CANopen Programmer's Manual - Maccon.de

CANopen Programmer’s Manual 1: Introduction
The CAN Message
Overview
CANopen messages are transmitted within CAN messages (a CAN message is also known as a
communication object or COB).
CAN Message Format
CAN messages are communicated over the bus in the form of network packets. Each packet
consists of an identifier (CAN message ID), control bits, and zero to eight bytes of data.
CRC Error Checking
Each packet is sent with CRC (cyclic redundancy check) information to allow controllers to identify
and re-send incorrectly formatted packets.
CAN Message ID
Every CAN message has a CAN message ID (also known as COB-ID). The message ID plays two
important roles.
It provides the criteria by which the message is accepted or rejected by a node.
It determines the message’s priority, as described below.
CAN Message Priority
The priority of a CAN message is encoded in the message ID. The lower the value of the
message ID, the higher the priority of the message. When two or more devices attempt to transmit
packets at the same time, the packet with the highest priority succeeds. The other devices back
off and retry.
This method of collision handling allows for a high bandwidth utilization compared to other network
technologies. For instance, Ethernet handles collisions by requiring both devices to abort
transmission and retry.
For More Information
For more information on the CAN protocol, see CAN Specification 2.0, Robert Bosch GmbH, and
ISO 11898, Road Vehicles, Interchange of Digital Information, Controller Area Network (CAN) for
high-speed communication.
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