CAN Technical introduction How does it function Principles of data exchange Non-destructive bitwise arbitration Efficiency of bus allocation Message frame formats Detecting and signalling errors Data reliability of the CAN protocol Extended format CAN message Implementations of the CAN protocol Overview CAN controller with intermediate buffer CAN controller with object storage CAN slave controllers for I/O functions Physical CAN connection How does it function Principles of data exchange When data are transmitted by CAN, no stations are addressed, but instead, the content of the message (e.g. rpm or engine temperature) is designated by an identifier that is unique throughout the network. The identifier defines not only the content but also the priority of the message. This is important for bus allocation when several stations are competing for bus access. If the CPU of a given station wishes to send a message to one or more stations, it passes the data to be transmitted and their identifiers to the assigned CAN chip ("Make ready"). This is all the CPU has to do: To initiate data exchange. The message is constructed and transmitted by the CAN chip. As soon as the CAN chip receives the bus allocation ("Send Message") all other stations on the CAN network become receivers of this message ("Receive Message"). Each station in the CAN network, having received the message correctly, performs an acceptance test to determine whether the data received are relevant for that station ("Select"). If the data are of significance for the station concerned they are processed ("Accept"), otherwise they are ignored. A high degree of system and configuration flexibility is achieved as a result of the content-oriented addressing scheme. It is very easy to add stations to the existing CAN network without making any hardware or software modifications to the existing stations, provided that the new stations are purely receivers. Because the data transmission protocol does not require physical destination addresses for the individual components, it supports the concept of modular electronics and also permits multiple reception (broadcast, multicast) and the synchronization of distributed processes: measurements needed as information by several controllers can be transmitted via the network, in such a way that it is unnecessary for each controller to have its own sensor.

Broadcast transmission and acceptance filtering by CAN nodes Principle of non-destructive bitwise arbitration Non-destructive bitwise arbitration For the data to be processed in real time they must be transmitted rapidly. This not only requires a physical data transfer path with up to 1 Mbit/s but also calls for rapid bus allocation when several stations wish to send messages simultaneously. In real-time processing the urgency of messages to be exchanged over the network can differ greatly: a rapidly changing dimension (e.g. engine load) has to be transmitted more frequently and therefore with less delays than other dimensions (e.g. engine temperature) which change relatively slowly. The priority at which a message is transmitted compared with another less urgent message is specified by the identifier of the message concerned. The priorities are laid down during system design in the form of corresponding binary values and cannot be changed dynamically. The identifier with the lowest binary number has the highest priority. Bus access conflicts are resolved by bitwise arbitration on the identifiers involved by each station observing the bus level bit for bit. In accordance with the "wired and" mechanism, by which the dominant state (logical 0) overwrites the recessive state (logical 1), the competition for bus allocation is lost by all those stations with recessive transmission a