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RD4 CANopen®
7.3 Communication messages
Four different kinds of communication messages are used in a CANopen®
network:

Network management
NMT
service: through node control services, the NMT
Master controls the NMT state of the NMT Slaves; see “7.4 NMT messages”
section on page 36.

Process Data Object PDO service: the real-time data transfer is performed
by means of “Process Data Objects (PDO)”. The PDOs correspond to entries in
the object dictionary and provide the interface to the application objects;
see “7.6 PDO messages” section on page 37.

Service Data Object SDO
service: SDOs used to provide direct access to
entries of a CANopen® device “Object dictionary” (page 46); they allow to
read and set parameters; see “7.7 SDO messages” section on page 45.

Special Function Object
services:
- SYNC: synchronization object provides the basic network synchronization
mechanism and is used by the Master to enable the Slave devices to
transmit process data (position and velocity; see on page 47);
- Emergency EMCY: emergency objects are triggered by the occurrence of a
CANopen® device internal error situation, see on page 67;
- Node guarding protocol: it is used to detect remote errors in the network,
see on page 68.
Relation between device states and communication objects:
Initialization Pre-operat. Operational Stopped
NMT X X X
PDO X
SDO X X
SYNC X
EMCY X X
Boot-up X
Nodeguard X X X
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