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The SDO are communication channels with two basic characteristics:

  • Client / Server relationship
  • It provides access to the dictionary of CANopen objects of the device.

The SDO are used to transfer multiple object content simultaneously (each with an arbitrary amount of information) from client to server and vice versa.

SDO are transferred as a sequence of segments. Before sending the segments there is an initialization process in which the server and clients prepare themselves to send the segments. However, it is also possible to send information (up to 4bytes) during the initialization process. This mechanism is called SDO expedited transfer.

There are two mechanisms for SDO transfer:

  • Expedited transfer: Used for data objects up to 4 bytes in length.
  • Segmented transfer: For objects with length larger than 4 bytes.

The first byte of the first segment contains the necessary flow control information including a toggle bit to overcome the well-known problem of doubly received CAN frames. The next three byte of the first segment contains index and sub-index of the Object Dictionary entry to be read or written. The last four bytes of the first segment are available for user data. The second and the following segments (using the very same CAN identifier) contain the control byte and up to seven byte of user data. The receiver confirms each segment or a block of segments, so that a peer-to-peer communication can be established.

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