Chapter 8 - Section 5: Controller Area Network (CAN) Bus

Question 1

What are the data rates for a CAN bus?

Answer 1

• 1 Mbps for cable lengths <40m
• As cable length increases, the data rate falls, 125 kbps for 500 metres in length

Question 2

How are data collisions avoided on the bus?

Answer 2

Carrier Sense Multiple Access (CSMA)/Arbitration based on Message Priority (AMP) technique

Question 3

What does CSMA ensure?

Answer 3

A terminal will transmit only when the bus is quiet (no carrier)

Question 4

What happens if two or more terminals try to transmit at the same time?

Answer 4

The AMP bus arbitration logic will connect the terminal with the higher-priority message

Question 5

What is the normal limit to the number of terminals connected to the bus?

Answer 5

32 to avoid data delay

Question 6

What is the data encoding of the CAN bus?

Answer 6

Non-return to zero

Question 7

What are the advantages of the CAN bus?

Answer 7

• No separate master bus controller
• No need to retransmit a message due to non-destructive bitwise arbitration
• Only one message on the bus at any one time
• CAN bus will not collapse or lock up

Question 8

Why is there no SDI on a CAN bus?

Answer 8

Each LRU has a filter to accept only those messages relevant to it