CAN
CX8050, CX8051 - Embedded-PCs for
CANopen and CAN
67
Version: 1.4
Bus access procedures
CAN utilizes the Carrier Sense Multiple Access (CSMA) procedure, i.e. all participating devices have the
same right of access to the bus and may access it as soon as it is free (multi-master bus access). The
exchange of messages is thus not device-oriented but message-oriented. This means that every message is
unambiguously marked with a prioritized identifier. In order to avoid collisions on the bus when messages
are sent by different devices, a bit-wise bus arbitration is carried out at the start of the data transmission. The
bus arbitration assigns bus bandwidth to the messages in the sequence of their priority. At the end of the
arbitration phase only one bus device occupies the bus, collisions are avoided and the bandwidth is optimally
exploited.
Configuration and parameterization
The TwinCAT System Manager allows all the CANopen parameters to be set conveniently. An "EDS" file (an
electronic data sheet) is available on the BECKHOFF website (
parameterization of BECKHOFF CANopen devices using configuration tools from other manufacturers.
Certification
The BECKHOFF CANopen devices have a powerful implementation of the protocol, and are certified by the
CAN in Automation Association (
).
9.2
Protocol description
9.2.1
Network Management
Simple Boot-Up
CANopen allows the distributed network to boot in a very simple way. After initialization, the modules are
automatically in the
Pre-Operational
state. In this state it is already possible to access the object directory
using service data objects (SDOs) with default identifiers, so that the modules can be configured. Since
default settings exist for all the entries in the object directory, it is in most cases possible to omit any explicit
configuration.
Only one CAN message is then required to start the module: Start_Remote_Node: Identifier
0
, two data
bytes: 0x01, 0x00. It switches the node into the
Operational state
.
Network Status
The states and the state transitions involved as CANopen boots up can be seen from the state diagram:
Содержание CX8050
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