CANopen application layer Version 4.1

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Version 4.1 of the CANopen application layer (CiA 301) has recently been released by the non-profit arm of CAN in Automation, CiA e. V. This specification is - for the time being - available to CiA members only.

The international users' and manufacturers' group has been maintaining the higher-layer protocol since it was developed in 1994 in an Esprit project. CANopen devices, especially programmable ones, may implement further functions of the application layer described in the specification CiA 302. Further functions beside the SDO Manager are the Flying NMT Master, the Configuration Manager as well as the usage of redundant bus lines.

These specifications have been partly edited and reworked and are available to members of CiA as version 4.0.

CANopen is a serial bus system that is based upon the internationally standardized protocol Controller Area Network (CAN). The first version of the CANopen application layer (CiA 301) was published in November 1994. Version 4.0 published in 1999 was expanded by just a few functions: Counter in Sync messages and the Dyn-bit in the SDO (service data object) COB-ID (communication object identifier).

Of course the specification CiA 301 then was also reworked regarding structure and language. Text passages that were mistakable and often misunderstood were edited. Partly the specification was re-arranged to simplify reading and understanding. Especially the model descriptions upon which the specification is based were enhanced.

The Sync counter is one byte long. It is used to implement several virtual Sync messages in a network. This is a requirement e.g. for applications that need an evenly distributed bus load. One group of devices reacts to every even Sync message and the other group to the uneven Sync messages. The starting value to which a synchronous process data object (PDO) is to react, is configured in sub-index 6 of the respective communication parameter set.

The maximum counter value (the spillway value) can be configured in object 0x1019. For this it is important to remember that the value must be a multiple of the highest synchronous PDO cycle. This guarantees that the lowest frequency synchronous PDO will be transmitted in one counter cycle. With the Sync counter it is also possible to close control loops via the network. However, here one must consider the maximum delay of messages.

The SDO Manager described in the specification CiA 302-5 is able to establish static or dynamic SDO connections between devices. The Dyn bit (bit 30) was introduced, an additional bit in the 32-bit communication object identifier (COB-ID) for service data objects (SDO). With it the SDO Manager may establish, which SDO connections are dynamic and which are static. If either of the two COB-ID entries of an SDO connection has set the Dyn bit, the COB-ID values must not be saved in non-volatile memory so that in case of a power failure or a reset the dynamic SDO connections will be erased. Dynamic SDO connections are thus only temporary. Configured as static SDO connections they are still active even after a reset if they were permanently saved before the reset.

20 October 2006

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