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10.3.2 The CANopen Communication Object
The CANopen communication protocol contains the following communication objects.
1. PDO (process data object)

The PDO provides the direct channel for the device application object, and transmits the real-time data.
It has high priority. Every byte in the PDO CAN message data list transmits data, and the message
usage rate is high.

There are two kinds of uses for PDOs; data transmission and data reception. They are distinguished
by Transmit-PDOs (TxPDOs) and Receive-PDOs (RxPDOs). Devices supporting TxPDOs are called
PDO producers, and devices that receive PDOs are called PDO consumers.

The PDO is described by the “producer/consumer mode”. The data transmits from one producer to one
or many consumers. The data that can be transmitted is limited to between 1-byte and 8-byte data.
After the producer transmits the data, the consumer does not need to reply to the data. Every node in
the network detects the transmitted data and decides whether to process the received data .

Every PDO is described by two objects in the object dictionary: the PDO communication parameters
and the PDO mapping parameters
PDO communication parameters: the COB-ID used by PDO, the transmission type, the prohibition
time, and the counter cycle
PDO mapping parameters: the object list in an object dictionary. These objects are mapped into the
PDO, including the data length (in bits). To explain the contents of the
PDO, the producer and the consumer both have to understand the
mapping.
The PDO transmission modes: synchronous and asynchronous
Synchronous mode: synchronous periodic and synchronous non-periodic
Asynchronous: The producer transmits the PDO when the data changes, or after a trigger.