The next bit is the Remote Transmission Request. This repeats until all the messages have been sent in the order of priority, as shown below: CAN message priority competition Once this message has been sent, the other lower priority transmitters restart the process. This continues until only one transmitter is left to send its message. This tells the affected transmitter to delay transmitting, as it is not the highest priority.
If multiple transmitters send a message simultaneously, they detect this when they detect they are sending recessive logic level 1 but the actual Arbitration Field bit on the bus is a dominant level 0. The transmitters identify a message priority when they start to send a message. This is followed by the Arbitration Field, which indicates the priority of the data.
Usually, the CAN bus will be in an idle state (1), so to identify the beginning of the message, the dominant 0 signal is sent, which wins over recessive 1. The message begins with the Start Frame, which indicates the start of the message. In the CAN differential transmission lines, the dominant logic level is low or 0, while the recessive logic level is high or 1.īelow is a complete CAN protocol frame for a standard 11-bit message: CAN data message frame Electromagnetic disturbance affecting both differential pair wires Because there is no reference ground in a differential pair, the CAN bus voltage is measured from the difference between the paired differential transmission lines. This is because the two signal wires are kept very close to each other, so when the electromagnetic interference affects one transmission line, it will equally affect the other. Simple CAN wiring example with 120 Ω termination resistors at the endsĪ differential pair transmission line is much more robust and immune from environmental interference and noise. 120 Ω termination resistors are used at each end of the differential lines, as shown in the circuit diagram below. Flexible Data Rate - 15 Mbps data rate and 10 meters maximum bus lengthĬAN bus uses differential transmission lines and therefore does not require a ground connection.High Speed (or Hi-Speed) - 1 Mbps data rate and 40 meters maximum bus length.Low Speed - 125 kbps data rate and 500 meters maximum bus length.
There are three different speed types for CAN buses, which are: Also, the nodes determine each message’s priority if several messages are sent at the same time.
In a CAN bus, a transmitting device sends a message to all the CAN nodes, and each node decides how to react to the received message.
This article is part of the series Serial Communication Protocols. In this article, we will be looking at the popular CAN and LIN protocols together.