Mitsubishi Evolution X. Manual — part 113

SYSTEM OPERATION

TSB Revision

CONTROLLER AREA NETWORK (CAN)

54C-5

SYSTEM OPERATION

M2542000300272

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 10-10000 msec
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 simulta-
neously), 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
specific time, new data (data frame content) is
created 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.

.

AC206267

ECU-1

A

A

A

A

B

B

B

C

C

A

B

Interval "a"

Interval "b"

Interval "c"

C

C

C

B

B

A

A

A

ECU-1

ECU-2

CAN bus

Data frames

AC206267AD

Transmission
suspended by
mediation

Re-
transmission

SYSTEM OPERATION

TSB Revision

CONTROLLER AREA NETWORK (CAN)

54C-6

VOLTAGE TRANSFORMATION ON THE CAN-B BUS and CAN-C BUS

The voltage transformation (output signal) when the
data frame is sent to the CAN-C bus line has a dis-
tinctive CAN-C profile. The transmitting ECU through
the CAN_H and CAN_L bus lines sends 2.5 to 3.5 V
signals to the CAN_H side and 2.5 to 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 "Domi-
nant" refers to the state where CAN_H is under the
3.5 V state and CAN_L is under the 1.5 V state. On
the other hand, the voltage transformation (output
signal) when the data frame is sent to the CAN-B bus
line has a distinctive CAN-B profile. The transmitting
ECU through the CAN_H and CAN_L bus lines

sends 0 to 4 V signals to the CAN_H side, and 1 to 5
V signals to the CAN_L side. "Recessive" is a state
in which CAN_H is at 0 V and CAN_L is at 5 V, and
"Dominant" is a state in which CAN_H is at 4 V and
CAN_L is at 1 V. 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 V 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.

.

AC507623

CAN_H

3.5 V

2.5 V

1.5 V

0

1

5 V

4 V

1 V

0 V

0

1

AB

CAN_L

<CAN-C>

<CAN-B>

VOLTAGE
TRANSFORMATION
IN CAN_L

STATE:

LOGIC VALUE:

RECESSIVE

RECESSIVE

DOMINANT

VOLTAGE
TRANSFORMATION
IN CAN_H

RECESSIVE

RECESSIVE

DOMINANT

SYSTEM OPERATION

TSB Revision

CONTROLLER AREA NETWORK (CAN)

54C-7

DATA FRAMES

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

SOF (Start of Frame)

• Indicates the start of the frame

ID (Identifier) region

• Identifies the data content while specifying prior-

ity rank in case of mediation

Control region

• Specifies the frame type, data length, etc.

Data region

• Values used for data control, etc.

Cyclic Redundancy Check (CRC) 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."

AC504950

SOF

EOF

AB

ID

REGION

CRC

REGION

ACK

REGION

CONTROL
REGION

DATA REGION

DATA FRAMES

SELF-DIAGNOSIS

TSB Revision

CONTROLLER AREA NETWORK (CAN)

54C-8

SELF-DIAGNOSIS

M2542000400235

• CAN self-diagnosis is performed by each ECU

connected to the CAN bus.

• diagnosis codes related to communication are

named with the capital letter U, and are called
"U-codes."

• A summary of the CAN self-diagnosis system is

presented below. For further details on each
diagnosis code stored by the ECUs.

.

TIME-OUT

Each ECU transmits data frames periodically. If the
data frame is not received within the specified period,
the intended receiving ECU transmits a diagnosis
code indicating communication time-out for the ECU
that failed to transmit.

.

BUS OFF

Related to a communication error that persists even
after the transmitting ECU attempts recovery for a
specified number of attempts or that persists for a
specified period after recovery. Communication is
stopped and this diagnostic code is issued.

CAN BUS DIAGNOSTICS

M2542000500254

As ECUs are connected via CAN bus (including scan
tool), always diagnose CAN bus to confirm its nor-
mality when inspecting. Simply by performing scan
tool screen operations, the following inspections can
be performed automatically, and the result can be
used to verify the CAN bus status.

• Confirmation of diagnosis code of ETACS-ECU

• Confirmation of communication of all ECUs