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About CANopen |
Introduction |
CANopen, a comprehensive communication
and application standard for distributed systems
CANopen is a standardized application for distributed automation systems based on CAN (Controller Area Network) offering the
following performance features:
- Transmission of time-critical process data according to the producer consumer principle
- Standardized device description (data, parameters, functions, programs) in the form of the so-called "object dictionary". Access to all "objects" of a device with standardized transmission protocol according to the client-server principle.
- Standardized services for device monitoring (node guarding/heartbeat) and network coordination ("network management")
- Standardized system services for synchronous operations (synchronization message), central time stamp message, emergency messages
- Standardized help functions for configuring baud rate and device identification number via the bus
- Standardized assignment pattern for message identifiers for simple system configurations in the form of the so-called "predefined connection set"
A comprehensive introduction to the system concepts of CAN and CANopen is given in:
Etschberger et. al.: "CAN - Grundlagen, Protokolle, Bausteine".
Hanser-Verlag. Third edition ,2002, ISBN 3-446-21776-2 or
Etschberger et. Al. "Controller Area Network, Basics, Protocols,
Chips and Applications". IXXAT Press, 2001, ISBN 3-00-007376-0
The document CiA Draft Standard 301 "CANopen Application Layer and Communication Profile" in the
current version 4.02 is the CANopen basic specification and is available via the user
organization "CAN-in-Automation e.V." in Erlangen, Germany (http://www.can-cia.com/services/  ).
Extended communication mechanisms are described in DSP 302: "Framework for Programmable Devices". This standard represents a very important extension of the communication mechanisms described in DS 301 for so-called "intelligent", i.e. programmable devices , in particular PLCs, HMIs or CANopen tools.
The specification proposals DR 303, DSP 305 and DSP 306 are also worth mentioning, in which standards and recommendations for cables, pin assignments, SI units, the so-called "layer setting services" and the specification of an electronic data sheet (EDS) are described.
Further specifications for specific requirements are DSP 304 ("Framework for safety-relevant communication") and DSP 307 ("Framework for maritime electronics").
An overview of the many different CANopen device and application profiles is given in the section "Device and application profiles".
All CANopen specifications CANopen were developed by member companies of CAN-in-Automation and are freely available without usufruct.
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Physical structure of a CANopen network |
Two basic conditions must be fulfilled in order that a CANopen network can function from a physical point of view:
- All nodes must be configured to the same bit rate and
- No node-ID may exist twice.
Unfortunately there are no mechanisms with which these conditions can be automatically ensured, so that the system integrator must check the bit rate and node-ID of every single network node when wiring a network and adjust if necessary. Normally the node-ID is configured directly on the device via DIP-switches or hexadecimal rotary switches. Alternative solutions require setting of these parameters via two reserved CAN identifiers by software with the aid of the so-called "LSS-service" (layer setting service). Here the device to be configured is to be connected 1:1 with a configuration device.
As generally required for CAN networks, both ends of the network must be terminated with a 120-Ohm terminal resistance between wire 2 (CAN_L) and wire 7 (CAN_H). In addition, the maximum permissible branch lengths for connection of the individual network nodes are to be observed.
The recommended permissible bit rates for a CANopen-network are given in DS-301: 10 kbps, 20 kbps, 50 kbps, 125 kbps, 250 kbps, 500 kbps, 800 kbps and 1000 kbps. In DS-301 a recommendation for the configuration of the bit timing is also given.
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