OPERATION

The CAN communication system is described below.

Each ECU communicating with CAN periodically sends several sensors’ information on CAN bus as data frame (called periodical sending data). For further details, consult the data frame section.
ECUs requiring data on CAN bus can receive data frames sent from each ECU simultaneously.
The data sent from each ECU conducting CAN communication is transmitted at 10-1000msec interval depending on necessity of data.

note In the figure above, the data frame A is transmitted in "a" intervals, while the data frames B and C are transmitted at intervals "b" and "c," respectively.
A single ECU transmits multiple data frames.
When data frames conflict with one another (when plural ECUs transmit signals simultaneously), data is prioritised for transmission by mediation, therefore, plural data frames are not sent simultaneously. For further details, consult the mediation section.
Data is transmitted not by the conventional voltage-using method but by voltage potential difference. For further details, consult the section on CAN bus voltage transformation.
Reliability of each ECU transmitting signals via CAN communication is secured by several error detection and recovery processes. For further details, consult the sections on error detection and system recovery.

MEDIATION

Because each ECU transmits data independently on the CAN bus, there are cases of data collision when multiple data frames that ECUs attempt to transmit simultaneously (if multiple ECUs transmit at nearly the same moment). At this moment, processing of the ECUs attempting transmission is performed in the following way.

Data frame with high priority is transmitted first according to ID codes memorized in data frames.
Transmission of low-priority data (data frames) is suspended by the issuing ECUs until the bus clears (when no transmission data exists on the CAN bus).

note If the suspended state continues for a specific time, new data (data frame content) is created and sent.
ECU containing suspended data frames transmits the data when the bus becomes available.

note

There is enough capacity on the CAN bus, which never prevents data frames from being sent.

VOLTAGE TRANSFORMATION ON THE CAN BUS

Data frame transmission through the CAN bus line involves voltage transformation (for output signals) in the distinctive CAN profile as follows. The ECU transmitting through the CAN_H and CAN_L bus lines sends 2.5 - 3.5V signals to the CAN_H side and 2.5 - 1.5V signals to the CAN_L side. The receiving ECU reads the data from the CAN_H and CAN_L potential difference. "Recessive" refers to the state where both CAN_H and CAN_L are under the 2.5 V state, and "Dominant" refers to the state where CAN_H is under the 3.5 V state and CAN_L is under the 1.5 V state. By transformation mainly to 2.5 V, even in cases when voltage is rendered 0 from faulty grounding or the like (causing a problem of an approximate 0.5 voltage increase on the communications line), communication can be continued uninterrupted. Employing dual communications lines improves reliability to prevent the presence of noise, compared to the conventional communication method.

DATA FRAMES

The data frame, which is sent from each ECU (or sensor) to the CAN bus line, consists of the fields bellow.
SOF (Start of Frame)

Indicates the start of the frame

ID (Identifier) region

Identifies the data content while specifying priority rank in case of mediation

Control region

Specifies the frame type, data length, etc.

Data region

Values used for data control, etc.

CRC (Cyclic Redundancy Check ) region

Region where to check for errors in sent data. The transmitting ECU calculates data regions by applying prescribed operations and stores the results. The receiving ECU detects erroneous communication by comparing the CRC region with the data region.

ACK (Acknowledge) region

Region where to conform the reception of sent data

EOF (End of Frame)

Indicates the end of the frame

ERROR DETECTION AND RECOVERY

CAN protocol secures its reliability of communication by providing several error detection function such as CRC shown in data frame, and the recovery function (recovery is performed by resending, from abnormal state such as transmission errors). If an error is detected but it is not resolved even after recovery, communication is stopped. This state is called "Bus Off."