CAN Networks

Brief description of technical details of the CAN vehicle networking protocol.

1. VEHICLE
NETWORKS
Introduction to CAN
Joseph Holly
jholly4@ gmail.com

2. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
NETWORK PROTOCOLS
There are multiple vehicle networking protocols,
each with its own history and function.
The most common protocol in light
vehicles is Controller Area Network
(CAN).
Even a single vehicle is likely to contain
multiple networks and multiple protocols.

3. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
The CAN Protocol
HISTORY - Originated by Bosch, 1983
- Released at SAE Congress, 1986
- Adopted by ISO, 1993
NOW - Multiple ISO Standards
- Numerous Derivatives
- Used in family vehicles, farm equipment, trucks,
medical equipment, industrial controls

4. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
CAN Features
 Low Data Overhead
- No addressing—all nodes receive all messages
- Easily add or delete nodes
 Efficient for Small Networks
- Effective error-trapping
- Automatic message arbitration
 Flexibility
- Multiple speeds possible on same network

5. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
Types of CAN Networks
 High-Speed (2-wire)
- up to 1 Mbps
- Used for: Engine, Suspension, Transmission, Anti-Skid
 Single-Wire
- up to 33 kbps, 88 kbps in High-Speed Mode
- Used for: Seat Controls, Mirror Controls
 Low-Speed (2-wire)
- up to 125 kbps
- Used for: Lights, Climate Control, IP Cluster, Doors

6. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
FOUR FRAME TYPES
- DATA
- REMOTE
- ERROR
- OVERLOAD

7. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
DATA Frame
Two Types: STANDARD and EXTENDED
 STANDARD
- 11-bit identifier
 EXTENDED
- 29-bit identifier

8. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
Structure of a DATA FRAME
START OF FRAME (SOF)
- Indicates a new transmission to all receiving nodes
- Also used by all receiving nodes for synchronization
SOF ARB DATA ACKCRCCONTROL EOF

9. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
Structure of a DATA FRAME
ARBITRATION FIELD
- Signals identity of originating source (ARB ID)
- 11-bit identifier in Standard Frame, 29 bit-identifier in Extended Frame
- Used to determine message priority
SOF ARB DATA ACKCRCCONTROL EOF

10. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
Structure of a DATA FRAME
CONTROL FIELD
- Six-Bit Field with two components:
- two one-bit flags
- last four bits identify length of the following data field, up to 8 bytes
SOF ARB DATA ACKCRCCONTROL EOF

11. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
Structure of a DATA
FRAME
DATA FIELD
– Actual message data
SOF ARB DATA ACKCRCCONTROL EOF

12. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
Structure of a DATA FRAME
CYCLIC REDUNDANCY CHECK (CRC)
– Error-trapping method used by controller to verify successful data
transmission. Every data frame is validated, retransmitted if necessary.
SOF ARB DATA ACKCR
C
CONTROL EOF

13. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
Structure of a DATA FRAME
ACKNOWLEDGEMENT FIELD
– Field in which receiving nodes signal that message was properly
received. If ACK is not received, originating node retransmits message.
SOF ARB DATA AC
K
CRCCONTROL EOF

14. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
Structure of a DATA
FRAME
END OF FRAME (EOF)
- Indicates end of transmission from originating node
- Signals beginning of Intermission period.
SOF ARB DATA ACKCRCCONTROL EO
F

15. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
REMOTE Frame
Used by any node to request data from any other node.
Same structure as Data Frame, except:
- Remote Frame has no message in Data Field
- Control Field used to indicate size of expected return data

16. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
ERROR Frame
Two Types: ACTIVE and PASSIVE
When a transmitting node sends an Error Frame, it is detected by
all the other nodes on the network. They respond by sending their
own error flags.

17. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
Structure of an ERROR
FRAME
ERROR FLAG ERROR
DELIMITER
ERROR FLAG
– Unique message sent by the node detecting an error. Any receiving
node will respond with its own error flag transmission.
ERROR DELIMITER
– “quiet” period following initial error flag transmission.

18. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
ERROR TYPES
Greater-than-allowed number of consecutive bits of
the same polarity trigger an Error Flag.
Stuff Error
Sending nodes verify their own transmissions and
re-send if there’s an inappropriate bit.
Bit Violation
A dominant bit in EOF, Interframe Space, CRC
Delimiter or ACK Delimiter triggers an Error.
Form
Violation
Every receiving node should acknowledge message.
If no ACK received, originating node generates Error
Flag and re-sends message.
ACK Error
If the checksum fails at any node, the Error Flag will
cause the originating node to re-send.
CRC Error

19. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
OVERLOAD Frame
Structurally similar to the Error Frame, but sent during the
Intermission period.
Originally used to allow an early ECU chip to keep up with
network traffic. Not usually needed by modern processors.

20. Introduction to CAN
Joseph Holly
jholly4@ gmail.com
CAN NETWORKS ERRORS DETECTED
(none shown)
MSG DATA
(shown in hex)
ARB ID
MSG LENGTH
CAN DATA on viewing software