Curtis 1353 - 5 - SDO Communications; SDO Master Request (SDO-MOSI) Format

5 — SDO COMMUNICATIONS
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Return to TOC Curtis 1353 CANopen Expansion Module Manual – June 2017
5 — SDO COMMUNICATIONS
CANopen uses Service Data Objects (SDOs) to change and view all internal parameters, or “objects.”
e SDO is an 8-byte packet that contains the address and sub-address of the parameter in question,
whether to read or write the parameter, and the parameter data (if it is a write command). SDOs are
sent infrequently and have a low priority on the CANbus.
SDOs are designed for sporadic and occasional use during normal runtime operation. ere are
two types of SDOs: expedited and block transfer. e 1353 does not support large le uploads or
downloads (using the block transfer), so all the SDOs used by the 1353 are expedited SDOs.
e SDOs in the 1353 are used to set up and parameterize the module. ey are also used to retrieve
basic module information (such as version or manufacture date), review the fault log, and monitor
a few key internal variables (mostly for system debug purposes).
SDO Master Request (SDO-MOSI)
An SDO transfer always starts with a request message from the master. Each SDO request message
consists of one control byte, a two-byte CAN Object index, a one-byte CAN Object sub-index, and
up to 4 bytes of valid data. is format is CANopen compliant.
SDO-MOSI (received from the system master)
Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6 Byte 7 Byte 8
Control CAN Object
Index
Sub-index Data Data Data Data
e rst data byte contains R/W message control information.
Action Byte 1 Value
Read 0x4x
Write 0x2x
e next two data bytes hold the CAN Object index. e LSB of the index appears rst, in byte 2,
and the MSB appears in byte 3. For example, if the index is 0x3021, byte 2 holds the 0x21 and byte
3 holds the 0x30.
Data byte 4 holds the CAN Object sub-index. When there is only one instance of a parameter or
value type, this value is 0. If there are several related parameters or values, the sub-index is used.
e last four data bytes hold the data that is to be transferred. In the case of a single-byte transfer,
the data is placed into data byte 5, with bytes 6 through 8 being undened (set to 0). In the case of a
16-bit transfer, the lower 8 bits appear in data byte 5 and the upper 8 bits appear in data byte 6; bytes
7 and 8 are undened (set to 0). e case of a 32-bit transfer follows the same strategy, with the least
signicant byte placed in data byte 5 and the most signicant byte placed in data byte 8.