Appendix B: CANopen Overview | VFD-VJ
Apx B - 5
SDO (Service Data Objects)
Use SDO to access the Object Dictionary in every CANopen node using the Client/Server
model. One SDO has two COB-IDs (request SDO and response SDO) to upload or download
data between two nodes. There is no data limit for SDOs to transfer data, but it must transfer
data by segment when the data exceeds four bytes with an end signal in the last segment. The
VJ series does not currently support segment transmission.
The Object Dictionary (OD) is a group of objects in a CANopen node. Every node has an OD in
the system, and OD contains all parameters describing the device and its network behavior.
The access path in the OD is the index and sub-index; each object has a unique index in the
OD, and has a sub-index if necessary. The following shows the request and response frame
structure of SDO communication:
PDO (Process Data Object)
PDO communication can be described by the producer/consumer model. Each node of the
network listens to the messages of the transmission node and distinguishes whether the
message has to be processed or not after receiving the message. A PDO can be transmitted
from one device to one another device or to many other devices. Every PDO has two PDO
services: a TxPDO and an RxPDO. PDOs are transmitted in a non-confirmed mode. All
transmission types are listed in the following table:
Type Number
PDO
Cyclic Acyclic Synchronous Asynchronous RTR only
0
○ ○
1-240
○ ○
241-251 Reserved
252
○ ○
253
○ ○
254
○
255
○
Type number 0 indicates the synchronous aperiodic message between two PDO transmissions.
Type number 1-240 indicates the number of SYNC message between two PDO transmissions.
Type number 252 indicates the data is updated (but not sent) immediately after receiving SYNC.
Type number 253 indicates the data is updated immediately after receiving RTR.
Type number 254: Delta CANopen doesn’t support this transmission format.
Type number 255 indicates the data is an asynchronous aperiodic transmission.
All PDO transmission data must be mapped to the index with Object Dictionary.
EMCY (Emergency Object)
When errors occur inside the hardware, an emergency object is triggered. An emergency
object is only sent when an error occurs. As long as there is nothing wrong with the hardware,
there is no emergency object warning of an error message.
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