CANopen communication
Process data transfer (PDO transfer)
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KHB 13.0003-EN 2.0
5.3
Process data transfer (PDO transfer)
Process data objects (PDOs) can be used, for instance, for the fast event-controlled transfer
of data. The PDO transfers one or several parameters specified in advance. Unlike with an
SDO, the transfer of a PDO is not acknowledged. After the PDO activation, all receivers
must therefore always be able to process any arriving PDOs. This usually means a
considerable software load on the master. However, this disadvantage is compensated by
the advantage that the master does not need to cyclically poll the parameters transferred
by a PDO, which results in a significant reduction of the CAN bus load.
Example:
The master wants to know when the drive controller has completed the positioning from
A to B.
When SDOs are used for this purpose, the master continuously (e.g. every millisecond) has
to poll the
status word
object, i.e. the load on the bus is high.
When a PDO is used, right from the start of the application the drive controller is
parameterised in such a way that it transmits a PDO containing the
status word
object as
soon as the
status word
object changes.
Instead of polling continuously, the master automatically receives a corresponding
message as soon as the event has occurred.
The following types of process data telegram are distinguished
ƒ
Process data telegrams
to
the drive controller: Receive PDO (RPDOx)
ƒ
Process data telegrams
from
the drive controller: Transmit PDO (TPDOx)
