Chapter 1
Introduction
©
National Instruments Corporation
1-3
Figure 1-1.
Example of CAN Arbitration
CAN Frames
In a CAN network, the messages transferred across the network are called
frames. The CAN protocol supports two frame formats as defined in the
Bosch version 2.0 specifications, the essential difference being in the
length of the arbitration ID. In the standard frame format (also known as
2.0A), the length of the ID is 11 bits. In the extended frame format (also
known as 2.0B), the length of the ID is 29 bits. The ISO 11898 specification
supports only the standard frame format. Figure 1-2 shows the essential
fields of the standard and extended frame formats, and the following
sections describe each field.
Figure 1-2.
Standard and Extended Frame Formats
Start of Frame (SOF)
Start of Frame is a single bit (0) that marks the beginning of a CAN frame.
S
S
Device B Loses Arbitration
Device A Wins Arbitration and Proceeds
Device A
ID = 11001000111 (647 hex)
Device B
ID = 11011111111 (6FF hex)
S= Start Frame Bit
Standard Frame Format
Extended Frame Format
S
O
F
R
T
R
I
D
E
A
C
K
11-Bit
Arbitration ID
DLC
0-8 Data bytes
15-Bit CRC
End of Frame
S
O
F
I
D
E
R
T
R
High 11 Bits
of Arbitration ID
Low 18 Bits
of Arbitration ID
DLC
0-8 Data bytes
A
C
K
15-Bit CRC
End of Frame
