CAN
CX8050, CX8051 - Embedded-PCs for
CANopen and CAN
71
Version: 1.4
Heartbeat: Node Monitoring without Remote Frame
In the heart beat procedure, each node transmits its status message cyclically on its own initiative. There is
therefore no need to use remote frames, and the bus is less heavily loaded than under the guarding
procedure.
The master also regularly transmits its heartbeat telegram, so that the slaves are also able to detect failure of
the master.
Heartbeat procedure
Protocol
The toggle bit is not used in the heart beat procedure. The nodes send their status cyclically (s). See
Guarding.
9.2.2
Process Data Objects (PDO)
Introduction
In many fieldbus systems the entire process image is continuously transferred - usually in a more or less
cyclic manner. CANopen is not limited to this communication principle, since the multi-master bus access
protocol allows CAN to offer other methods. Under CANopen the process data is not transferred in a master/
slave procedure, but follows instead the producer-consumer model. In this model, a bus node transmits its
data, as a producer, on its own accord. This might, for example, be triggered by an event. All the other nodes
listen, and use the identifier to decide whether they are interested in this telegram, and handle it accordingly.
These are the consumers.
The process data in CANopen is divided into segments with a maximum of 8 bytes. These segments are
known as process data objects (PDOs). The PDOs each correspond to a CAN telegram, whose specific CAN
identifier is used to allocate them and to determine their priority. Receive PDOs (RxPDOs) and transmit
PDOs (TxPDOs) are distinguished, the name being chosen from the point of view of the device: an input/
output module sends its input data with TxPDOs and receives its output data in the RxPDOs.
This naming
convention is retained in the TwinCAT System Manager.
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