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STOBER
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02/2020 | ID 442516.07
10.3
Communication objects
Based on CANopen, the following communication objects are of key importance for data transmission as part of EtherCAT:
§
Process data objects (PDO)
... transmit real-time data such as set and actual values, control commands or status information based on events or
objectives, cyclically or upon request.
§
Service data objects (SDO)
... grant access to the object directory, enabling a device configuration.
§
Emergency objects (EMCY)
... are triggered in the event of faulty state transitions or device-internal errors. The messages contain error codes and
causes.
10.3.1
Process data objects – PDO
Process data objects normally transmit cyclical data that is required for controlling and observing the ongoing process, such
as target positions, travel velocities or acceleration information. They are generally used for data exchange in real time.
They also enable simultaneous access to multiple drive parameters.
The specific communication elements that are sent and received in a specific PDO can be freely selected. In a PDO
transmission, contents are transmitted directly instead of addressing specific objects.
PDOs are generally exchanged over the process data channel with high priority. A distinction is made, from the perspective
of the respective nodes, between receive PDOs (= RxPDO) and transmit PDOs (= TxPDO).
10.3.1.1
PDO mapping
In PDO mapping, the communication parameters to be transmitted are mapped to PDOs and the associated transmission
channel. This determines which communication objects are transferred over the process data channel.
PDO communication allows for 4 PDO channels to be operated simultaneously per transmission and sending direction. A
maximum of 6 parameters can be transmitted in a defined sequence per channel. The process data can be configured in
any way.
In order to guarantee error-free communication between the controller and drive controller, STOBER offers application-
dependent pre-assignment of the channels, which can be changed at any time.
10.3.2
Service data objects – SDO
Service data objects are used to transmit data that is not time-critical. They enable read and write access to the
configuration parameters of an EtherCAT slave.
SDO frames are transmitted over the mailbox channel acyclically during ongoing cyclical EtherCAT operation, without
impairing PDO communication.
10.3.2.1
Addressing axis-dependent parameters
When addressing axis-dependent parameters of logical axes using SDO, the axis must be preselected using parameter
A11.1. The parameter is addressed according to the access rules described in the attachment (see the chapter
