Beckhoff CX1500-M510 Manual page 104

Product overview
Variable mapping
As a rule, the default mapping of the process data objects already satisfies the requirements. For special
types of application the mapping can nevertheless be altered: the Beckhoff CANopen Bus Couplers, for
instance, thus support variable mapping, in which the application objects (input and output data) can be
freely allocated to the PDOs. The mapping tables must be configured for this: as from Version 4 of
CANopen, only the following procedure is permitted, and must be followed precisely:
1. First delete the PDO (set 0x1400ff, or 0x1800ff, sub-index 1, bit 31 to "1")
2. Set sub-index 0 in the mapping parameters (0x1600ff or 0x1A00ff) to "0"
3. Change mapping entries (0x1600ff or 0x1A00ff, SI 1..8)
4. Set sub-index 0 in the mapping parameters to the valid value. The device then checks the entries for
consistency.
5. Create PDO by entering the identifier (0x1400ff or 0x1800ff, sub-index 1).
Dummy Mapping
A further feature of CANopen is the mapping of placeholders, or dummy entries. The data type entries stored
in the object directory, which do not themselves have data, are used as placeholders. If such entries are
contained in the mapping table, the corresponding data from the device is not evaluated. In this way, for
instance, a number of drives can be supplied with new set values using a single CAN telegram, or outputs on
a number of nodes can be set simultaneously, even in event-driven mode.
2.3.8.3 PDO Parameterization
Even though the majority of CANopen networks operate satisfactorily with the default settings, i.e. with the
minimum of configuration effort, it is wise at least to check whether the existing bus loading is reasonable:
80% bus loading may be acceptable for a network operating purely in cyclic synchronous modes, but for a
network with event-driven traffic this value would generally be too high, as there is hardly any bandwidth
available for additional events.
Consider the Requirements of the Application
The communication of the process data must be optimized in the light of application requirements which are
likely to be to some extent in conflict. These include
• Little work on parameterization - useable default values are optimal
• Guaranteed reaction time for specific events
• Cycle time for regulation processes over the bus
• Safety reserves for bus malfunctions (enough bandwidth for the repetition of messages)
• Maximum baud rate - depends on the maximum bus length
• Desired communication paths - who is speaking with whom
The determining factor often turns out to be the available bus bandwidth (bus load).
Baud Rate
Baud Rate
We generally begin by choosing the highest baud rate that the bus will permit. It should be borne in mind that
serial bus systems are fundamentally more sensitive to interference as the baud rate is increased. The
following rule therefore applies: just as fast as necessary. 1000 kbit/s are not usually necessary, and only to
be unreservedly recommended on networks within a control cabinet where there is no electrical isolation
between the bus nodes. Experience also tends to show that estimates of the length of bus cable laid are
often over-optimistic - the length actually laid tends to be longer.
Determine the Communication Type
Once the baud rate has been chosen it is appropriate to specify the PDO communication type(s). These
have different advantages and disadvantages:
104 Version: 1.0 CX1500-M510, CX1500-B510