SYSTEM OPERATION
CONTROLLER AREA NETWORK (CAN)
54C-5
• When data frames conflict with one another
(when plural ECUs transmit signals simultane-
ously), 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 volt-
age-using method but by voltage potential differ-
ence. 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.
• For major communication signals (transmitting
signals) among ECUs.
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.
1. Data frame with high priority is transmitted first
according to ID codes memorized in data frames.
2. 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 spe-
cific time, new data (data frame content) is cre-
ated and sent.
3. ECU containing suspended data frames trans-
mits 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
− 3.5 V signals to the CAN_H side
and 2.5
− 1.5 V 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
earthing or the like (causing a problem of an approxi-
mate 0.5 voltage increase on the communications
line), communication can be continued uninter-
rupted. Employing dual communications lines
improves reliability to prevent the presence of noise,
compared to the conventional communication
method.
AC204753
Voltage
transformation
in CAN_L
State:
Logical value:
Recessive
Recessive
Dominant
3.5 V
2.5 V
1.5 V
0
1
AE
Voltage
transformation
in CAN_H