CANopen Programmer's Manual - Maccon.de

1: Introduction CANopen Programmer’s Manual
Accessing the Object Dictionary
Two Basic Channels
CANopen provides two ways to access a device’s object dictionary:
The Service Data Object (SDO)
The Process Data Object (PDO)
Each can be described as a channel for access to an object dictionary.
SDOs and PDOs
Here are the basic characteristics of PDOs and SDOs.
SDO PDO
The SDO protocol allows any object in the object dictionary
to be accessed, regardless of the object's size. This comes
at the cost of significant protocol overhead.
One PDO message can transfer up to eight bytes of data in
a CAN message. There is no additional protocol overhead
for PDO messages.
Transfer is always confirmed. PDO transfers are unconfirmed.
Has direct, unlimited access to the object dictionary. Requires prior setup, wherein the CANopen master
application uses SDOs to map each byte of the PDO
message to one or more objects. Thus, the message itself
does not need to identify the objects, leaving more bytes
available for data.
Employs a client/server communication model, where the
CANopen master is the sole client of the device object
dictionary being accessed.
An SDO has two CAN message identifiers: a transmit
identifier for messages from the device to the CANopen
master, and a receive identifier for messages from the
CANopen master.
Employs a peer-to-peer communication model. Any
network node can initiate a PDO communication, and
multiple nodes can receive it.
Transmit PDOs are used to send data from the device, and
receive PDOs are used to receive data.
SDOs can be used to access the object dictionary directly. A PDO can be used only after it has been configured using
SDO transfers.
Best suited for device configuration, PDO mapping, and
other infrequent, low priority communication between the
CANopen master and individual devices. Such transfers
tend to involve the setting up of basic node services; thus,
the term service data object.
For more information about SDOs,
see SDOs: Description and Examples, p. 22.
Best suited for high-priority transfer of small amounts of
data, such as delivery of set points from the CANopen
master or broadcast of a device’s status. Such transfers
tend to relate directly to the application process; thus, the
term process data object.
For more information about PDOs,
see PDOs: Description and Examples, p. 24.
For help deciding whether to use an SDO or a PDO see SDO vs. PDO: Design Considerations, p. 27.
20 Copley Controls