The contents of the Arbitration Field is, per the Standard, used to determine the message’s priority on the bus. The difference between these two concepts is small but significant. A contents-addressed message is like “Here’s a message containing data labeled X”. CAN messages are said to be “contents-addressed”.Ī conventional message address would be used like “Here’s a message for node X”. Instead, the contents of the messages is identified by an identifier which is present somewhere in the message. In fact, there is no notion of message addresses in CAN. Each CAN controller will pick up all traffic on the bus, and using a combination of hardware filters and software, determine if the message is “interesting” or not. It is worth noting once again that there is no explicit address in the CAN messages. So the node will continue to transmit forever, at least until someone acknowledges the message. By an special rule in the error confinement algorithm, the transmit error counter is not further increased if the node is error passive and the error is an ACK error. This will happen 16 times then the transmitter will go error passive. But when the time comes for acknowledging… no node will send a dominant bit during the ACK slot, so the transmitter will sense an ACK error, send an error flag, increase its transmit error counter by 8 and start a retransmission. Q: What happens if a node is alone on the bus and tries to transmit?Ī: The node will, of course, win the arbitration and happily proceeds with the message transmission. Since the bus is wired-and and a Dominant bit is logically 0, it follows that the message with the numerically lowest Arbitration Field will win the arbitration. There is one exception to this rule: if the message contains no data, then any node may transmit that message. No time is lost in the arbitration process.Īn important condition for this bit-wise arbitration to succeed is that no two nodes may transmit the same Arbitration Field. The other potential transmitters will try to retransmit their messages when the bus becomes available next time. This node continues the transmission as if nothing had happened. The arbitration is performed over the whole Arbitration Field and when that field has been sent, exactly one transmitter is left on the bus.
If a node detects a dominant level when it is sending a recessive level itself, it will immediately quit the arbitration process and become a receiver instead. The transmitting nodes monitor the bus while they are sending. The conflict is resolved in the following way. This may result in two or more controllers starting a message (almost) at the same time. The message arbitration (the process in which two or more CAN controllers agree on who is to use the bus) is of great importance for the really available bandwidth for data transmission.Īny CAN controller may start a transmission when it has detected an idle bus. Of course, a “Full CAN” controller can communicate with a “Basic CAN” controller and vice versa. Today, most CAN controllers allow for both programming models, so there is no reason to use the terms “Full CAN” and “Basic CAN” – in fact, these terms can cause confusion and should be avoided. To distinguish between the two programming models, people for some reason termed the Intel way as “Full CAN” and the Philips way as “Basic CAN”. Then came along Philips with the 82C200 which used a FIFO- (queue-) oriented programming model and limited filtering abilities. Once upon a time there was the Intel 82526 CAN controller which provided a DPRAM-style interface to the programmer. The terms “Basic CAN” and “Full CAN” originate from the childhood of CAN.
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