Beckhoff FC5101, Документация

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CANopen Communication
FC5101 and FC5102 52 Version: 2.0
default in the device profile of analog inputs, and must be turned on specifically. Time windows for the
transmit PDOs can be set using progress timers: the telegram is not sent again before the
inhibit time
[
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44] has elapsed, and not later than the time required for the progress timer to complete.
• The communication type is parameterized by means of the transmission type [
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44] .
It is also possible to combine the two PDO principles. It can, for instance, be helpful to exchange the set and
actual values of an axis controller synchronously, while limit switches, or motor temperatures with limit values
are monitored with event-driven PDOs. This combines the advantages of the two principles: synchronicity for
the axis communication and short reaction times for limit switches. In spite of being event-driven, the
distributed limit value monitoring avoids a constant addition to the bus load from the analog temperature
value.
In this sample it can also be of value to deliberately manipulate the identifier allocation, in order to optimize
bus access by means of priority allocation: the highest priority is given to the PDO with the limit switch data,
and the lowest to that with the temperature values.
Optimization of bus access latency time through modification of the identifier allocation is not, however,
normally required. On the other hand the identifiers must be altered if masterless communication is to be
made possible (
PDO linking [
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44] ). In this sample it would be possible for one RxPDO for each axis to be
allocated the same identifier as the limit switch TxPDO, so that alterations of the input value can be received
without delay.
Determining the Bus Loading
It is always worth determining the bus loading. But what bus loading values are permitted, or indeed
sensible? It is first necessary to distinguish a short burst of telegrams in which a number of CAN messages
follow one another immediately - a temporary 100% bus loading. This is only a problem if the sequence of
receive interrupts that it caused at the CAN nodes cannot be handled. This would constitute a data overflow
(or CAN queue overrun). This can occur at very high baud rates (> 500 kbit/s) at nodes with software
telegram filtering and relatively slow or heavily loaded microcontrollers if, for instance, a series of remote
frames (which do not contain data bytes, and are therefore very short) follow each other closely on the bus
(at 1 Mbit/s this can generate an interrupt every 40 µs; for example, an NMT master might transmit all its
guarding requests in an unbroken sequence). This can be avoided through skilled implementation, and the
user should be able to assume that the device suppliers have taken the necessary trouble. A burst condition
is entirely normal immediately after the SYNC telegram, for instance: triggered by the SYNC, all the nodes
that are operating synchronously try to send their data at almost the same time. A large number of arbitration
processes take place, and the telegrams are sorted in order of priority for transmission on the bus. This is
not usually critical, since these telegrams do contain some data bytes, and the telegrams trigger a sequence
of receive interrupts at the CAN nodes which is indeed rapid, but is nevertheless manageable.
Bus loading most often refers to the value averaged over several primary cycles, that is the mean value over
100-500 ms. CAN, and therefore CANopen, is indeed capable of managing a bus loading of close to 100%
over long periods, but this implies that no bandwidth is available for any repetitions that may be necessitated
by interference, for asynchronous error messages, parameterization and so on. Clearly, the dominant type of
communication will have a large influence on the appropriate level of bus loading: a network with entirely
cyclic synchronous operation is always in any case near to the worst case state, and can therefore be
operated with values in the 70-80% range. The figure is very hard to state for an entirely event-driven
network: an estimate must be made of how many events additional to the current state of the system might
occur, and of how long the resulting burst might last - in other words, for how long the lowest priority
message will be delayed. If this value is acceptable to the application, then the current bus loading is
acceptable. As a rule of thumb it can usually be assumed that an event-driven network running with a base
loading of 30-40% has enough reserve for worst-case scenarios, but this assumption does not obviate the
need for a careful analysis if delays could have critical results for the plant.
The BECKHOFF FC510x CANopen master cards / EL6751 CANopen master terminal display the bus load
via the System Manager. This variable can also be processed in the PLC, or can be displayed in the
visualization system.
The amount data in the process data objects is of course as relevant as the communication parameters: the
PDO mapping. [
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