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Printed in the Federal Republic of Germany
TR-Electronic GmbH 2016, All Rights Reserved
09/29/2020
TR-EMO-BA-GB-0022 v03
Page 27 of 153
4.7 Transmission of SDO messages
The entries of the object dictionary can be read or written with the SDO services. The
SDO Transport Protocol allows the transmission of objects of any size. The EtherCAT
SDO Protocol is equivalent to the CANopen SDO Protocol, in order to guarantee the
re-use of existing protocol stacks.
The first byte of the first segment contains the necessary control information. The next
three bytes of the first segment contain the index and sub-index of the object
dictionary entries to be read or written. The last four bytes of the first segment are
available for useful data. The second and following segments contain the control byte
and useful data. The recipient confirms each segment or a block of segments, so that
Peer-To-Peer communication (client/server) takes place.
In CAN-compatible mode the SDO protocol comprises 8 bytes, in order to correspond
to the CAN data size. In extended mode the useful data are simply extended, without
changing the protocol header. In this way the increased data volume of the EtherCAT
mailbox is adapted to the SDO protocol, accelerating the transmission of large data
volumes accordingly.
In addition, the mode “Complete Access” has been added which makes it possible to
transmit the complete data of an index from the object dictionary in a single process.
The data of all sub-indices are subsequently transmitted. The access is controlled via
the
“Initiate SDO Download / Upload” – write/read services, which also are supported
by the drive. In the ESI file this is indicated with the mailbox attribute entry
CompleteAccess="1"
. If older EtherCAT master do not support this access, this
entry must be set to “0”.
Services with confirmation (Initiate SDO Upload, Initiate SDO Download, Download
SDO Segment, and Upload SDO Segment) and services without confirmation (Abort
SDO Transfer) are used for the execution of Segmented/Expedited transmission of
Service Data Objects.
The so-called SDO Client (master) specifies in its "Request" the parameter, the
access type (read/write) and the value if applicable. The so-called SDO Server (slave
or drive) executes the write or read access and answers the request with a
"Response". In the case of error, an abort code (Abort SDO Transfer) provides
information on the cause of the error.
Normally the EtherCAT master provides appropriate mechanisms for the SDO
transfer. Knowledge of the protocol structure and internal sequences is therefore not
required.
This also applies to read out the complete CoE object directory. The drive supports
the so-called
“SDO Information Service”. In online mode about this service it is
possible to read out all supported objects in one read service.