CANopen protocol of CAN bus

Abstract: This paper analyzes some characteristics of CAN bus and the application of state in the country, proposed the introduction of internationally accepted high-level application protocol CAN Bus, in order to enhance the level of the domestic application of the recommendations of CAN system, and a brief description of a CAN high-level protocol-CANopen protocol. The last part of the article describes some of the more advanced international CAN products and development methods.

In the design of embedded systems, especially distributed embedded systems, solve the system unit between the reliable, effective communication is the key to success or failure of system design, real-time reliability and safety even more so in terms of high network . There are various options to solve this problem, such as RS232/485 serial bus, CAN, ProfitBus, FF, WorldFIP, LonWorks, and other types of field bus, there are embedded Ethernet. Which, despite the RS485 serial bus protocol's performance is not high, but because of its simple hardware costs and the development of the great advantages of upper, is still the wide of the bus Ying Yong.

With the development of embedded system applications, RS485 deficient in the performance is gradually revealed, has not met to design a high performance, real-time systems. Despite the proven need to practice, but the author after years of observation and practice, felt CAN bus is one of the most promising of success.

Select CAN bus to realize

Communication because

Select CAN bus as the best candidates, mainly based on the following reasons:

CAN serial bus with high-performance

CAN transmission distance can reach 10 km; communication rates up to 1Mbps; has perfect error detection mechanisms; use "multiple access violation arbitration" mechanism, frame transmission can be guaranteed no loss of information; each frame can transmit up to 8 bytes of data, can provide real-time high and so on. Performance advantage can be applied to ensure the CAN in many areas, in the automobile industry, ship transportation, machine control, factory automation, building automation and so you can see the application of CAN.

CAN hardware costs has a competitive advantage

In addition to performance, and compared to other field bus, CAN bus, the hardware cost is also a great advantage. From the hardware chip, the smart nodes to send and receive information needs of a CAN controller and a CAN transceiver. After 20 years of 發(fā)展, CAN Yi Jing received international major semiconductor Zhi Zao Shang strong support of the most important of Tuiguangzuzhi Ju CAN CIA (automatic CAN) statistics there are now 20 species of CAN controllers and transceivers for Xuanze , on-chip CAN controller integrated single chip for more than 100 species. CAN in the development of cost advantage is also evident

 

Currently, from the widely used 8-bit / 16 bit microcontroller, to the DSP and 32-bit PowerPC, ARM and other embedded processors, are contained in the chip CAN bus hardware interface unit. Therefore, from a hardware perspective, CAN field bus can not compare with other high-integration advantages and broad market base of support.

CAN's development platform is simple, and users who choose MCU with CAN controller general development of the CAN microcontroller development platforms, and common development platform for exactly the same; If you select an on-chip CAN controller, microcontroller developed, then As long as a change in support of the microcontroller emulator on it, exactly the same as other developing devices. CAN also need to develop the appropriate driver. Users can choose their own according to the CAN controller development driver.

CANopen protocol of CAN bus

Figure 1 CANopen protocol communication model

CANopen protocol of CAN bus

Figure 2 CANopen device model

By using the application of high-level agreement will push the deepening CAN

And compared to the other's field bus, CAN only defines the physical layer and link layer of the Gui Fan Shu Ju (Zunxun OSI Biaozhun), Zhezhongsheji and the CAN specification defines O'clock in the history of Tiaojianyouguan it will allow CAN to get a broader Shiyingbutong application conditions, but will inevitably cause some inconvenience to the user application. CAN protocol in the application when the user must define their own high-level protocol.

How CAN protocol applications to a deeper level, while meeting the compatibility and interoperability of products? Prevailing international approach on the development of high-level application protocol based on CAN, only in the application level of different companies product before Keneng interoperate good application layer protocol Geng will be available both system performance of the leap.

CAN bus protocol in the 20 years of rapid development, many areas have developed CAN in this field is used when high-level protocol specification. Among them, the more famous the U.S. Society of Automotive Engineers (SAE) J1939 specifications developed by other vehicle communications. The specification of the CAN protocol and played an important role to promote, but, overall, the modularity of the agreement are not very good, generally only used in specific areas. In order to be able to spread to more areas of CAN, the company introduced some European CAL (application layer CAN) protocol, although correct in theory, CAL, and can be put into application in industry, but each user must design a new child agreement, because CAL is a true application layer protocol. CAL CAN can be seen as an application program necessary theoretical step, but it will not be promoting this area. Since 1993, led by the Bosch company a prototype of the European institutions to work out an agreement, thus developed into a CANopen specifications.

 

CANopen is a CAL-based sub-agreement, adopt the idea of object-oriented design with good modular properties and high adaptability, through the expansion can be applied to a large number of applications. Basically completed in the CANopen specification, Bosch will be handed over to the CIA of its organization, its maintenance and development. In 1995, CIA published a full version of the CANopen communication sub-agreement; in just 5 years, it has become embedded in Europe's most important network standards.

CANopen not only defines the application layer and communication sub-agreements, but also for programmable systems, various devices, interfaces, substructure 協(xié)議 defines a number of OK rules, follow these lines rules developed out of the CANopen Shebei will Nenggou achieve different company product interoperability. In addition, CANopen-free license agreement, and any organizations and individuals can support the CANopen protocol developed equipment without paying royalties, and this is the rapid development of CANopen be one of the important reasons. CANopen control system is now in the automobile industry, public transport systems, medical equipment, marine electronic equipment and building automation systems have made wide range of applications is to push the deepening of CAN application ideal.

CANopen protocol adopted

To achieve communication

CANopen protocol includes a standard application-layer specification and communication standard, the communication model shown in Figure 1. In the CANopen application layer, communication between devices through the exchange of objects to communicate with each other. Well-layered and object-oriented design will give the user a clear communication model.

CANopen Device Model

A CANopen device module can be divided into three parts, shown in Figure 2.

Communication interface and protocol software to send and receive communications objects in the bus service. CANopen communication between different devices is accomplished through the exchange of communication objects. This part of the direct operation for the CAN controller.

Object Dictionary describes the equipment used by all data types, communication objects and application objects. Is the core of a CANopen device. Object Dictionary in the communication process and applications, provide the interface to the application, the application to operate on the object dictionary can be achieved CANopen communications. Understand the concept of the object dictionary to understand the key CANopen model.

Application prepared by the user, including the functional part and communication part. Communication part of the operation through the implementation of the CANopen object dictionary to communicate, and functional part of the implementation by the user according to application requirements.

CANopen 網(wǎng)絡(luò) and manage communications traffic through a different object to accomplish, in order to achieve communication, network management, emergency management and other functions, CANopen specification defines a standard communication objects Silei:

* Process Data Objects (PDO)

The first type of communication object to process data objects. PDO is mapped to a single CAN frame, the use of all of the 8 bytes of data field to transmit application object. Each PDO has a separate identifier and a node can only be sent, but it can be more than one node to receive, this model is called producer / consumer communication model. PDO can be transmitted through various modes, internal events, external clock, a remote frame request and receive from a particular node can start the synchronization messages sent PDO.

* Service Data Objects (SDO)

The second type of communication object to service data object, which can transmit more than 8 bytes of configuration information. In other words, SDO transfer protocol to allow transmission of arbitrary length objects. The recipient will acknowledge receipt of each section of information between sending and recipient to establish point to point communication, called client / server model. The future, CANopen allows fast transfer of SDO, do not need to send confirmation of each segment are as long as the whole object is carried out after transfer to confirm.

* Network management objects (NMT)

The third type of communication object is the network management objects, including objects and NMT object node alert. Node object alert NMT master node by sending a remote request for data with 1 byte CAN frames, a byte of data contains a trigger bit, and 7 nodes for that state of data bits. NMT master node, the node will periodically send alert object. Transmission period (guard time), the length specified in the object dictionary and can be configured via SDO. In addition, the system also defines a life guard time, NMT master node to life after the warning time to send a remote request from node NMT. This mechanism ensures that even if the NMT master node is gone, the other nodes in the system can also respond to user-defined manner.

* Special function objects

CANopen also synchronized, states of emergency and the time tag transmission defines three specific object. Synchronization objects can be synchronized to the network periodically broadcast producers, the object will provide a basic clock. When the device a serious internal error, a state of emergency related to the client object sends a state of emergency. Timestamp object applications will provide a common reference time frame.

To understand the CANopen specification, the core is to understand the CANopen device model and the various types of communication objects. After both mastered through the use of various standard device description can be developed in line with international standards of a CANopen device.

Outlook

Recent period, the domestic development and application of CAN systems are progressively increasing, the research on the CAN protocol is also deepening. In many areas, such as the development of hybrid electric vehicles and 863 vehicles a major issue, has a standard vehicle CAN communication protocol determined. Electric power, aerospace and other departments have also made no small CAN application performance.

CAN applications in the booming, we should clearly see that, despite the CAN protocol has been developed in Europe and the United States for 20 years, the application layer protocol development for almost 10 years, but most domestic applications are still based on CAN2 .0 B specification development, and can not at the application layer level further to say that this is very regrettable. In addition, the domestic research, development CAN protocol, in particular high-level CAN protocol organization and personnel is also too small, this CAN promotion in China is very unfavorable, I sincerely hope that more insight can be cut to join the ranks of .