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Introduction to CANopen

The profile family CANopen defines a stardardized application for distributed industrial automation systems based on CAN as well as the communication standard CAL. CANopen is a standard of CAN-in-Automation (CiA) and has already, soon after its release, found a broad acceptance. Especially in Europe CANopen can be considered the leading standard for CAN based industrial system solutions.

The CANopen profile family is based on a so called "Communication Profile", which specifies the basic communication mechanisms and their description.

The most important device types such as digital and analog I/O modules, drives, operating devices, controllers, programmable controls or encoders, are described by so called "Device Profiles". The device profiles define the functionality of standard devices of the corresponding types. The configuration of devices through the bus is the foundation of the preferred manufacturer-independent configuration by means of the profile family.

The central element of the CANopen standard is the description of the device functionality through an object dictionary (OD). The object dictionary is divided in two sections. The first section contains general device information like device identification, manufacturer name, etc., as well as communication parameters. The second section describes the specific device functionality.

A 16-bit index and an 8-bit sub-index identify the entry ("object") in the object dictionary. The entries in the object dictionary provide the standardized network access to the "Application Objects" of a device, such as input and output signals, device parameters, device functions or network variables.

The functionality and characteristics of a CANopen device can be described by means of an "Electronic Data Sheet" (EDS) using the ASCII-format. In this case the EDS must be understood as a kind of template. The actual device setting are described by the so called "Device Configuration File (DCF)". EDS and DCF can be provided in form of a data carrier, which can be downloaded from the Internet or stored inside the device. Similar to other well-known field bus systems CANopen also distinguishes two basic data transfer mechanisms: The high-speed exchange of small process data portions through so called "Process Data Objects (PDO)" as well as the access to entries in the object dictionary through so called "Service Data Objects (SDO)". The ladder ones are primarily used for the transmission of parameters during the device configuration as well as in generalfor the transmission of larger data portions. Process data object transmissions are generally event triggered, cyclic or requested as broadcast objects without the additional protocol overhead. A PDO can be used for the transmission of a maximum of 8 data bytes. In connection with a synchronization message, the transmission as well as the acceptance of PDOs can be synchronized through the entire network ("Synchronous PDOs"). The assignment of application objects to a PDO (Transmission Object) is adjustable through a structure description ("PDO Mapping") which is stored in the object dictionary, thus allowing the adjustment of a device to the corresponding application requirements.

The transmission of SDOs is performed as a confirmed data transfer with two CAN objects in form of a peer-to-peer connection between two network nodes. The addressing of the corresponding object dictionary entries is accomplished by providing the index and the sub-index of the object dictionary entry. Transmitted messages can be of very large length. The transmission of SDO messages involves an additional protocol overhead.

Standardized event-triggered "Emergency Messages" of high priority are reserved to report device malfunctions. A common system time can be provided through a central timing message. The required functionality for the preparation and coordinated start of a distributed automation system is compliant to the under CAL network management (NMT) defined mechanisms. The same applies to the cyclic "Node Guarding".

Alternatively, it is possible to display the communication capability of a CANopen device through a so called "Heartbeat Message".

The assignment of CAN message identifiers to PDOs and SDOs is possible by direct modifications of identifiers inside the data structure of the object dictionary or, for simple system structures, through the use of pre-defined identifiers.

For more than 10 years IXXAT/stzp is involved in the specification and development of higher layer CAN based protocols. Besides other activities, IXXAT has provided essential contributions to the specification of the CANopen standard. First implementations of our CANopen software were already available by 1995. The membership in the CiA and the active cooperation with all important workgroup as well as the know-how gained through many projects surely makes IXXAT/stzp one of the most competent partners for the development of CANopen products and system solutions.

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