What is arbitration field in CAN BUS . Does it use to identify each nodes on CAN Bus?



CAN bus has two signals, CAN high (CANH) and CAN low (CANL)

In diagram CAN uses an 11-bit identifier, which comes to a total of 211, i.e. 2048,

 

The arbitration field is used for collision avoidance. If two transmitters start at the same time and have the same sequence of dominant and recessive bits up to say the 5th bit, but at bit 6 one transmitter sends a recessive bit and the other sends a dominant bit, then the transmitter with the dominant bit wins the arbitration and the other one will back off and retry the transmission later. In numerical terms the identifier with the lowest number will win the arbitration because '0's correspond to dominant bits.

In order for the network to function smoothly the identifier space has to be divided up in such a way that it is impossible for multiple nodes to transmit the same identifier. How this is done distinguishes the many different varieties of CAN based networks.

 

Next is the remote transmission request (RTR) bit. Each receiving node in a CAN network, when detecting a high RTR bit, will now know that the received message is a Remote Frame.

What is remote frame in CAN bus ?

 

As near as we could tell from the original documents written by Bosch, it was intended to be used in conjunction with a zero length data field by a "master" device to request a data transmission from each node that received the message if it met the "mask and match" criteria. We made the conscious decision in DeviceNet not to use this feature at all -- EVER. In practice this meant that no transmitter should set the bit and all receivers should ignore the bit. The reason for this was to prevent a large number of devices from flooding the network with traffic. Yes, I know that the arbitration scheme will eventually let all the traffic get through, but we felt that a network with a greater level of determinism would have fewer problems than the alternative. I think we were right.