CANopen Programmer's Manual - Maccon.de

CANopen Programmer’s Manual 1: Introduction
SDO vs. PDO: Design Considerations
Differences Between SDO and PDO
As stated earlier, SDOs and PDOs can both be described as channels through which CAN
messages are sent, and both provide access to a device’s object dictionary, but each has
characteristics that make it more appropriate for certain types of data transfers.
Here is a review of the differences between SDOs and PDOs, and some design considerations
indicated by those differences:
SDO PDO Design Considerations
The accessed device always confirms
SDO messages. This makes SDOs
slower.
PDO messages are unconfirmed. This
makes PDOs faster.
One SDO transfer can send long blocks A PDO transfer can only send small
of data, using as many CAN messages amounts of data (up to eight bytes) in a
as required.
single CAN message. Mapping allows
very efficient use of those eight bytes.
Asynchronous. Synchronous or asynchronous. Cyclic
or acyclic.
The SDO employs a client-server
communication model. The CANopen
master is the client. It reads from and
writes to the object dictionaries of
devices. The device being accessed is
the server.
All communications can be performed
through the SDO without using any
PDOs.
The PDO employs a peer-to-peer
communication model. Any device can
send a PDO message, and a PDO
message can be received and
processed by multiple devices.
The CANopen master application uses
SDO messages to map the content of
the PDO, at a cost of increased CPU
cycles on the CANopen master and
increased bus traffic.
To transfer 8 bytes or less at real-time
speed, use a PDO. For instance, to
receive control instructions and transmit
status updates.
To transfer large amounts of low priority
data, use the SDO.
Also, if confirmation is absolutely
required, use an SDO.
Use PDO when synchronous or
broadcast communications are
required. For instance, to communicate
set points from the master to multiple
devices for a multi-axis move, or to
have a device broadcast its status.
If the application does not benefit from
the use of a PDO for a certain transfer,
consider using SDO to avoid the extra
overhead. For instance, if an object’s
value is updated only once (as with
many configuration objects), the SDO is
more efficient. If the object’s value is
updated repeatedly, a PDO is more
efficient.
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