suspension fork and so on. The electrically controlled system increases, although enhanced passenger vehicle’s power, the efficiency and comfortableness, but increases along with it the compound circuit also reduced automobile’s reliability, increased the service difficulty. From the wiring angle analysis, the traditional electron gas system mostly uses the point-to-point sole mailing address, unusually relates, creates like this inevitably favors the big wiring system. Therefore, on one new concept - - automobile electronic controller confined network CAN, also arises at the historic moment. In order to enable the different factory production the spare part to be coordinated the work on the identical automobile, must formulate the standard. According to the ISO related standard, the CAN topology is the main line type, therefore is called the CAN main line. The CAN main line is designed takes in the automobile environment the micro controller correspondence, exchanges the information in Che Zaige between electronic control ECN, exchanges the information in Che Zaige between electronic control ECN, forms the automobile electronic control network.
Controller local area network CAN (Controller Area Network) is more than one kind of host’s side-like serial telecommunications bus, the basic design code requirement has the high bit rate, high anti-electromagnetic interference, moreover can examine any mistake which produces. CAN on automobile’s application, has many profession standards or is the international standard, for instance International Standardization Organization (ISO) ISO11992, ISO11783 as well as automotive engineering association (Society of Automotive Engigeers) SAE J1939. The CAN main line already included in automobile’s complete bikes design as automobile’s one kind of standard equipment.
Figure 1
1 CAN main line characteristic and communication protocol
1.1 CAN main line synopsis
The CAN communication protocol has stipulated 4 kind of different frame forms, namely data frame, long-distance frame, wrong frame and overload frame. Carries on the correspondence based on the following several fundamental rules to be coordinated: The main line visit, the arbitration, the code/decoding, make a mistake labelling and ultra cut labelling. CAN complies with the OSI model. Has three according to the OSI datum model: The physical level, the data link layer and implores the level, but the application layer must user define. The CAN main line takes one kind to support the distributional control or the real-time control serial communication network effectively, the application scope spreads from the high speed network to the low cost multi-line network. For example: CAN in automobile’s engine control part, ABS, application and so on anti-slippery in system bit rates may reach as high as 1Mbps. At the same time, it may use in inexpensive the transportation carrier electric system, for example the electrical window, the light bunching, the chair adjustment and so on, substitute the hardware connection which needs. Its transmission lies between the sanction is the twisted pair line, the correspondence speed is highest may reach 1Mbps/40m, the direct transmission is away from far may reach 10km/5kbps, hangs meets the equipment number to be most may reach 110. CAN is the multi-host working, the mailing address is nimble, does not need node information and so on station address, uses the non-destructive bus arbitration technology, satisfies the real-time request. Moreover, CAN uses the short frame structure transmission signal, the transmission time is short, has the strong antijamming ability.
The CAN main line mainly has two aspects with other communication protocol’s difference: First, the text transmission does not contain the goal address, it is take the entire net broadcast as the foundation, in various receiving stations basis text reflected that the data nature the identifier filtration text, its characteristic is may the online surfer bottom wire, namely insert namely uses with the multi-station receive; Other aspect strengthened the data security specially, satisfied the control system and other high data demand system’s demand.
1.2 J1939 communication protocol
The J1939 agreement was (29 identifiers) above has realized the application layer specifically in CAN main line communication protocol 2.0B, was SAE is the heavy load truck and the passenger train formulation communication protocol; Take CAN 2.0B as the foundation, the physical level standard and the ISO11898 standard compatible and uses conforms to this standard CAN controller and the transceiver. The J1939 agreement divides the CAN identifier into the following several parts: Priority (P), data page (PGN), protocol data unit (PDU) form, PDU special localization (PS) and source address (SA). The J1939/71 application layer documents have defined vehicle control each kind of parameter and order PGN.
Thus it can be seen, J1939 and the CAN communication protocol’s difference lies in 29 identifiers (ID), the data field is the same. J1939 (or name distinguished the CAN 29 identifiers recognition) to carry on the detailed physical definition. Through PDU the CAN standard format seal is the J1939 agreement form. The PDU information frame by priority P, guarantees retains R, data page DP, protocol data unit PF, expansion unit PS, source address SA and the data field DATA seven parts is composed, namely the CAN 29 identifiers add on the data.
2 system designs
2.1 automobile electrically controlled network architecture
In the automobile between the ECU data transmission characteristic main difference lies in the data transmission frequency, when for example engine high speed movement, what carries on is the high frequency data transmission, every several other ms transmits 1 time; But when low speed movement, what carries on is the low frequency data transmission, every several other dozens ms and even several hundred ms only then transmit 1 time. However to satisfy the timely request, requests in the automobile each control unit to realize automobile public data sharing as far as possible, but because also each control unit to the timely request is because of data update rate and control period different and different. This requests its data exchange network is based on the priority competition pattern, 
and itself has the high correspondence speed. The CAN main line is precisely to satisfy these requests to design. CAN had the international standard, namely high speed situation’s ISO11898 and uses in the low speed situation ISO11519-3.
The automobile network architecture usually uses many different speeds the main lines to connect the different type separately the node, and uses the gateway server to realize complete bikes’s information sharing and the network management. If according to American automobile Engineer the Association (SAE) vehicles network committee standard SAE J2057, the automobile data transmission network division is three kinds. Here available chart 1 simple declaration. And the gateway is the automobile internal communication core, may realize through it in CAN on the main line information sharing as well as realizes the automobile internal network management and the failure diagnosis function; Each data bus’s on information feedback to the display board unit’s in display monitor, driver’s through display board on information may know that each electric control installs whether normal work.
2.2 communications platform hardware design
In the design, master control chip CPU selects 51 series monolithic integrated circuits. CAN the communication controller execution complete CAN agreement, completes the correspondence function, including message buffer and receive filter, therefore the CAN controller selects Philips SJA1000. Selects PCA82C250 to take the CAN main line’s transceiver, PCA82C250 is between the CAN agreement controller and the physical main line’s connection, has anti-transient, the anti-radio frequency and the anti-electromagnetic interference performance in the movement environment, the internal current limiting electric circuit has the electric circuit short-circuits when to the transmission output stage carries on the protection the function. The transmission medium uses the shield electrical bill, joins the light pair electric circuit between the observation node and the medium, enhances the bus interface antijamming ability. Figure 2 is the communications platform hardware diagram, Figure 3 is the controller and the transceiver connection connection.
To further enhance system’s reliability, needs to consider system’s redundancy design. Because the automobile environment is bad, and the disturbance factor are many, distinguished the parallel two electric capacities in CAN_H and between the CAN_L holding wire and the grounding, filtered out the noise, caused the signaling to be steady. The double shield electrical bill may suppose two set, simultaneously carries on the intelligence transmission on two sets of medium. The receiving end only uses a medium, and the non-redundant section’s connection critical point carries on the main line cut in the redundancy.
2.3 software designs
Puts briefly, this communications platform must realize the function is, causes each node (ECU) to realize through the CAN main line corresponds mutually, the transmission receive order, the information and so on, and realize data sharing, thus enhances the respective control performance and the operating efficiency. On the automobile CAN data bus’s each node (ECU) has own address and the name corresponds. The ECU address has pointed out the data transfer destination, but its name has marked the ECN basic function. On the node continuous monitoring main line is sending out each kind of data. When receives the data address value when tallies with own address, this node obtains the token. In the communication protocol, obtains the token this node to be authorized only to transmit the data, prevents two or two above nodes simultaneously transmits the data to cause the confusion. Simultaneously each node has the opportunity to obtain the token, completes the data transmission.
This software design is based on Keil the C language compilation. The procedure mainly by the main module, the interrupt processing module and the data communication module and so on is composed, as shown in Figure 4.
Figure 4
The main module completes to the hardware initialization, register’s disposition, the SJA1000 initialization and so on; Interrupt processing module including data interrupt transmission, receive, error processing and warning processing and so on; The data communication module completes the data the request, the transmission, the receive and so on.
When a node A transmission data request text (long-distance frame), to other node B request message (reply frame), after node B receives the request, the process distinguished that then transmits the data (reply frame). Because the data request does not have the data field, therefore relative data frame length small many. After analysis confirmation receives the data with node B to be the same, requested that the data procedure obtains the confirmation.
Conclusion
In the modern automobile’s design, the CAN main line already became the construction automobile network one tendency; But the automobile network takes directly the transmission which, the data transmission and sharing connects with automobile interior each ECU and is responsible to order, its reliable and stable and complete bikes’s performance is closely related. This article design development is in the simulation platform which builds based on the test condition, between the node correspondence is through the coordinated CAN communication node carries on. The experiment indicates its performance characteristic stably reliable, but the practical application still needed the further research and the improvement, and procedure correspondence handling ability, the error correction and the fault-tolerant ability wait for the further enhancement.
