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22 Fieldbus interface
3 Basics IL•1F CANopen DS301
0198441113586, V2.01, 11.2008
CAN message Data is exchanged via the CAN bus in the form of CAN messages. A
CAN message transmits the communication object and a variety of ad-
ministration and control information.
Figure 3.5 CAN message and simplified representation of CANopen mes-
sage
CANopen message For work with CANopen objects and for data exchange, the CAN mes-
sage can be represented in simplified form because most of the bits are
used for error correction. These bits are automatically removed from the
receive message by the data link layer of the OSI model, and added to
a message before it is transmitted.
The two bit fields "Identifier" and "Data" form the simplified CANopen
message. The "Identifier" corresponds to the "COB ID" and the "Data"
field to the data frame (maximum length 8 bytes) of a CANopen mes-
sage.
COB ID The COB ID (Communication OBject Identifier) has 2 tasks as far as
controlling communication objects is concerned:
• Bus arbitration: Specification of transmission priorities
• Identification of communication objects
An 11 bit COB identifier as per the CAN 3.0A specification is defined for
CAN communication; it comprises 2 parts
• Function code, 4 bits
• Node address (node ID), 7 bits.
Figure 3.6 COB ID with function code and node address
111 1 111 7
End-Bits
Acknowledge
CRC
Data
Control
RTR-Bit
Identifier
Start-Bit
>=36160..8 Byte
COB-ID
data carrier
11 Bit
7 Bit4 Bit
0..8 Byte
1 2 3 4 5 6 70
CANopen message (simplified)
CAN message
1
COB-ID
2 3 4 1 2 3 4 5 6 7
Function code
0...15
Node-ID
0...127
Bit:10 0