CAN
CX8050, CX8051 - Embedded-PCs for
CANopen and CAN
74
Version: 1.4
Polled
The PDOs can also be polled by data request telegrams (remote frames). In this way it is possible to get the
input process image of event-driven inputs onto the bus, even when they do not change, for instance through
a monitoring or diagnostic device brought into the network while it is running. The time behavior of remote
frame and answer telegrams depends on what CAN controller is in use (Fig. 8). Components with full
integrated message filtering ("FullCAN") usually answer a data request telegram immediately, transmitting
data that is waiting in the appropriate transmit buffer - it is the responsibility of the application to see that the
data there is continuously updated. CAN controllers with simple message filtering (BasicCAN) on the other
hand pass the request on to the application which can now compose the telegram with the latest data. This
does take longer, but does mean that the data is up-to-date. BECKHOFF use CAN controllers following the
principle of Basic CAN.
Since this device behavior is usually not transparent to the user, and because there are CAN controllers still
in use that do not support remote frames at all, polled communication can only with reservation be
recommended for operative running.
Synchronized
It is not only for drive applications that it is worthwhile to synchronize the determination of the input
information and the setting the outputs. For this purpose CANopen provides the SYNC object, a CAN
telegram of high priority but containing no user data, whose reception is used by the synchronized nodes as
a trigger for reading the inputs or for setting the outputs.
PDO transmission types: Parameterisation
The PDO transmission type parameter specifies how the transmission of the PDO is triggered, or how
received PDOs are handled.
Содержание CX8050
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