Basic concepts of CANopen bus protocol

Nowadays, many communication electronic devices are highly intelligent. To some extent, it is comparable to computers. In the communication settings, we will introduce the use of the CANopen bus. Considering that CANopen bus is a good open and low-cost Field Bus supported by most European and American manufacturers, it is necessary to enhance the study of CANopen bus protocol.

The CANopen bus protocol includes the Communication sub-Protocol Communication Profile) and the Device sub-Protocol Device Profile ). Communication Sub-Protocol describes the main form of the object dictionary and objects in the communication sub-Protocol area in the object dictionary, communication parameters, and descriptions of CANopen communication objects. This sub-Protocol applies to all CANopen devices, like CIA-DS301. The sub-Protocol defines objects in the object dictionary for various types of devices, and describes the functions, names, indexes, sub-indexes, and Data Types of each object in the object dictionary, and whether the object is required or optional. The object is read-only, write-only, or readable. Currently, multiple sub-protocols are available, such as DS401 and DS402.

The core part of the CANopen standard is to describe the functions of a device through Object Dictionary. The object dictionary portal allows you to access the basic network of a device's "Application Object". The "Application Object" of a device can be input and output signals, device parameters, device functions, and network variables. The functions and features of a CANopen device are described in the form of EDS. The actual device settings are described in the device configuration file DCF.

Most important device types, such as digital and analog input/output modules, drivers, operating devices, controllers, programmable controllers or encoders, all are described in protocols called "device subset. A subset of devices defines different types of standard devices and their functions. With the support of the CANopen bus protocol set, devices of different vendors can be configured through the bus.

The core of CANopen bus protocol standards is to describe the functions of devices through Object Dictionary. The object dictionary consists of basic device information, such as device ID, manufacturer, and communication parameters. The second part describes special device functions.

A 16-bit index and an 8-bit sub-index uniquely determine the object dictionary entry. You can use the object dictionary entry to access the basic network of the device's "Application Object". The "Application Object" of the device can be an input/output signal, device parameter, device function, and network variable.

The functions and features of the CANopen device are described in the form of electronic data sources (EDS). EDS adopts the ASCII format and can be understood as a form of table. The actual device settings are described through the so-called Device Configuration File DCF. Both EDS and DCF can be downloaded from the Internet and stored on devices.

Like other well-known fieldbus systems, the CANopen bus protocol is also divided into two basic data transmission mechanisms: through the process data object (PDO) for small data, perform high-speed data exchange and access the object dictionary through the Service Data Object SDO. The latter is mainly used to transmit parameters and transmit big data blocks during device configuration. Generally, process data objects are sent through event triggering, loop, or request. As broadcast objects, there is no additional protocol at the upper layer.

A pdo can transmit up to 8 bytes of data. When a synchronous packet is connected, the entire network can transmit and receive the PDO in synchronous mode (Synchro-nous PDOs ). By using the PDO Mapping Structure Stored in the Object dictionary, you can adjust the Transmission Object from the application Object to the PDO Object, this ensures that devices can adjust different application requirements.

Through the transmission of SDO, reliable data transmission CAN be achieved by two CAN objects through point-to-point communication between two network nodes. By transmitting the index and sub-index of the object dictionary, you can locate the corresponding object dictionary entry. Messages sent through SDO are not limited by the length, but the extra protocol overhead is required for transmitting SDO packets.

High-priority emergency messages triggered by standardized events are used to report device failures. The system clock of the entire system can be provided through the central clock report. The functions required to prepare and coordinate the establishment of a distributed automation system are suitable for CAL Network Management NMT) defined mechanisms, and can also be applied to periodic node maintenance (node guarding ).

You can select "Heartbeat Message" to display the communication capability of the CANopen bus protocol. The CAN message identifiers of PDO and sdo can be directly allocated through the entry identifier of the data structure of the object dictionary, or pre-defined identifiers CAN be used in a simple system.