Beckhoff CX8050, Documentation

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CAN
CX8050, CX8051 - Embedded-PCs for
CANopen and CAN
66 Version: 1.4
9 CAN
9.1 Introduction
CANopen is a widely used CAN application layer, developed by the CAN in Automation association (CiA,
http://www.can-cia.org ), which has meanwhile been adopted for international standardization.
Device Model
CANopen consists of the protocol definitions (communication profile) and of the device profiles that
standardize the data contents for the various device classes.
Process data objects (PDO) [ } 71] are used for
fast communication of input and output data. The CANopen device parameters and process data are stored
in a structured object directory. Any data in this object directory is accessed via service data objects (SDO).
There are, additionally, a few special objects (such as telegram types) for network management (NMT),
synchronization, error messages and so on.
Communication Types
CANopen defines a number of communication classes for the input and output data (process data objects):
• Event driven [ } 71] : Telegrams are sent as soon as their contents have changed. This means that the
process image as a whole is not continuously transmitted, only its changes.
• Cyclic synchronous [
}
71] : A SYNC telegram causes the modules to accept the output data that was
previously received, and to send new input data.
• Requested [
}
71] : A CAN data request telegram causes the modules to send their input data.
The desired communication type is set by the Transmission Type [ } 71] parameter.
Device Profile
The BECKHOFF CANopen devices support all types of I/O communication, and correspond to the device
profile for digital and analog input/output modules (DS401 Version 1). For reasons of backwards
compatibility, the default mapping was not adapted to the DS401 V2 profile version.
Transmission Rates
Nine transmission rates from 10 kbaud up to 1 Mbaud are available for different bus lengths. The effective
utilization of the bus bandwidth allows CANopen to achieve short system reaction times at relatively low data
rates.
Topology
Topology [
}
26]
CAN is based on a linear topology. The number of devices participating in each network is logically limited by
CANopen to 128, but physically the present generation of drivers allows up to 64 nodes in one network
segment. The maximum possible size of the network for any particular data rate is limited by the signal
transit time required on the bus medium. For 1 Mbaud, for instance, the network may extend 25 m, whereas
at 50 kbaud the network may reach up to 1000 m. At low data rates the size of the network can be increased
by repeaters, which also allow the construction of tree structures.