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RD4 CANopen®
7.7 SDO messages
SDO messages are used to set new values to and read values from the device.
These parameters are described in the “Object dictionary” (see on page 46).
4 bytes at the most are used for CAN data, other 4 bytes are used for Command,
Index and Sub-index fields. SDO messages always need confirmation from the
Slave. It follows that when the Master sends a SDO message to the Slave, the
Slave always sends a reply (and a warning, should an error occur).
SDO structure:
IDENTIFIER from 4 to 8 CAN data bytes
COB-ID(hex) 0 1 2 3 4 5 6 7
F.C. Node-ID Com Index Sub Data
1 byte LSB MSB 1 byte LSB … … MSB
Com
command
Index
parameter index
Sub
parameter sub-index
Data parameter value (set or read value)
7.7.1 Command
Command byte contains the type of COB telegram transmitted to the CAN
network.
Three types of telegram are available:

Set
: used to send configuration parameters to the Slave;

Req
: used by the Master to read data from the Slave;

Warnings
: used by the Slave to send error messages to the Master (e.g.
“Index does not exist”, “Process data value not valid”, etc.).
Command COB COB type Data length
22h Set M  S request not specif.
23h Set M  S request 4 byte
2Bh Set M  S request 2 byte
2Fh Set M  S request 1 byte
60h Set S  M confirmation 0 byte
40h Req M  S request 0 byte
42h Req S  M reply non specified
43h Req S  M reply 4 byte
4Bh Req S  M reply 2 byte
4Fh Req S  M reply 1 byte
41h Req S

M reply segmented SDO
80h Warning S

M reply 4 byte
MAN RD4 CB E 1.5 CANopen® interface 45 of 72