Transmission Of Sdo Messages - TR-Electronic LP-46 Bedienungsanleitung

3.6 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, a mode 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.
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 measuring system) executes the write or read access and answers the request with
a "Response" In the case of error, an error 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.
However, for troubleshooting it can be important to know the principal sequence
of SDO transfers. For this reason, the services Initiate SDO Download Expedited
and Initiate SDO Upload Expedited are dealt with in more detail below. Up to four
bytes can be written and up to four bytes read via these services. This is
sufficient for most objects.
Printed in the Federal Republic of Germany
01/14/2022 TR-ELA-BA-DGB-0020 v04
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