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Short description of CANopen
D301087 1211 - BL20 CANopen 3-4
Process Data Objects (PDOs)
Process Data Objects (PDOs) are mapped to a single CAN frame using up to 8 bytes of the data field to
transmit application objects. Each PDO has a unique identifier and is transmitted by only one node, but
it can be received by more than one (producer/consumer communication).
PDO transmissions
PDO transmissions may be driven by an internal event, by an internal timer, by remote requests and by
the Sync message received:
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Event- or timer-driven:
An event (specified in the device profile) triggers message transmission. An elapsed timer
additionally triggers the periodically transmitting nodes.
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Remotely requested:
Another device may initiate the transmission of an asynchronous PDO by sending a remote
transmission request (remote frame).
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Synchronous transmission:
In order to initiate simultaneous sampling of input values of all nodes, a periodically transmitted
Sync message is required. Synchronous transmission of PDOs takes place in cyclic and acyclic
transmission mode. Cyclic transmission means that the node waits for the Sync message, after
which it sends its measured values. Acyclically transmitted synchronous PDOs are triggered by a
defined application-specific event.
Special Function Objects
CANopen also defines three specific protocols for synchronization, emergency indication, and time-
stamp transmission.
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Synchronization object (Sync)
The Sync Object is broadcast periodically by the Sync Producer. The time period between Sync
messages is defined by the Communication Cycle Period, which may be reset by a configuration
tool to the application devices during the boot-up process. There can be a time jitter in transmission
by the Sync Producer due to some other objects with higher prior identifiers or by one frame being
transmitted just before the Sync message. The Sync message is mapped to a single CAN frame with
the identifier 128 by default. The Sync message does not carry any data.
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Emergency object (Emcy)
The Emergency message is triggered by the occurrence of a device internal error situation and are
transmitted from an Emergency producer on the concerned application device. This makes them
suitable for interrupt type error alerts. An Emergency message is transmitted only once per ‘error
event’. As long as no new errors occurs on a device, no further Emergency message can be
transmitted. Zero or more Emergency consumers may receive these. The reaction of the Emergency
consumer is application-specific. CANopen defines several Emergency Error Codes to be
transmitted in the Emergency message, which is a single CAN frame with 8 data byte.
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Time stamp object (Time)
By means of Time-Stamp, a common time frame reference is provided to application devices. It
contains a value of the type Time-of-Day. This object transmission follows the producer/consumer
push model. The associated CAN frame has the pre-defined identifier 256 and a data field of 6-byte
length.