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CANopen communication
Process data transfer (PDO transfer)
Telegram structure
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25 KHB 13.0002−EN 4.1
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)
5.3.1 Telegram structure
The telegram for process data has the following structure:
11 bits 4 bits User data (up to 8 bytes)
1st byte 2nd byte 3rd byte 4th byte 5th byte 6th byte 7th byte 8th byte
Identifier
Data
length
Data 0 Data 1 Data 2 Data 3 Data 4 Data 5 Data 6 Data 7
5.3.2 Available process data objects
The drive controller is provided with two transmit and two receive PDOs.
Almost all objects of the object directory can be entered in (mapped to) the PDOs, i.e. the
PDO contains for instance the actual speed value or actual position value as data. The drive
controller must know in advance which data is to be transferred because the PDO only
contains user data and no information about the type of the parameter.
In this way almost all kinds of data telegrams can be defined. The settings required are
described in the following chapters.