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26 Fieldbus interface
3 Basics IL•1F CANopen DS301
0198441113586, V2.01, 11.2008
3.3 Service data communication
3.3.1 Overview
Service Data Object(SDO: Service Data Object) can be used to access
the entries of an object dictionary via index and subindex. The values of
the objects can be read and, if permissible, also be changed.
Every network device has at least one server SDO to be able to respond
to read and write requests from a different device. A client SDO is only
required to request SDO messages from the object dictionary of a dif-
ferent device or to change them there.
The T_SDO of an SDO client is used to send the request for data ex-
change; the R_SDO is used to receive. The data frame of an SDO con-
sist of 8 bytes.
SDOs have a higher COB ID than PDOs and therefore are sent over the
CAN bus at a lower priority.
3.3.2 SDO data exchange
A service data object (SDO) sends parameter data between two de-
vices. The data exchange conforms to the client-server relationship. The
server is the device to whose object dictionary an SDO message refers.
Figure 3.10 SDO message exchange with request and response
Message types Client-server communication is triggered by the client to send parameter
values to the server or to get them from the server. In both cases, the cli-
ent starts the communication with a request and receives a response
from the server.
Client
COB-ID data
COB-ID
data
Server
R_SDO
(request)
(response)
CAN
T_SDO
R_SDO T_SDO