1. Why is CAN called a Asynchronous protocol?

Asynchronous in communication means the sender transmits data using its own timing source rather than sharing a timer or clock with the receiver. So, since CAN uses its own timing source and the bit timing parameters are calculated using this timing source. it is called as a Asynchronous protocol.

2. Why is CAN called a serial protocol?

Serial in communication means sending one bit followed by another bit, rather than multiple bits over multiple wires simultaneously. Since, can sends data over one wire one bit after another bit it is called as a serial protocol.

3. Why is CAN called a half-duplex protocol?

CAN is called a half duplex protocol as only one node has access to the bus at a time.

4. What is the meaning of CSMA in CAN?

CSMA- Carrier Sense Multiple Access means each node must wait for a prescribed amount of period of inactivity before attempting to send a message.

5. What is CD+AMP?

CD+AMP- Collision Detection with Arbitration on Message Priority means the collisions are detected and resolved using bitwise arbitration.

6. What is Dominant bit in CAN?

The Dominant Bit represents “0” and has logical voltage range from 1.5 to 3.5V.

7. What is Recessive bit in CAN?

The Recessive Bit represents “1” and has logical voltage of 2.5V.

8. Explain different properties of CAN.

1. Prioritization of messages – Higher priority message will be transmitted first. The priority of messages is decided using CAN arbitration.
2. Multicast reception with time synchronization – Messages will go to all nodes, but nodes will decide whether to act on it.
3. System wide data consistency – Every node is informed about error, due to this destroyed message will be retransmitted again until it is successful, thus data consistency is maintained.
4. Multi-master – Multiple ECU’s will try to gain the bus access but ECU message with highest priority will get the bus access.
5. Error Detection & signaling – Any node can detect the error in message & tell that using the error frame.

8. How much data can a single CAN message hold?

A single CAN message can hold up to 64 signals if each signal is of size 1 bit. Therefore the size of 1 CAN message is equal to 8 bytes.

9. Explain CAN Arbitration process.

The priority of transmission of messages in CAN is based on identifier. The lower the value of the identifier, the priority is higher and vice versa.

Example–

ECU1 : Transmits a message with Identifier: 0x1A1

ECU2 : Transmits a message with Identifier: 0x100

ECU3 : Transmits a message with Identifier: 0x21A

Here the priority will be given to ECU2, since it has the identifier with lowest vale.

This is how the process of arbitration takes place in CAN.

10. Why is CAN bus termination done with a 120 ohm resistor?

CAN is designed to be used with shielded twisted cable and with 120 ohm characteristic impedance.

The termination resistance of 120 ohm is used to prevent wrong interpretation of data because of electronic interference and impedance mismatch i.e. Transmitter transmits dominant bit but the receiver receives recessive bit.

The 120 ohm resistance absorbs signals so they do not reflect back to the source.

Without the 120 ohm resistance there is a possibility that error frames are present in the CAN trace window.

With the 120 ohm resistance we can see that there wont be any error frames in the CAN trace window.

11. What is Wired AND logic in CAN?

There are only two logic state in CAN i.e. “0” or “1”.

So Dominant Bit i.e. “0” always prevails above Recessive Bit i.e. “1”.