Owner manual

CANopen communication

Process data transfer (PDO transfer)

Telegram structure

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.