Communication Busses

Question 1

What’s the format of a CAN frame

Answer 1

Start of Frame, 11 bit identifier, RTR, Control, Data (8 bytes), CRC, Ack, Eof

Question 2

What’s the format of a CAN FD Frame

Answer 2

29 bit identifier instead of 11, adds

Question 3

What’s Vector CAN

Question 4

What kind of signaling does CAN use?

Answer 4

2 wire - differential. C_low, C_high. Dominant (0) signal is CANH at 3.5V and CANL at 1.5V. Recessive (1) is both at 2.5V (driven by 120ohm resistor). Passive transition (dominant to recessive) is slower and depends on lenght of the CAN network

Question 5

What’s the topology of a CAN bus?

Answer 5

multi-master linear bus, multi-drop. Multi-drop meaning each node has a unique id it uses to determine whether messages are for it

Question 6

What’s the topology of an Ethernet network?

Answer 6

Ethernet supports multiple topologies

Question 7

What’s special about a CAN cable?

Answer 7

It uses differential signaling (2.5V recessive (1), 3.5 and 1.5V dominant (0)) and 120ohm termination resistors

Question 8

Can any device on a CAN bus transmit whenever it wants to?

Answer 8

Yes, CANBus is not master-slave. It allows multi-masters and collisions are arbitrated by transmitting identifier

Question 9

How are collisions handled in CAN

Answer 9

Lower message IDs are let through first

Question 10

Why do CAN cables have termination resistors

Answer 10

To return bus to recessive state (2.5V for low and high)