Can Frames; Start Of Frame (Sof); Figure 1-1. Example Of Can Arbitration; Figure 1-2. Standard And Extended Frame Formats - National Instruments NI-CAN User Manual

CAN Frames

Standard Frame Format
S
11-Bit
O
Arbitration ID
F
Extended Frame Format
S
High 11 Bits
O
of Arbitration ID
F
© National Instruments Corporation
Device A
ID = 11001000111 (647 hex)
Device B
ID = 11011111111 (6FF hex)
S= Start Frame Bit
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.
R I
T D
DLC 0-8 Data bytes
R E
I R
Low 18 Bits
D T
of Arbitration ID
E R

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

Figure 1-1. Example of CAN Arbitration

A
C
15-Bit CRC
K
0-8 Data bytes
DLC
1-3
Chapter 1
Device B Loses Arbitration
Device A Wins Arbitration and Proceeds
End of Frame
A
C
15-Bit CRC End of Frame
K
NI-CAN User Manual
Introduction